Evaluation of Carbon Capture Methodologies,Mechanisms,and Improvements for Sustainable Carbon Dioxide Mitigation Using Microalgae

Abstract

This review article presents a comprehensive evaluation of carbon capture (CC) methodologies, with a focus on sustainable carbon dioxide (CO₂) mitigation using microalgae. It highlights the urgency of addressing escalating atmospheric CO₂ levels owing to their significant contribution to global warming and explores various CC techniques. Special emphasis is placed on microalgae-based strategies, which offer a promising solution for biologically sequestering CO₂ efficiently and cost-effectively while converting it into valuable biomass, which then can be used for various applications that include bioenergy. The article examines various mechanisms identified and involved in CO₂ assimilation, methodologies, and possible improvements in CO₂ capture and biomass conversion by microalgae. It also assesses the economic feasibility of microalgae cultivation for CC, suggesting that large-scale implementation could sequester substantial amounts of CO₂ annually and yield significant biomass for applications such as integrating wastewater treatment and flue gas utilization, thereby contributing to a sustainable global bioeconomy and mitigating climate change impacts.

Introduction

The period marked by revolutionary industrial transformations and relentless energy requirements has resulted in an unparalleled surge in greenhouse gas (GHG) emissions, posing a grave global threat. Carbon dioxide (CO₂), one of the eight principal GHGs, significantly contributes to this surge in emissions. The escalation in GHG emissions is persistently rising at an alarming pace, having already surpassed 412 parts per million (ppm) of CO₂ equivalent in 2022 and projected to ∼685 ppm of CO₂ equivalents by 2050.^1,2 The negative effects of excessive CO₂ emissions on the environment can range from complex climate shifts to increased air pollution, faster global warming, receding glaciers, ocean acidification, unpredictable drought patterns, and lower crop yields.³ In addition to the obvious dangers to human health, these occurrences have a multiplicity of negative effects, including a decline in agricultural output, a loss of biodiversity in aquatic habitats, and heavy economic losses.

The predominant strategies for carbon management in recent years have heavily relied on carbon capture (CC) and storage or CC, storage, and utilization technologies as the primary solution to mitigate CO₂ emissions from human activities.⁴ Current carbon management practices involve extracting CO₂ industrial exhaust gases through solvent-based physical or chemical absorption methods. These methods include the use of amine-based solvents such as Rectisol and Selexol, which absorb CO₂ from the gases. Solid adsorbents such as calcium oxide are also employed for CO₂ capture.⁵ The captured CO₂ is separated and concentrated through thermal regeneration or calcination processes. In thermal regeneration, heating the CO₂-laden solvent releases and collects the CO₂. Calcination heats the solid adsorbent to release and capture the CO₂ for further utilization or storage.⁶ Underground geological reservoirs store intense CO₂. It can boost recovery in oil and gas operations and produce stable carbonates in saline aquifers and basaltic rocks.⁷ Postcombustion is a widely studied and established technology for efficiently removing CO₂ from industrial emissions. It is one of several methods available for CC, storage, and utilization. It efficiently isolates CO₂ from industrial exhaust gases by absorption, making it widely used in industry.⁸ The CO₂ extraction from these sources requires enrichment owing to the low concentration (3–20%) and partial pressure (0.03–0.2 bar) of flue gases. Several chemical solvents, including as monoethanolamine (MEA), diethanolamine (DEA), 2-amino-2-methyl-1-propanol, and liquid ammonia, have been effectively used for this CO₂ collection and concentration stage.⁹

Another method of CO₂ capture is to use physical adsorbents such as methanol diethyl ethers, polyethylene glycol, and N-methyl-2-pyrrolidone. These materials successfully collect CO₂ by adsorbing it to their surfaces.^10,11 Furthermore, precombustion technologies turn fossil fuels into syngas, a blend of hydrogen and carbon monoxide, which facilitates CO₂ absorption. Oxy-fuel combustion uses pure oxygen rather than air, resulting in more effective CO₂ condensation. Chemical looping uses metal oxides to react with fuel, which aids in CO₂ capture and separation.¹² Advanced CO₂ extraction methods include membrane-based separation, cryogenic separation, and hydrate-based gas capture. Selective membranes filter CO₂ efficiently yet cost a lot to make and maintain. Liquefying CO₂ by cryogenic separation is efficient yet energy-intensive.^12,13 Hydrate-based gas separation absorbs CO₂ in the form of hydrates, offering a unique but difficult technique. Each approach has advantages and disadvantages, with notable difficulties such as the energy-intensive adsorbent renewal process and the high operational costs of vaporizers and reactors.¹⁴ Although CC, storage, and utilization techniques are used in a variety of industries, geological CO₂ storage necessitates ongoing risk assessment and monitoring to address concerns about oceanic acidification and potential CO₂ leakages while ensuring environmental safety and effectiveness.¹⁵

The use of biological techniques for the CC and storage has emerged as an efficient means of achieving a reduction in the overall amount of CO₂ emissions. These techniques encompass afforestation, oceanic fertilization, and microalgae-based CO₂ fixing. Nevertheless, afforestation and oceanic fertilization are frequently not preferred because they demand significant amounts of time, large areas of land or ocean, and may unintentionally impact marine biodiversity. Microalgae-based CO₂ fixation, in contrast, provides a swifter and more effective alternative, rendering it a more feasible choice for CC and storage on a wide scale.¹⁶ Addressing these constraints, microalgae-based CC, utilization, and storage technologies present a compelling solution, providing an efficient, cost-effective, and secure method for biologically sequestering CO₂ while simultaneously converting it into bioenergy and value-added byproducts.¹⁷ Harnessing microalgae-based carbon fixation offers vast potential to establish an innovative framework for tackling the hurdles of achieving net negative CO₂ emissions, preserving endangered biodiversity, mitigating air pollution, and fostering a sustainable global bioeconomy. Thus, this review presents an in-depth exploration of microalgae-based CC technologies, emphasizing their preference over other methods, advancements in CO₂ uptake, diverse biomass production, and CO₂ assimilation mechanisms from flue gas and wastewater, offering crucial insights for sustainable CO₂ mitigation efforts.

Methodologies for Carbon Capture

Methods for CC and sequestration of CO₂ have evolved over the years, with various physicochemical and biological approaches being developed (Table 1). Physicochemical methods for CC include chemical absorption, solid adsorption, gas separation membranes, and cryogenic distillation.²⁹ Biological approaches for CC and sequestration have gained significant attention as a promising solution. These encompass methods such as afforestation, oceanic fertilization, and microalgae-based CO₂ fixation (Fig. 1). Biological approaches for CC and sequestration, such as microalgae-based CO₂ fixation, have emerged as the most promising solution owing to their efficiency, cost-effectiveness, and potential for simultaneous bioenergy production and generation of valuable byproducts.³⁰ Furthermore, microalgae-based CC technologies have the advantage of being able to use flue gas and wastewater as sources of CO₂, making them highly versatile and adaptable in different industrial settings.²⁷ These methods have shown great promise in achieving sustainable CO₂ mitigation, providing a pathway toward net negative emissions and the establishment of a circular carbon economy.⁵,³¹ Table 1 demonstrates the three primary categories of CO₂ capture and the technology that correlate to each of them. There is a vast range of different separation techniques that can be used.

Table 1.

A Comparative Analysis of Various Carbon Capture Technologies

METHOD	MECHANISM	ADVANTAGES	SHORTCOMINGS	REFERENCE
Physical CO₂ capture
Absorption	This method involves using a liquid solvent to absorb CO₂ from a gas stream. The CO₂-rich solvent is then separated and can be stored.	High CO₂ capture efficiency Reliable and controllable	High cost Energy intensive	¹⁸
Adsorption	Adsorption process uses solid materials like activated carbon or zeolites to capture CO₂ from a gas stream by adsorbing it on the surface of materials.	High selectivity Low energy consumption	Limited capacity Complex process	¹⁹
Membrane separation	Membranes with selective permeability can separate CO₂ from other gasses based on differences in molecular size.	Energy efficiency Simple operation	Capital cost Limited application	^20,21
Chemical CO₂ capture
Amine-based absorption	This method uses amine solutions to absorb CO₂ from flue gas. The absorbed CO₂ is then released by heating the amine solution allowing for carbon storage.	High carbon capture efficiencyRegenerability	Energy intensiveCorrosion and degradation	²²
Solid sorbents	Solid sorbents such as metal organic frameworks or solid amines can adsorb CO₂ from gas streams and release under controlled conditions.	High selectivityLow energy consumption	Limited capacityCost sensitive to impuritiesComplex	²³
Cryogenic separation	This method involves cooling the gas stream to very low temperatures, causing CO₂ to condense and separate from other gasses.	High purityNo use of chemicals	Limited applications	²⁴
Geological CO₂ capture
Oil and gas reservoirs	This method involves injecting captured CO₂ into depleted oil and gas reservoirs. The CO₂ displaces hydrocarbons and remains trapped underground.	Economic benefitsInfrastructure development	Environmental impactEnvironmental accidents	⁷
Saline aquifer storage	In this method, CO₂ is injected into deep saline aquifers, which are porous rock formations with brine, and it is stored in pore spaces of the rock.	Large storage capacityMinimize land useStable geological conditions	High initial costRisk of leakage	⁷
Deep coal seams	CO₂ can be stored in deep coal seams by injecting into previously mined streams. The CO₂ is adsorbed onto coal surfaces and stored in a coal matrix.	Economic viability	Energy intensiveRisk of leakage	²⁵
Biological CO₂ capture
Afforestation	It involves growing of trees, farming the crops, and livestock.	No use of chemicals	Time consumingRequire large area
Oceanic fertilization	Oceans are enriched with iron and other nutrients which promote the increase of CO₂ uptake by phytoplankton.	Potential for CO₂ capture.	High costMay affect the marine life	²⁶
Microalgae-based carbon capture and utilization	Conversion of CO₂ into biofuels and other products by photosynthesis.	No requirement of landFaster growth rate than plantsHighly effective method inCO₂ concentration.	Sensitive at extreme conditions	^27,28

Fig. 1.

Various methodologies for CO₂ capture.

PHYSICAL METHOD OF CO₂ CAPTURE

The separation of CO₂ from gas streams is accomplished by the use of physical processes that include adsorption, absorption, and membrane separation in the technique of physical CO₂ capture technologies (Fig. 2). Several typical methods of physical CO₂ capture include the following:

Fig. 2.

Physical method of CO₂ capture: (a) adsorption, (b) absorption, and (c) membrane separation.

Adsorption

Adsorption involves the adhesion of CO₂ molecules onto a solid surface, typically a porous material like activated carbon or zeolites. The CO₂ molecules are captured on the surface of the adsorbent material, allowing for their separation from the gas stream (Fig. 2a).¹⁹

Absorption

Absorption is a process where CO₂ is dissolved in a liquid solvent, such as amines or ammonia, to form a solution. The CO₂-rich solvent is then separated from the gas stream, and the CO₂ can be released by heating the solvent in a process known as desorption (Fig. 2b).¹⁸

Membrane separation

In the process of membrane separation, selective membranes are used to extract CO₂ from gas mixtures on the basis of differences in permeability. The membranes allow CO₂ to pass through while inhibiting the passage of other gases, which enables the separation of CO₂ from the gas stream (Fig. 2c).²⁰

These physical CO₂ capture methods offer advantages such as energy efficiency, scalability, and potential for integration with existing industrial processes. However, they also have limitations such as high capital costs, energy requirements for regeneration, and challenges in achieving high purity CO₂ capture.

CHEMICAL METHOD OF CO₂ CAPTURE

Chemical methods for CO₂ capture involve the use of chemical reactions to selectively capture and separate CO₂ from gas streams (Fig. 3). Some common chemical CO₂ capture methods include the following:

Fig. 3.

Chemical method of CO₂ capture: (a) amine scrubbing, (b) carbonation, and (c) chemical looping.

Amine scrubbing

Amine scrubbing is a widely used chemical method where CO₂ is absorbed by aqueous amine solutions, for example, MEA or DEA. The CO₂ reacts with the amines to form stable carbamate compounds, which are then regenerated by heating to release the captured CO₂ (Fig. 3a).²²

Carbonation

Carbonation involves the reaction of alkaline materials, such as calcium oxide (lime) or magnesium oxide, with CO₂ to form stable carbonates. This process can be used in industrial applications for CO₂ capture and storage (Fig. 3b).³²

Chemical looping

Chemical looping is a process where a solid oxygen carrier, such as metal oxides, is used to transfer oxygen for the combustion of a fuel. In the context of CO₂ capture, chemical looping combustion can enable the separation of CO₂ from flue gasses without the need for energy-intensive separation processes (Fig. 3c).³³

Chemical CO₂ capture methods offer advantages such as high capture efficiency, potential for CO₂ utilization in the form of stable carbonates, and applicability to a wide range of industrial processes. However, challenges include the energy requirements for regeneration, solvent degradation, and cost considerations.^33,34

GEOLOGICAL METHOD OF CO₂ CAPTURE

Geological CO₂ capture, storage, and utilization (CCSU) involve the capture of CO₂ from industrial bases, transportation to geological storage sites, and injection into underground reservoirs for long-term storage or utilization. Some key aspects of geological CO₂ capture include the following:

Capture

The CC from industrial sources, for example, power plants or industrial facilities, is accomplished by the use of a variety of capture methods, including postcombustion capture, precombustion capture, and oxy-fuel combustion.³⁵ The captured CO₂ is then compressed for transportation.

Storage

Pipelines or ships are used to carry the captured CO₂ to geological storage sites that are suited for the purpose. These geological storage sites may include depleted oil and gas reserves, saline aquifers, or deep basalt formations.³⁶ The CO₂ is injected into these geological formations for secure storage.

Monitoring and verification

Monitoring techniques, for example, seismic surveys, pressure monitoring, and geochemical analysis, are engaged to ensure the integrity of the storage sites and verify the long-term containment of injected CO₂.^37,38 This is crucial for assessing the effectiveness and safety of geological CO₂ storage.

To reduce GHGs emissions and slow the progression of climate change, geological CC and storage present a potentially useful alternative. It is possible for geological CCSU to contribute to the achievement of climate targets and help avoid the release of CO₂ into the atmosphere by storing it safely beneath.

BIOLOGICAL METHOD OF CO₂ CAPTURE

Afforestation, oceanic fertilization, and microalgae-based biological methods aimed at capturing and storing CO₂ to mitigate climate change.

Afforestation

Afforestation involves the establishment of forests on lands that were previously not forested. Trees and vegetation naturally capture CO₂ from the air through photosynthesis process, storing CO₂ in their biomass. Afforestation projects help increase carbon sequestration, enhance biodiversity, improve soil health, and provide ecosystem services.³⁹

Oceanic fertilization

To encourage the growth of phytoplankton, oceanic fertilization makes use of the addition of nutrients to ocean waters. These nutrients include iron and nitrogen, among others. Microscopic marine algae known as phytoplankton are responsible for a significant portion of the carbon sequestration process that occurs through photosynthesis. The goal of oceanic fertilization is to increase the intake of CO₂ from the atmosphere and to boost the biological pump that carries carbon to the deep ocean. This is accomplished by achieving this goal through the enhancement of phytoplankton growth.²⁶

Microalgae-based method

One prominent biological method is microalgae-based CO₂ capture, which uses microalgae to sequester CO₂ and produce biomass. Here is a brief explanation of microalgae-based CO₂ capture:

Microalgae Cultivation

Microalgae are microscopic photosynthetic organisms that can grow in various aquatic environments, such as ponds, bioreactors, or open tanks. These microorganisms have a high CO₂ fixation capacity and can convert CO₂ into organic compounds through photosynthesis.

CO₂ Sequestration

Through the process of photosynthesis, microalgae are able to take in CO₂ from the surrounding air or from industrial sources such as flue gases. Microalgae are able to successfully sequester carbon in their biomass by converting CO₂ into carbohydrates, lipids, and proteins with their own metabolic processes.⁴⁰

Biomass Production

The biomass produced by microalgae can be harvested and processed to extract valuable products such as biofuels, animal feed, nutraceuticals, and bio-based chemicals. This biomass can also be employed for CC and storage applications.⁴¹

Microalgae-based CO₂ capture offers several advantages, including high CO₂ sequestration rates, rapid growth rates, and the potential for producing valuable bioproducts.⁴² In addition, microalgae cultivation can be integrated with wastewater treatment processes, providing a sustainable solution for CO₂ mitigation and resource recovery.⁴³

Mechanism of CO₂ Capture by Microalgae

Microalgae have the unique ability to capture and use CO₂ through a process called photosynthesis.⁴⁴ During photosynthesis, microalgae use energy from sunlight to convert CO₂ and water into oxygen and carbohydrates.³¹ These carbohydrates serve as a source of energy for the growth and reproduction of microalgae, whereas the oxygen is released back into the atmosphere. This process not only helps to reduce the levels of atmospheric CO₂ but also contributes to the production of biomass, which can be used for various bioproducts and biofuels. Furthermore, microalgae have developed various mechanisms to optimize their CO₂ capture efficiency. These mechanisms include the ability to concentrate CO₂ within their cells, the utilization of efficient carbon fixation pathways, and the optimization of light and nutrient availability for optimal photosynthesis and CO₂ fixation.

PHOTOSYNTHESIS

Photosynthesis is the fundamental process by which microalgae capture and convert CO₂ into biomass. During photosynthesis, microalgae use sunlight as an energy source to convert CO₂ and water into oxygen and organic compounds. These organic compounds, mainly in the form of carbohydrates, serve as the primary source of energy for microalgae’s growth and metabolic processes⁴⁵ (Fig. 4).

Fig. 4.

Photosynthesis process that takes place in microalgae and higher plants (exported from Kyoto Encyclopedia of Genes and Genomes).

OXYGEN PRODUCTION AND CO₂ FIXATION

During the process of photosynthesis, microalgae release oxygen back into the atmosphere as a byproduct.⁴⁴ Simultaneously, they fix CO₂ from the environment and incorporate it into their biomass. This dual function of microalgae, oxygen production, and CO₂ fixation makes them highly valuable in the context of mitigating climate change and reducing GHGs emissions.

CARBON CONCENTRATION MECHANISM

Microalgae have developed a specialized carbon concentration mechanism to optimize their CO₂ capture efficiency. This mechanism allows microalgae to actively accumulate and concentrate CO₂ within their cells, even in environments with low CO₂ concentrations. This carbon concentration mechanism involves the active transport of CO₂ across the cell membrane and its subsequent conversion into bicarbonate ions within the cells. These bicarbonate ions are then converted back into CO₂ at the site of carbon fixation, such as within the chloroplasts.³¹ This specialized mechanism enables microalgae to efficiently capture and use atmospheric CO₂, maximizing their potential as CO₂ fixers in biorefinery processes and other applications. Their ability to concentrate and use CO₂, coupled with their efficient carbon fixation pathways and optimization of light and nutrient availability, allows microalgae to maximize their photosynthetic efficiency and CO₂ fixation capacity.⁴⁴ Overall, microalgae are efficient in capturing and converting CO₂ through photosynthesis.³¹

MULTIPLE ROUTES OF CARBON ASSIMILATION

In addition to their efficient carbon concentration mechanism and photosynthetic pathways, microalgae have multiple routes of carbon assimilation. These routes allow microalgae to effectively assimilate and use carbon from various sources, including atmospheric CO₂, organic compounds, and dissolved inorganic carbon sources. These multiple routes of carbon assimilation enhance the versatility of microalgae as CO₂ fixers in biorefinery processes.³¹ The combination of a specialized carbon concentration mechanism, efficient photosynthetic pathways, and multiple routes of carbon assimilation makes microalgae highly effective in capturing and using CO₂ from various sources.⁴⁴ The ability of microalgae to competently capture and use CO₂ through their specialized carbon concentration mechanism, efficient photosynthetic pathways, and multiple routes of carbon assimilation makes them a promising tool for CO₂ capture and mitigation strategies. Microalgae have a specialized carbon concentration mechanism that allows them to efficiently capture CO₂ from the environment and convert it into biomass through photosynthesis. Their efficient carbon fixation pathways and multiple routes of carbon assimilation make microalgae highly effective in capturing and using atmospheric CO₂, making them a promising tool for CO₂ capture and mitigation strategies in biorefinery processes and other applications. Microalgae’s specialized carbon concentration mechanism, efficient photosynthetic pathways, and multiple routes of carbon assimilation make them a highly effective tool for capturing and converting CO₂.³¹

PHOTOSYNTHESIS CASCADE

Light dependent

During the light-dependent (LD) phase of photosynthesis, microalgae use light energy to generate adenosine triphosphate (ATP) and nicotinamide-adenine dinucleotide phosphate (NADPH), which are key molecules in driving the carbon assimilation process. During the LD phase of photosynthesis, microalgae use light energy to generate ATP and NADPH, which are crucial molecules for driving the carbon assimilation process in microalgae.⁴⁴ During the LD phase of photosynthesis, microalgae use light energy to generate ATP and NADPH, which are crucial for driving the carbon assimilation process in microalgae (Fig. 5). This process plays a vital role in the conversion of CO₂ into biomass and the production of valuable bioproducts in microalgae-based biorefineries.³¹ This process plays a vital role in the conversion of CO₂ into biomass and the production of valuable bioproducts, making microalgae-based biorefineries an environmentally friendly and sustainable solution for CO₂ capture and utilization. Microalgae’s efficient photosynthetic pathways and their ability to use different light wavelengths make them highly productive in capturing energy from sunlight for the conversion of CO₂ into biomass.

Fig. 5.

Light-dependent pathway of CO₂ fixation.

Dark reactions

In the dark reactions of photosynthesis, microalgae use ATP and NADPH generated during the LD phase to fix CO₂ into organic molecules such as sugars and lipids.⁴⁶ These organic molecules serve as the building blocks for biomass production and can be further processed to obtain various value-added products, including biofuels, food supplements, and pharmaceuticals. In addition, microalgae have the ability to use multiple routes of carbon assimilation, such as the Calvin cycle and alternative carbon fixation pathways (Fig. 6).⁴⁷ Overall, the photosynthetic process of microalgae plays a crucial role in capturing and converting CO₂ into biomass, offering a sustainable solution for CO₂ capture and utilization.⁴⁴ The efficient photosynthetic pathways of microalgae allow them to capture energy from sunlight and convert CO₂ into biomass through the LD and dark reactions of photosynthesis. The efficient photosynthetic pathways of microalgae allow them to capture energy from sunlight and convert CO₂ into biomass through the LD and dark reactions of photosynthesis.⁴⁸

Fig. 6.

Calvin cycle of carbon fixation pathway.

ALTERNATIVE MODES OF CO₂ ASSIMILATION

Microalgae demonstrate a remarkable capacity to use alternative pathways for CO₂ assimilation, surpassing the conventional Calvin cycle. These alternative modes, including the C4 and Crassulacean Acid Metabolism pathways, equip microalgae with the ability to thrive in diverse environmental settings while optimizing their efficiency in fixing CO₂.⁴⁹ Through these nuanced modes of CO₂ assimilation, microalgae effectively harness and transform CO₂ into biomass even amid formidable environmental challenges.⁵⁰

FLEXIBILITY AND ADAPTABILITY

Microalgae have emerged as a prime focus of research studies owing to their inherent flexibility and adaptability to different CO₂ concentrations. For instance, Vale et al.⁵¹ discuss microalgae’s rapid adaptability and high specific growth rates, which make them effective CO₂ absorbers compared with terrestrial plants.⁵¹ Wang et al.³¹ have provided extensive insights into the photosynthetic CO₂ fixation capabilities of microalgae, emphasizing their resilience and adaptability to variable CO₂ levels.³¹ These research efforts illuminate the potential of microalgae, positioning them as a versatile solution in the realm of CO₂ bioremediation and an instrumental component toward achieving carbon net-zero emissions.

SURVIVABILITY IN DIFFERENT CO₂ CONCENTRATIONS

Microalgae have exhibited varying degrees of adaptability and CO₂ fixation capabilities under different CO₂ concentrations, as highlighted by several research works.⁵² Patil and Kaliwal discussed the impacts of CO₂ concentrations on the biomass productivity and CO₂ fixation rates of several microalgae species.⁵³ Furthermore, the survival and growth efficiency of these microorganisms was scrutinized by Wang et al.,³¹ who examined the importance of process regulation to enhance fixation efficiency while optimizing CO₂ and light conditions for various strains. These studies illustrate a keen interest in using microalgae not only for their robust CO₂ bioremediation potential but also in advancing toward a sustainable future through micro algal biorefinery approaches.

Methodology of CO₂ Capture Using Microalgae

The methodology of CO₂ capture using microalgae involves several key steps.⁴⁴ These steps include selecting suitable microalgae strains with high CO₂ fixation potential, optimizing cultivation conditions to enhance CO₂ capture efficiency, implementing genetic and metabolic modifications to further enhance CO₂ fixation capability, and designing efficient photobioreactor systems for large-scale cultivation.

IDENTIFICATION AND SELECTION OF THE HIGH CO₂-TOLERANT ALGAL STRAINS

Within the framework of the CO₂ capture process, the initial phase involves the discovery and selection of algal strains that have a high tolerance to CO₂.³¹ These strains are characterized by their ability to thrive in environments with high levels of CO₂, making them ideal candidates for efficient CO₂ fixation. They are typically selected based on their biomass productivity, CO₂ fixation rates, and lipid potential. Furthermore, their tolerance to high CO₂ concentrations is crucial to ensure their survivability and optimal performance in CO₂ capture processes. In an investigation, a selection of river algal species was isolated and recognized for their potential in CO₂ sequestration, specifically species such as Spirogyra, Oscillatoria, and Chlorella, which were collected from a freshwater pond in a coal mining area. The blue–green alga, Oscillatoria, was a focus for the CO₂ capture study.⁵⁴ The CO₂ fixation potential of these selected strains was evaluated, and it was found that they exhibited a high capacity for CO₂ capture and biomass production. These findings highlight the importance of selecting microalgae strains with high CO₂ fixation potential for effective CO₂ capture. Furthermore, the choice of high CO₂-tolerant algal strains is crucial in ensuring their suitability for large-scale cultivation and maximizing their CO₂ capture potential.^31,55

OPTIMIZATION OF PROCESS PARAMETERS FOR CO₂ CONVERSION

The subsequent phase in the technology of CO₂ capture using microalgae involves fine-tuning the process parameters to enhance CO₂ conversion. Optimizing process parameters is essential for increasing the efficiency of CO₂ conversion during microalgal cultivation. This involves fine-tuning various environmental and operational conditions to maximize the photosynthetic capabilities of microalgae, thereby improving their CO₂ uptake and conversion into biomass. One key parameter is the CO₂ concentration supplied to microalgae. Elevated CO₂ levels can stimulate growth and increase the carbon fixation rate; however, it is essential to determine the optimal range as excessively high concentrations can inhibit growth. For example, studies have shown that microalgae such as Chlorella vulgaris exhibit an increase in growth rate and CO₂ sequestration efficiency within certain elevated CO₂ concentration ranges.⁵⁵

Light intensity and photoperiod are also pivotal factors. Microalgae require light for photosynthesis, but the optimal quantity and duration of light exposure must be meticulously calibrated. Varied species of microalgae have distinct light requirements, with some thriving under high light intensity, whereas others prefer lower levels. Adjusting the photoperiod can regulate the light/dark cycle to suit the metabolic rhythm of specific strains, enhancing CO₂ conversion efficiency. For instance, the studies conducted by Razzak et al.⁵⁶ have demonstrated the significance of optimizing light intensity and duration for different microalgal strains, showcasing substantial improvements in CO₂ conversion and biomass production under carefully controlled light conditions. In another study by Zhao et al.,⁵⁷ the researchers investigated the effect of light intensity on the growth and lipid production of Scenedesmus obliquus and found that a moderate light intensity led to the highest biomass and lipid content, indicating the importance of optimizing light conditions for specific desired outcomes.

Apart from CO₂ and light, nutrient availability also plays a vital role in the optimization process. Microalgae require essential nutrients such as nitrogen, phosphorus, and micronutrients for their growth and metabolism. Balancing the nutrient levels in the growth medium is critical to promoting CO₂ conversion efficiency and biomass production. In a study, researchers discovered that modifying the nutrient composition of the growth medium, specifically adjusting the nitrogen-to-phosphorus ratio, resulted in improved CO₂ conversion efficiency and biomass production for Chlorella sp.⁵⁸ Furthermore, research has shown that the addition of micronutrients, such as iron and trace metals, can enhance the growth and lipid accumulation of microalgae, further enhancing their ability to mitigate CO₂ and contribute to biomass production.⁵⁵

pH levels in the growth medium have been identified as a critical factor in the regulation of CO₂ fixation. Maintaining the appropriate pH range is crucial for the enzymatic activity involved in CO₂ uptake and carbon fixation during photosynthesis. Studies have demonstrated the significance of controlling pH to optimize CO₂ biofixation in microalgae. For instance, research by Qiu et al.⁵⁹ showcased the impact of pH regulation on the CO₂ biofixation efficiency of C. vulgaris, emphasizing the role of culture pH in governing the metabolic pathways responsible for CO₂ assimilation. Furthermore, the regulation of temperature and culture mixing can significantly influence CO₂ fixation. Temperature affects the metabolic rates of microalgae and the solubility of CO₂ in the growth medium, thus impacting CO₂ uptake and utilization. Studies have highlighted the importance of maintaining an optimal temperature range to support robust CO₂ fixation in microalgae.^49,50,52,60 In addition, efficient culture mixing is essential to ensure uniform CO₂ distribution and prevent CO₂ limitation in the growth medium, maximizing the overall CO₂ capture efficiency.^49,50,60

Process regulation also extends to the management of potentially inhibitory substances, such as oxygen and reactive oxygen species, which can negatively impact CO₂ fixation.⁴⁵ Oxygen evolution during photosynthesis can compete with CO₂ fixation, leading to reduced carbon assimilation. Strategies to mitigate the inhibitory effects of oxygen, such as adjusting the culture aeration rate or employing oxygen-scavenging mechanisms, are essential for enhancing CO₂ conversion efficiency.⁵¹ Nutrient availability, including sources of nitrogen, phosphorus, and trace minerals, is necessary to support algal growth and metabolism. The composition of the culture medium must be optimized to provide a balanced nutrient supply while avoiding excesses that can lead to eutrophication and inhibit CO₂ conversion.⁴³ Stirring and mixing are operational parameters that should not be overlooked. Proper mixing ensures that cells receive uniform light exposure and CO₂ access while preventing the settling of algal biomass. It also aids in temperature control and nutrient distribution throughout the culture medium. Hence, by effectively optimizing these parameters, microalgae can be engineered to become more efficient converters of CO₂, thereby not only mitigating GHGs emissions but also producing valuable biomass for biofuel, food supplements, and other bioproducts. For example, Arias et al.⁶¹ found significant CO₂ removal efficiency using a photobioreactor with optimized conditions for C. vulgaris, achieving up to 74% CO₂ removal from the airstream.

CO₂ CAPTURE IN PHOTO BIOREACTOR SYSTEMS

One of the prime factors to maximize CO₂ capture is the cultivation of microalgae in suitable photo bioreactor systems. The open pond system, the closed-culture system, and the tubular photobioreactor are some of the different types of photobioreactor systems that are available (Fig. 7). A variety of criteria, including scalability, cost-effectiveness, and the specific requirements of the microalgae species that are being farmed, are taken into consideration when selecting a photo bioreactor system. Each type of photo bioreactor system has both advantages and disadvantages for its operation. Figure 7 shows some of the common types of photo bioreactor systems used for CO₂ capture in microalgae cultivation. Open pond systems are cost-effective and easy to scale up, but they have limitations in terms of controlling light exposure, temperature, and contamination. Several researchers have focused on improving the design and operation of open pond systems to address these limitations, implementing strategies such as shading devices, mixing systems, and advanced monitoring and control techniques to optimize CO₂ capture and microalgae growth.^31,49,61,62 Wang et al.³¹ explored the use of a modified open pond system with baffles and paddlewheels for efficient CO₂ removal, achieving a CO₂ capture efficiency of 68%. Similarly, researchers implemented a novel approach by integrating an open pond system with a thin film photobioreactor to enhance CO₂ capture and microalgae growth, achieving a remarkable CO₂ capture efficiency of 72%.^31,63

Fig. 7.

Different photobioreactor systems: (1) Open system—(A) raceway pond (B) circular pond (C) rectangular stirred pond; (2) Closed system—(D) flat plate reactor (E) column reactor (F) Tubular reactor.

Closed-culture systems, on the contrary, provide better control over environmental factors such as light exposure, temperature, and contamination.⁶⁴ These systems are often more suitable for cultivating specific microalgae strains with precise requirements. The enclosed nature of closed-culture systems also reduces the risk of contamination, making them an ideal choice for high-value microalgae products such as pharmaceutical compounds and nutraceuticals.⁵⁸ Recent advancements in closed-culture systems, including the use of light-emitting diodes to provide tailored light spectra and intensities, have significantly improved CO₂ capture efficiency and microalgae growth rates.^28,65 In addition, advancements in tubular photobioreactor have led to enhanced CO₂ capture and microalgae cultivation efficiency. These systems offer a favorable surface-to-volume ratio, allowing for efficient light exposure and CO₂ utilization.⁴⁶ The tubular design also enables easy scale-up and operational flexibility, making it suitable for large-scale microalgae cultivation for biofuel and biomass production. Researcher Xu et al.⁶⁶ demonstrated the effectiveness of a tubular photobioreactor system in achieving high CO₂ capture efficiency and microalgae biomass productivity, highlighting its potential for sustainable carbon mitigation and bioenergy generation.

When selecting a photobioreactor system for CO₂ capture, it is essential to consider factors such as the specific requirements of the cultivated microalgae species, operational scalability, and cost-effectiveness. Each type of system offers distinct advantages and limitations, and the choice should align with the intended goals of CO₂ mitigation and biomass production. To further enhance CO₂ capture efficiency and microalgae cultivation, ongoing research is focusing on innovative design modifications and process optimization for all types of photobioreactor systems. These efforts aim to address challenges related to light exposure, temperature control, and contamination while maximizing the potential of microalgae as a sustainable solution for CO₂ biofixation and bioresource generation.

Improvements in CO₂ Uptake by Microalgae

Improvements in CO₂ uptake by microalgae can be achieved through various approaches, including physicochemical engineering of cultivation systems to optimize CO₂ delivery and availability. Genetic engineering of microalgae is also used to enhance their CO₂ fixation efficiency.⁵⁰ In addition, the design of photobioreactor and baffles aims to maximize light exposure and CO₂ absorption.⁴⁷ Finally, supplementation of growth media with nutrients that promote CO₂ uptake and biomass production contributes to the enhancement of CO₂ uptake by microalgae.⁶⁷ Adapted strains with higher CO₂ fixation capacity are developed through genetic engineering for improved CO₂ uptake capabilities, whereas optimizing cultivation conditions further maximizes CO₂ delivery and availability.⁶³ Chemical methods as well as hybrid methods for delivering and capturing CO₂ from the atmosphere or industrial sources are explored, along with solvent-based and membrane-based techniques for capturing and concentrating CO₂ from microalgae cultivation systems.⁵¹ Table 2 summarizes these methods and approaches for improving CO₂ uptake by microalgae.

Table 2.

Improvements in CO₂ Uptake by Microalgae

METHODS	DESCRIPTION	ADVANTAGES	DISADVANTAGES	REFERENCE
Physicochemical engineering	It is broadly classified into open ponds and closed photobioreactor. OPEN PONDS: These involve the usage of circular ponds where microorganisms such as algae are grown, used in large-scale cultivation.CLOSED PHOTOBIOREACTOR: Theseare enclosed, controlled systemswhere microorganisms are cultivated,used in smaller-scale production.They maintain sterility, uniformmixing, and prevent evaporationloss.	Low costEasy operationImprove gas transfer for high biomass productionEnhance CO₂ utilization	It requires a large area.Contamination riskHigh costControlled gas transfer	⁵⁰
Design of photobioreactor and baffles	This method is crucial for improving CO₂ fixation and algal growth. Strategies include creating vortices, adding grooves to draft tubes, and well-designed baffles. These boost CO₂ fixation rate and contribute for carbon capture and utilization.	Improve CO₂ fixationIncrease algal growth	Complex designScale-up challenges	⁴⁷
Sparging system for CO₂ transfer	This method is used to introduce CO₂ into a liquid. It involves using a sparger to bubble or dissolve CO₂ into liquid, which includes CO₂ source, sparger, and flow control. These systems are used in beverage production and fermentation processes.	Efficient CO₂ transferEnhance fermentation and biotech processesCost effective for large-scale production	Energy consumptionRequire skilled operation	⁶⁷
Adapted strains with higher CO₂ fixation	This method involves enhancing the ability of microorganisms to capture and use CO₂. These can be achieved through genetic modification and selective breeding which are effective at fixing CO₂.	Mitigate climate change by reducing CO₂ levels. Potential for sustainable bioproduct production.	Complex and costlyStrains may have limited adaptation	⁶³
Genetically engineered strains	Genetic modifications and strategies used to enhance CO₂ uptake during photosynthesis, aiming to create artificial carbon sinks, and improve biofuel production. RNA interference and CRISPR-Cas9 are employed.	Increased CO₂ fixation can boost biofuel productionOptimization of enzymes efficiency and carbon assimilation pathways can lead to more efficiency	Developing and deploying genetically modified organisms may face regulatory and legal challengesLimiting funding and expertise	⁶³
Chemical and hybrid methods for CO₂ delivery	To enhance CO₂ transfer and bioavailability to microalgal cells is to chemically or biochemically convert CO₂ into soluble forms like sodium bicarbonate (NaHCO₃).A novel photobioreactor with an additional CO₂ absorption unit using nickel foam was used to capture CO₂ from the gaseous phase and convert to NaHCO₃.	Conversion of CO₂ into soluble forms such as NaHCO₃ makes CO₂ more readily available for microalgae. Demonstrated high CO₂ recovery efficiency	Poor separation of adsorbents from algal cultures can negatively affect the quality of valuable byproducts. Uses of certain absorbents such as monoethanolamide solutions may result in formation of toxic species	⁵¹

Improving CO₂ uptake by microalgae can lead to more efficient and effective CC and utilization, thereby contributing to mitigating climate change and transitioning to a sustainable future.⁵¹ The development and implementation of these strategies will not only enhance the economic viability of microalgae-mediated CO₂ biomitigation but also address the challenges of low sequestration efficiency and high energy consumption.⁴⁵ Crucially, these enhancements are essential for achieving more sustainable and effective CC and utilization technologies (Fig. 8). Focusing on continuous improvement in CO₂ uptake by microalgae is important for advancing these technologies while enhancing their efficiency.

Fig. 8.

Different approaches of improving carbon fixation efficiency in microalgae system.

Carbon Assimilation and Biomass Yield by Capturing CO₂ from Flue Gasses and Using Wastewater as Growth Media

Through the utilization of wastewater as a growing medium and the capture of CO₂ from flue gases, it is possible to achieve both the assimilation of carbon and the production of biomass.³¹ This sustainable approach provides an effective way to address environmental challenges while promoting resource efficiency in industrial processes. Through the utilization of these alternative resources, businesses have the ability to lessen their impact on the environment and make a contribution toward a more sustainable future for our world. In addition, implementing innovative technologies that optimize the utilization of renewable energy sources will further enhance sustainability efforts in various industries. Tables 3 and 4 show the source of wastewater and the CO₂ assimilation by microalgae, as well as the biomass yield. Several studies were conducted utilizing wastewater from various sources for carbon assimilation and biomass production by microalgae. The results indicated significant CO₂ assimilation and biomass yield using different types of wastewater as growth media.^43,64,78,79 Microalgae exhibited remarkable CO₂ assimilation from industrial flue gasses containing exceptionally high concentrations of CO₂ (150,000 ppm).⁴⁷ The biomass yield from this source was notable, highlighting the effectiveness of microalgae in capturing CO₂ from industrial emissions. Wastewater from the natural gas processing industry facilitated efficient CO₂ capture by the selected microalgae species.^56,80 The study demonstrated substantial biomass production using this wastewater, emphasizing the potential of microalgae in carbon assimilation and biomass generation in industrial settings. The wastewater from thermal power plants provided a conducive environment for CO₂ capture by the studied microalgae. The results revealed significant biomass yield, indicating the viability of utilizing wastewater from thermal power plants for sustainable biomass production through microalgal carbon assimilation.^43,80 Thus, the utilization of alternative resources such as industrial flue gasses and wastewater for carbon assimilation showcased promising results in terms of biomass yield. Furthermore, the study underscored the environmental significance of using microalgae for CO₂ capture and biomass production, contributing to the reduction of ecological footprints in industrial processes. As the demand for sustainable practices continues to grow, incorporating innovative technologies and leveraging renewable energy sources will be pivotal in driving environmental stewardship and resource efficiency across diverse industries. The findings from this study provide valuable insights into the potential of microalgae in addressing environmental challenges and promoting sustainable resource utilization, offering a pathway toward a more environmentally conscious and resource-efficient industrial landscape.

Table 3.

Biomass Production and CO₂ Fixation by Various Algal Species

SPECIES	SUPPLY METHOD	CO₂ SUPPLY (%V/V)	SCALE	BIOMASS PRODUCED (G/L/D)	CO₂ REMOVAL EFFICIENCY/BIOMASS PRODUCED (%)	CO₂ FIXATION RATE	REFERENCE
Botryococcus braunii	Sparging	0.03	1 L glass bottle	0.04	—	0.08	⁶⁸
Chlorella pyrenoids	Sparging	10	1.8 L bubble column	0.14	95.1	0.25	⁶⁹
C. sorokiniana	Sparging	1	2 L flask	0.29	—	0.58	⁶⁹
C. vulgaris	Sparging	10	1.8 L bubble column	0.07	95.3	0.13	⁷⁰
Scenedesmus obliquus	Sparging	10	1.8 L bubble column	0.15	94.7	0.27	⁷⁰
Spirulina platensis	Water-based culture	1	1L bubble column	0.5–2.5	93.4	0.03	⁷¹
Dunaliella salina	Agar medium	0.04	2 L bubble column	0.01	94.4	0.75	²¹

Table 4.

Carbon Assimilation and Biomass Yield by Capturing CO₂ from Flue Gasses and Using Wastewater as Carbon Source (as Growth Media)

MICROALGAE	WASTEWATER SOURCE	FLUE GAS SOURCE AND COMPOSITION	CARBON ASSIMILATION	BIOMASS YIELD/PRODUCTIVITY	REFERENCE
Nannochloropsis salina	N/A	Coal flue gas	Point source CO₂	1.13 g/L/d ± 0.12 (control) to 0.37 g/L/d ± 0.18	⁷²
Mixed culture	N/A	Coal flue gas	CO₂-rich source	0.242 g/L/d	⁷³
Dunaliella tertiolecta	Municipal wastewater	Natural gas flue gas	Point source CO₂	0.079 g/L	⁷⁴
Mixed culture	Municipal wastewater	Natural gas flue gas	Point source CO₂	0.1–2.3 g/L/d	⁷⁵
Chlorella vulgaris	Domestic wastewater	Synthetic flue gas	Point source CO₂	0.501–2.114 g/L/d	⁷⁶
Scenedesmus obliquus	Domestic wastewater	Natural gas flue gas	CO₂	0.58 g/L/d	⁷⁷

N/A, not applicable.

Biomass Valorization Through Circular Economy

It is a paradigm of sustainable development that substitutes the linear economy with a closed framework that focuses on extracting, reducing, reusing, and recycling biological resources to minimize waste formation. Microalgae biomass valorization through circular economy is an example of this concept.^70,81 The inherent CO₂ sequestration capacity of microalgal cells can be used for the development of a robust bioenergy system that uses CC, storage, and utilization technology. This technology offers advantages over traditional CC, storage, and utilization technology in terms of the amount of time it takes, the geological hazards it poses, and the CO₂ storage and transport capabilities it possesses. For the purpose of bolstering the circular bioeconomy, the integration of microalgal CC, storage, and utilization technology with energy generation necessitates the development of algal biorefinery systems that are both optimized and integrated (Fig. 9).

Fig. 9.

An encompassing scheme for microalgae utilization: CO₂ sequestration, wastewater remediation, bioenergy production, and synthesis of high-value coproducts, driving the circular bioeconomy.

Extensive research has been conducted to identify the function that the microalgae-based bioenergy with CC, storage, and utilization technology plays in supporting a circular bioeconomy. This research has focused on the net energy and cost effectiveness of the bioenergy with CC, storage, and utilization technology. Particularly, the focus has been on carbon sequestration by microalgae as well as their energy and cost efficiencies. Using flue gas from coal-fired power stations, Seambiotic Ltd. has been able to effectively develop a variety of marine microalgal strains since the year 2003. This has allowed the company to absorb CO₂ and produce algal bioethanol and biodiesel. In a similar vein, the use of waste gas from paper and pulp mills for the cultivation of mixed cultures of Scenedesmus, Chlorella, and Monoraphidium resulted in a much better biomass energy production per area.³¹ These cultivation concepts improved the net energy ratio to 0.25 while also enhancing lipid production with higher heating values of 20–23 MJ kg⁻¹. In addition to examining the energy efficiency and carbon fixation capabilities of microalgae-based bioenergy with CC, storage and utilization technology is an important economic assessment that highlights the viability of a biorefinery framework over a conventional CCSU model. The microalgae-based biorefinery for methanol and bio-oil exhibited high CO₂ capture efficiency (73%) with substantial revenue generation totaling $59.2 million USD alongside a net CO₂ capture cost much lower than amine-based CC and utilization predictions.³¹

In addition, microalgal biofilms show a high productivity of 30 g/m² per day and demonstrate thermal and electrical efficiencies (0.25 W/m²) comparable with direct air capture technology (ranging from 0.12 to 0.41 W/m²).⁸² This indicates the potential viability of using microalgae for bioenergy with CC, storage, and utilization technologies in a circular bioeconomy. The efforts to produce bioelectricity from algal biomass can be utilized to power the entire system for cultivating and harvesting algae, completing the loop of a biorefinery model. Furthermore, by coproducing various energy products such as biohydrogen, bioethanol, and biodiesel from waste sources within an algal biorefinery environment, it is feasible to significantly reduce the overall cost of algal energy production while contributing toward a circular economy.⁶²

Policy-making organizations in different countries have been progressively implementing carbon taxation systems to decrease carbon emissions and meet sustainable development goals by 2050. The carbon taxes have increased over time, going from $20 per ton of CO₂ in 2019 to $50 per ton of CO₂ in 2022. Projections indicate that the carbon taxes will continue rising to reach $170 per ton of CO₂ by 2030 as part of strict emission regulations.⁸³ This increase aims to encourage the adoption of sustainable technologies for capturing and using carbon instead of paying high carbon taxes. Among these options, microalgae-based bioenergy with CC, storage, and utilization technology is emerging as an attractive choice within a biorefinery model at a reasonable cost compared with chemical-based technology. In addition, microalgae-based bioenergy with CC, storage, and utilization technologies acts as a net-negative emission technology that not only reduces carbon emissions but also generates additional oxygen and valuable products or green energy within circular bioeconomy frameworks. To ensure its widespread applicability across industrial setups while maintaining an integrated system with industrial standards successfully achieving the target, essential considerations relating to space requirements and cultivation systems need further development efforts illustrating promising future prospects for microalgae-based bioenergy with CC, storage, and utilization technologies contribution toward circular economy objectives aligning with sustainability targets associated with energy–environmental nexus.

Future Perspective

Cultivating microalgae can help reduce CO₂ from the atmosphere and use the captured carbon in various products. This characteristic of algae has been recognized as a hopeful method for sustainable CO₂ sequestration and use, known as microalgae-based bioenergy with CC, storage, and utilization technology. Microalgae cells have approximately 400 times greater efficiency in capturing CO₂ than land plants, resulting in a higher fixation rate compared with traditional technologies.⁸⁴ Despite these advantages, the economic perspective on microalgae-based bioenergy with CC, storage, and utilization does not meet the needs of commercial markets and requires careful collaboration across multiple disciplines to translate progress into practical applications. The primary obstacle is the high initial operational cost of photobioreactors as well as the limited amount of land that is required for raceway pond placement. When considering the many options for cultivation, open raceway ponds and airlift photobioreactors provide exceptional performance and cost effectiveness. Nevertheless, the choice of the microalgal strain poses a constraint.⁸⁵ An algal strain possessing high biomass productivity, CO₂ tolerance, and adaptation to fluctuations in light and temperature can significantly improve the economic feasibility of the operation. Determining a robust strain necessitates a substantial investment of time and effort. Hence, innovative techniques that incorporate phenotyping, automated flow cytometry, and machine learning or artificial intelligence-based models are being developed to efficiently and accurately identify appropriate microalgal strains.³¹

Aside from conducting screenings, it is necessary to examine genetically modified strains that possess desired traits and have established metabolic pathways. Moreover, the use of nanotechnology for interventions exhibits promise from both economic and environmental perspectives. Nevertheless, the current constraint in genetic modification pertains to the limited genomics databases of microalgae.⁸⁶ Comprehensive omics-based studies on nonmodel and valued microalgal strains yield complete data and important targets for facilitating metabolic engineering.⁶³ These studies primarily focus on enhancing CO₂ fixation using chloroplast genes. Nevertheless, they necessitate costly techniques such as gene guns and transformation methods. Therefore, it is recommended to explore the utilization of microalgae-based CC, storage, and utilization to harness valuable energy products from carbon emissions. Specifically, within a microalgae-based bioenergy with CC, storage and utilization model, carbon integrated as biomass can be processed for lipid or carbohydrate production.⁶³ In the process of biodiesel synthesis, the leftover material obtained from lipids can be subjected to hydrolysis, which breaks it down into sugars. Alternatively, it can be subjected to anaerobic digestion in order to generate biogas and produce bioelectricity. By sequentially extracting different energy forms or isolating valuable molecules like pigments or platform chemicals, it is possible to significantly reduce costs in a microalgae-based bioenergy system that involves CC, storage, and utilization.³¹ Furthermore, the use of leftover biomass to produce biochar, biofertilizer, and liquid fertilizers has the potential to improve the economic value chain in a microalgae-based CC, storage, and utilization model.⁴⁷

A robust and methodical biorefinery necessitates a preexisting structure with an optimized workflow. Although some research have been conducted using models to anticipate numerous targets in microalgae-based bioenergy with CC, storage, and utilization technologies, there is still a lot of opportunity for experimental validation to increase prediction reliability and accuracy. Modeling the interplay of multiple contaminants in flue gases, in particular, can help discover resilient and CO₂-tolerant microalgae strains. Furthermore, additional research is needed on the techno-economic analysis of the microalgae-based bioenergy with CC, storage, and utilization biorefinery model in order to encourage association amid industries. Furthermore, to the techno-economic analysis, a life cycle assessment using a cradle-to-gate method for microalgae-based bioenergy with CC, storage, and utilization is advised. Considering factors such as the origin of industrial source flue gas, location, and climate conditions necessitates an integrated approach that combines advanced artificial intelligence tools such as geographic information systems and prediction models with experimental evidence, as well as economic and environmental considerations for microalgae-based bioenergy with CC, storage, and utilization.

Conclusion

In this review, the multifaceted role of microalgae in addressing the pressing issue of atmospheric CO₂ accumulation is explored. The evaluation of various CC methodologies has underscored the unique advantages of microalgae-based systems, which not only sequester CO₂ efficiently but also offer a sustainable pathway for biomass production with multiple applications. Microalgae have been identified as a promising biological agent for CO₂ capture, capable of converting this GHG into valuable resources through photosynthesis.⁴⁵ The mechanisms of CO₂ assimilation by microalgae, as well as the methodologies for enhancing CO₂ capture and biomass conversion, have been examined in detail. The economic feasibility of large-scale microalgae cultivation for CC has been assessed, demonstrating the potential for substantial CO₂ sequestration and biomass generation.⁶² Moreover, the integration of microalgae cultivation with wastewater treatment and flue gas utilization presents a synergistic approach that can enhance the environmental and economic benefits of these systems. This integration not only contributes to a sustainable global bioeconomy but also offers a promising avenue for mitigating climate change impacts. Thus, the development and implementation of microalgae-based CC technologies represent a significant step toward sustainable CO₂ mitigation. As we move forward, continued research and development are essential to optimize these systems, making them more efficient and economically viable. The future of microalgae in CC and utilization is bright, and with ongoing innovation, it could play a pivotal role in shaping a more sustainable and resilient planet.

Footnotes

Acknowledgment

The authors thankfully acknowledge Vignan’s Foundation for Science, Technology and Research, Vadlamudi, Guntur, Andhra Pradesh, India, for their support.

Author Contributions

Sankaran Krishnamoorthy: Conceptualization, Supervision, Writing–Original Draft Preparation, Writing–Review & Editing, Methodology, Project Administration. K. Chandrasekhar: Data Curation, Writing–Review & Editing, Formal Analysis, Investigation. M.R. Charan Raja: Visualization, Investigation, Writing, Resources. All authors have read and approved the final manuscript.

Author Disclosure Statement

All authors express no conflict of interest to disclose.

Funding Information

No funding was received for this article.

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Evaluation of Carbon Capture Methodologies,Mechanisms,and Improvements for Sustainable Carbon Dioxide Mitigation Using Microalgae

Abstract

Introduction

Methodologies for Carbon Capture

PHYSICAL METHOD OF CO2 CAPTURE

Adsorption

Absorption

Membrane separation

CHEMICAL METHOD OF CO2 CAPTURE

Amine scrubbing

Carbonation

Chemical looping

GEOLOGICAL METHOD OF CO2 CAPTURE

Capture

Storage

Monitoring and verification

BIOLOGICAL METHOD OF CO2 CAPTURE

Afforestation

Oceanic fertilization

Microalgae-based method

Microalgae Cultivation

CO2 Sequestration

Biomass Production

Mechanism of CO2 Capture by Microalgae

PHOTOSYNTHESIS

OXYGEN PRODUCTION AND CO2 FIXATION

CARBON CONCENTRATION MECHANISM

MULTIPLE ROUTES OF CARBON ASSIMILATION

PHOTOSYNTHESIS CASCADE

Light dependent

Dark reactions

ALTERNATIVE MODES OF CO2 ASSIMILATION

FLEXIBILITY AND ADAPTABILITY

SURVIVABILITY IN DIFFERENT CO2 CONCENTRATIONS

Methodology of CO2 Capture Using Microalgae

IDENTIFICATION AND SELECTION OF THE HIGH CO2-TOLERANT ALGAL STRAINS

OPTIMIZATION OF PROCESS PARAMETERS FOR CO2 CONVERSION

CO2 CAPTURE IN PHOTO BIOREACTOR SYSTEMS

Improvements in CO2 Uptake by Microalgae

Carbon Assimilation and Biomass Yield by Capturing CO2 from Flue Gasses and Using Wastewater as Growth Media

Biomass Valorization Through Circular Economy

Future Perspective

Conclusion

Footnotes

Acknowledgment

Author Contributions

Author Disclosure Statement

Funding Information

References

PHYSICAL METHOD OF CO₂ CAPTURE

CHEMICAL METHOD OF CO₂ CAPTURE

GEOLOGICAL METHOD OF CO₂ CAPTURE

BIOLOGICAL METHOD OF CO₂ CAPTURE

CO₂ Sequestration

Mechanism of CO₂ Capture by Microalgae

OXYGEN PRODUCTION AND CO₂ FIXATION

ALTERNATIVE MODES OF CO₂ ASSIMILATION

SURVIVABILITY IN DIFFERENT CO₂ CONCENTRATIONS

Methodology of CO₂ Capture Using Microalgae

IDENTIFICATION AND SELECTION OF THE HIGH CO₂-TOLERANT ALGAL STRAINS

OPTIMIZATION OF PROCESS PARAMETERS FOR CO₂ CONVERSION

CO₂ CAPTURE IN PHOTO BIOREACTOR SYSTEMS

Improvements in CO₂ Uptake by Microalgae

Carbon Assimilation and Biomass Yield by Capturing CO₂ from Flue Gasses and Using Wastewater as Growth Media