Abstract
This article aims to inform the development of biological perchlorate remediation schemes for the preparation of safer human Mars habitats and contaminant-free in situ resource utilization (ISRU) for crop production in these settings. No prior studies have attempted to remediate perchlorate from Martian regolith simulants. Thus, we draw from previous work on soil, sediment, and water biological remediation that we determined to be most relevant to Martian applications. Approaches include phytoremediation and microbial remediation. Phytoremediation utilizes terrestrial and aquatic plants for perchlorate removal, occurring by 3 different mechanisms: phytoaccumulation, phytodegradation, and rhizodegradation. We suggest potential plant candidates for phytoremediation. We discuss known microbial remediation processes utilizing both rhizosphere-derived microorganisms and extremophiles, and the most likely microorganism candidates for a successful microbial remediation of Martian regolith considering the harsh Martian environment. We also briefly discuss the economic implications of perchlorate remediation for ISRU farming viability. We recommend this article as a reference for future attempts to successfully and cost-effectively develop biological remediation technologies to remove perchlorate from Martian regolith, improving the viability of ISRU crop production.
INTRODUCTION
The perchlorate ion is abundant on the surface of Mars.1,2 Perchlorate is harmful to many life forms, and in humans, it affects the thyroid gland, causing hormone imbalances.2–5 The presence of perchlorate on Mars is a hindrance to any future crewed mission to the planet. 6 It is also a major roadblock for viable, efficient in situ resource utilization (ISRU) approaches to space agriculture. 7 Methods to neutralize or remove perchlorate from Martian soil are warranted. 2
Perchlorate remediation, including bioremediation, is well studied for terrestrial purposes, as numerous toxic perchlorate sites exist in the United States. Perchlorate is used to manufacture rocket propellants, explosives, fuels, and other industrial products.3,5,8–11 Phytoremediation has been demonstrated as an effective method of bioremediation of perchlorate, and there are many candidate plant species (Table 1). Microbial remediation of perchlorate from water and soil has also been successfully implemented in experiments at the bench scale (Table 2) and pilot scale. 12
Plant Candidates for Phytoremediation
Potential candidates for phytoremediation, including woody species, nonwoody terrestrial plants, and aquatic plants. These plants perform phytoremediation by 1, 2, or all 3 possible mechanisms.
Potential Microbial Candidates for Perchlorate Reduction of Regolith
See individual references for complete growth requirements for these organisms.
Salinity was estimated based on sodium % (w/v) in media recipes cited in these references.
CO, carbon monoxide.
The perchlorate ion (ClO4−) is composed of 1 chlorine atom bonded to 4 oxygen atoms in a tetrahedral arrangement. 2 Its formal charge is −1 and oxidation state is +7, and it is highly soluble in water. 13 Perchlorate occurs naturally on Earth in some arid regions but is more commonly found as a manufactured component of solid rocket propellants for use in the aerospace industry in the form of ammonium perchlorate.5,13 Soil and groundwater contamination of perchlorate is common at sites of perchlorate manufacture, use, and disposal. 5
Perchlorate exposure is dangerous to humans, as it affects uptake of iodide by the thyroid gland, leading to a reduction of thyroid hormones.4,5 As thyroid hormones are instrumental in metabolism, long-term exposure may cause lowered metabolic rates and slowed organ function.2,4 Perchlorate is harmful in the short term in small doses, causing irritation of the eyes, skin, and respiratory system, as well as nausea, vomiting, and diarrhea. The daily reference dose of perchlorate designated by the United States Environmental Protection Agency (EPA) is 0.7 μg·kg−1 of body weight.2,5
Perchlorate on Mars
The NASA Phoenix Lander detected perchlorate on the surface of Mars in May 2008. This perchlorate exists at a concentration of 0.4%–0.6% in Martian regolith and is present as the salts sodium perchlorate, Na(ClO4), and magnesium perchlorate, Mg(ClO4)2. These perchlorate salts are found in liquid water brines and ice on the Martian surface in addition to the regolith and dust.1,13,14 The presence of perchlorate is a severe hazard to astronauts. Dust inhalation, consumption of contaminated water, and ingestion of foods grown in Martian soil are the main hypothetical exposure pathways. There is evidence that fruits and vegetables are capable of accumulating perchlorate from contaminated soil even at low soil concentrations.15,16 Inhalation of several milligrams of dust alone would exceed the EPAs daily reference dose. 2 Due to the health risks posed to astronauts by perchlorate, remediation methods will be necessary for crewed missions to Mars.
Relevance of Earth-Based Perchlorate Bioremediation Studies to Mars
In addition to its interaction with regolith particles, biological perchlorate remediation using plants or microorganisms inevitably depends on solubility of these salts because of their reliance on aqueous environments. The perchlorate counter ion, thus, may impact removal efficiency. Most perchlorate remediation studies have been conducted on ammonium perchlorate, which is the principle form it takes in contaminated drinking water and soil. However, the perchlorate salts found on Mars are primarily sodium perchlorate, magnesium perchlorate, and to a lesser extent calcium perchlorate and potassium perchlorate 1 and at significantly greater concentrations than are found on Earth. Solubility of perchlorate salts differ,14,17 and their solubility is further altered by temperature (Table 3). It is unknown if Earth-based remediation studies will apply to Mars. Thus, it may be necessary for investigators to conduct remediation studies for removing perchlorate salts such as sodium or magnesium perchlorate and under environmental conditions that more closely resemble what is expected for the Martian surface.
Aqueous Solubility of Inorganic Perchlorate Salts at Various Temperatures 17
MW = molecular weight in atomic mass units, units of solubility are in g · 100 mL−1 of water.
ND, no data available.
There are currently no studies on the removal of perchlorate from regolith simulants, which are manufactured products meant to resemble what would be found on Mars.6,7 Removal of perchlorate from contaminated soil has been conducted primarily using biological methods such as microbial and plant-based remediation, rather than chemical or physical methods. This is likely due to the ubiquity of potential perchlorate-reducing microbes in soil, the high availability of organic carbon, as well as the obvious convenience associated with plant growth in soil. While physical and chemical perchlorate removal methods are also well studied for Earth remediation, they have so far been applied mostly to drinking water, 8 likely due to low availability of electron acceptors and organic carbon for microbial perchlorate reduction. 18 Even for water, other authors have found that physiochemical methods such as ion exchange, adsorbents, and membrane filtration are expensive and inefficient for large-scale processes.8,19 They are also not appropriate methods of remediating solid material such as regolith or soil because they depend on filtration of liquid with relatively little particulate matter. There are fewer physiochemical remediation studies available to draw from for potential Mars regolith applications. Therefore, biological perchlorate remediation using plants and/or microbes will be the focus of this review, drawing from both the vast perchlorate remediation literature for Earth industrial contamination of soil and water, which could be applicable to Mars settlements and ISRU.
PHYTOREMEDIATION
The phytoremediation of perchlorate is a well-studied bioremediation procedure for perchlorate contamination sites.3,5,10 Phytoremediation is the practice of using plants to remove perchlorate from the environment and occurs by 3 principle mechanisms: phytoaccumulation, phytodegradation, and rhizodegradation. Phytoaccumulation and phytodegradation typically occur in tandem, whereas rhizodegradation is an independent process. 3 Woody plants, particularly trees, are of great interest due to their capability in all 3 processes. Herbaceous (non-woody) terrestrial and aquatic plants are primarily useful for phytoaccumulation (Table 1).
Phytoaccumulation begins as perchlorate is absorbed by the tree and accumulates mostly in the leaves and branches. After accumulation of perchlorate in these regions, phytodegradation ensues as plant enzymes begin to process and reduce the ion. Rhizodegradation is the reduction of perchlorate in the rhizosphere, or root zone, of the tree by symbiotic rhizobial microbes.9,10 Although not all microbial inhabitants of the rhizosphere are capable of processing perchlorate, rhizodegradation has been demonstrated as an effective process. 20 The following sections will examine phytoaccumulation, phytodegradation, rhizodegradation, summarized in Table 1, and related microbial perchlorate reduction techniques, summarized in Table 2.
PHYTOACCUMULATION
Woody Plants
As phytoaccumulation requires the absorption of perchlorate and its subsequent deposition in leaves and branches, this process is dependent on the tree's uptake of water; therefore, it is typically a slow process.9,10 Willow, cottonwood, eucalyptus, hackberry, sweet gum, pine, and elm have been shown to accumulate perchlorate.3,9,10 Phytoaccumulation rates vary based on concentration levels, length of exposure, tree size and species, and the types of dissolved solids present. Tree size is of particular significance, as a tree's root zone extends and becomes denser with tree growth, allowing it to absorb and accumulate greater amounts of perchlorate. 10 Perchlorate phytoaccumulation rates are highest in aerobic environments with high nitrate concentrations.3,21 Phytoaccumulated perchlorate can leach back into the environment, however, increasing the importance of phytodegradation in the phytoremediation process. 9
Nonwoody Plants
A myriad of herbaceous plants, both aquatic (wetland) and terrestrial, have demonstrated the ability to phytoremediate perchlorate. These aquatic plants include watercress (Nasturtium sp.), parrot feather (Myriophyllum sp.), pickleweed (Salicornia sp.), water hyacinth (Eichhornia crassipes), water lily (Nymphaea odorata), Acorus calamus, Thalia dealbata, and Canna indica. Terrestrial plants include smartweed (Polygonum sp.), tobacco (Nicotiana tabacum), and the food crops cucumber, soybean, lettuce, and rice.9,16,22–24 Phytoaccumulation by crops is influenced by species as well as soil conditions such as pH and presence of nitrate fertilizers.16,21,25 These fertilizers likely act as preferred electron acceptors for microbial processes discussed in later sections of this article.
Aquatic plants have shown greater potential than their terrestrial counterparts for this purpose; this is most likely due to their ability to remove perchlorate from surface water in addition to sediment pore water.9,22 However, a combination of these plants would likely be best to maximize phytoremediation from both regions. 9 As in crops, aquatic plants' response to perchlorate is species-dependent in response to environmental factors such as perchlorate concentration, pH, nutrients available, and type of substrate, and it is believed that their primary phytoremediation method is phytoaccumulation into leaf tissue. 22 Species showing notable potential are parrot feather (Myriophyllum aquaticum) and water hyacinth (E. crassipes).9,22 Parrot feather has been shown to hyperaccumulate perchlorate in its tissue, 9 whereas water hyacinth has demonstrated superior perchlorate tolerance and accumulation in water with high-level perchlorate contamination. 22 These 2 species in particular should be researched further for potential Martian applications. Another potential candidate, beneficial due to its small size, is the free-floating aquatic plant organism Pistia, which was capable of remediation of water containing 5 mg·L−1 by 64% over 7 days using both phytoaccumulation and rhizodegradation. 26
PHYTODEGRADATION
Following phytoaccumulation, phytodegradation ensues as enzymes within the plant begin to reduce perchlorate ions upon their deposition in plant tissues. These enzymes metabolize the perchlorate to chloride, utilizing the reduction pathway of perchlorate to chlorate (ClO3−) to chlorite (ClO2−) to chloride (Cl−). 9 Other unknown leaf components are also responsible for some portion of phytodegradation. 20 Phytodegradation rates are dependent on tree species and size, exposure duration, contamination levels, and dissolved solids content. 9 However, without complete phytodegradation, there is potential for perchlorate to return to the soil in leaf litter or by leaching. 20 Experimental data suggest that phytodegradation rates never exceed phytoaccumulation rates, but some accumulation is inevitable. 3 Therefore, phytodegradation and phytoaccumulation do not fully diminish the threats posed by perchlorate.
RHIZODEGRADATION
Rhizodegradation as a phytoremediation technique is independent but interrelated with phytoaccumulation and phytodegradation. This process is done by ubiquitous perchlorate-degrading microbes that populate the rhizosphere. Organic matter serves as a carbon and electron source, microbes then oxidize this organic matter for energy, and perchlorate is then used as the terminal electron acceptor. 20 Of the 3 phytoremediation processes, rhizodegradation is the most rapid removal mechanism. Experiments have shown a total removal of 96.1% of initial perchlorate concentration utilizing this process. 3 While the macrophyte Pistia could only remove 64% of a 5 mg·L−1 concentration of perchlorate from water over 1 week, bacterial isolates from its root exudate, including a novel Acinetobacter species, could completely remediate a 100 mg·L−1 concentration of perchlorate in 48 h. 26
Rhizodegradation rates increase in anaerobic conditions and with low nitrate content, as perchlorate competes with nitrate as the terminal electron acceptor utilized by perchlorate-reducing microbes. 21 This utilization of perchlorate as a terminal electron acceptor is done by dissimilatory perchlorate-reducing bacteria during anaerobic energy production. In this process, perchlorate reductase (PcrABC) reduces perchlorate to chlorate, which is then reduced to chlorite by chlorate reductase, which is subsequently removed by chlorite dismutase (Cld), liberating oxygen and chloride ions. This oxygen is then respired by the microbe using cytochrome cbb3-oxidase.2,11
Additionally, the rhizosphere can be biostimulated using a carbon-based electron source; the perchlorate-reducing action associated with symbiotic root-zone bacteria could be greatly increased in the presence of dissolved organic carbon (DOC). Such stimulation importantly decreases the phytoaccumulation of perchlorate in plant tissues, as complete reduction of perchlorate is preferred to diminish ecotoxicological threats.3,5 In low nitrate conditions with DOC stimulation, phytoaccumulation is typically less than 5% of the initial perchlorate concentration in the rhizosphere, as rhizodegradation is the favored process. 3 Offering additional DOC electron sources has been shown to increase rhizodegradation rates by up to 2 orders of magnitude. 10 Overall, compared with phytoaccumulation and phytodegradation, the half-life of perchlorate rhizodegradation is hours instead of days. 3
MICROBIAL REMEDIATION
Microorganisms involved in rhizodegradation have inspired ex situ biological reduction experiments for decontaminating soil and water, although not for the expressed purpose of preparing regolith for crop production. These techniques and biochemical pathways involved are potentially useful for the remediation of regolith. Perchlorate's status as an environmental pollutant in water, soil, and crops has resulted in a significant body of research11,27–29 on microbial remediation of perchlorate-contaminated sites on Earth. Many rhizosphere-associated and other environmental bacteria are capable of performing perchlorate reduction.
30
Similar to phytoremediation and rhizodegradation, this pathway reduces perchlorate to chloride and oxygen, using an organic carbon source. An example of perchlorate reduction using succinate as a carbon source is shown in Equation 1. Other carbon sources may be used, with slightly different stoichiometry and removal efficiency.
19
Similar to the rhizodegradation pathways described above, enzymes that are involved in perchlorate reduction also include chlorite dismutase and a family of type II DMSO enzymes called perchlorate reductases.
31
Perchlorate naturally occurs in sediment in parts of the Great Salt Lake Desert of Utah, making it a useful analog to study Martian conditions. 16S rRNA sequencing of microcosm samples collected from the desert's Pilot Valley basin revealed the presence of endemic perchlorate-reducing microbes. 32 These include members of families Sporomusa, Azospira, Shewanella, Marinobacter, Pseudomonas, Haloarcula, and Acinetobacter. To test perchlorate-reducing ability, sediment microcosms were sampled and cultured in enrichment medium and exposed to 100 and 1,000 ppm sodium perchlorate solutions. Perchlorate reduction in all samples occurred, appearing to reduce perchlorate concentrations by up to half of the starting concentration over a period of 286–350 days.
Bench-scale experiments using perchlorate reduction bacteria have also been conducted.19,33–44 Relevant characteristics of these experiments are shown in Table 2. Rate of perchlorate reduction in vitro was dependent on temperature,19,36 carbon source,19,44 as well as pH.19,43,44 However, the optimal pH varies, although within a narrow range, by microorganism. Burkholderia sp. reduced perchlorate optimally at pH = 7 and had marked reduction at higher pH levels, 19 while a mixed microbial community from activated sludge reduced perchlorate in specialized growth media optimally at pH = 8. 44
Successful pilot-scale remediation 12 of potassium perchlorate-contaminated soil was achieved using a bioreactor colonized by Serratia marcescens (GenBank no. HM751096), Bacillus pumilus (GenBank no. JQ820452), and Micrococcus sp. (GenBank no. KJ410671). This system was held at a pH of 7 (±0.5) and was able to decontaminate 670 kg of contaminated (0.03%) garden soil (3% clay, 43% silt, and 33% sand, 10% moisture, 0.15 mg/kg nitrate) in 6.3 h, using ∼360 L of water, and a glucose/perchlorate ratio of 5:1. However, this perchlorate concentration is much lower than the 0.4%–0.6% detected in Martian regolith.1,2,45
CHALLENGES WITH MARTIAN REGOLITH CONDITIONS
Despite successful bench-scale and pilot-scale experiments, extreme Martian regolith conditions pose an additional challenge. Although it has been demonstrated that some can reduce perchlorate at 3%–7% salinity 46 or higher,12,35,37 many perchlorate-reducing microorganisms prefer mesophilic low-salinity conditions. 47 Generally, these conditions are not present on Mars. Perchlorate removal by mesophilic organisms would need to be performed under controlled conditions such as in a bioreactor. 12 For an organism to successfully remove perchlorate from regolith in the harsh environment of Mars, it needs to be able to (1) survive in a highly saline environment, (2) withstand cold temperatures, and (3) utilize electron donors and carbon sources that are found in regolith conditions (no organic matter) as opposed to soil (high organic matter).
Halophilic Perchlorate Reducers
Halophilic perchlorate-reducing microorganisms may be appropriate candidates for perchlorate reduction in this case and are also included in Table 2. Halophiles that reduce chlorate and perchlorate include Haloferax mediterranei,35,48,50,51 Haloferax denitrificans,49,50,51 Paracoccus halodenitrificans (reclassified and renamed to Halomonas halodenitrificans), 49 Moorella perchlorateireducens, 33 Haloferax gibbonsii,50,51 Haloarcula marismortui,50,51 and Haloarcula vallismortis.50,51 Al Soudi et al. 52 identified 16 perchlorate-tolerant organisms in the genera Marinococcus, Halomonas, Planococcus, Bacillus, Nesterenkonia, Terribacillus, and Salibacillus isolated from Hot Lake in Washington and The Great Salt Plains of Oklahoma, although none were tested for specifically for perchlorate-reducing ability—only perchlorate tolerance.
Cold Tolerance
Only 2 studies have identified potential cold-tolerant candidates for perchlorate reduction.36,53 Known psychrophilic halophile Rhodococcus sp. JG3, isolated from the permafrost of the Upper Dry Valleys of Antarctica, is able to survive in both −10°C and −20°C conditions, in 20% (w/v) magnesium perchlorate. 53 The purpose of this study was not to test perchlorate reduction, but to measure the expression of bacterial proteins that affect liquid to ice transition, so final perchlorate concentrations in the growth solution were not measured. Halotolerant Planococcus halocryophilus appears to be able to evolve over time to tolerate gradually increasing concentrations of various chloride and perchlorate salts at either 4°C or 25°C. 36 However, tolerance of perchlorate does not indicate ability to reduce perchlorate. Final perchlorate concentrations were not measured in this study.
Bioengineering
One possible solution is the bioengineering of psychrophilic microorganisms for the bioremediation of perchlorate. No research at this time has explored bioengineered cold-tolerant microbes for perchlorate-reducing ability. Past research has explored the bioengineering of cold-tolerant Pseudoalteromonas haloplanktis TAC125 (isolated from Antarctic seawater) for the bioremediation of aromatic compounds. 54 Parrilli et al. 54 demonstrated that the pbTAC/tou cells can be engineered for the conversion of aromatic compounds.
To the best of our knowledge, there are 5 published studies specifically looking at genetically modified microbes' ability to remediate chlorine compounds, although all involve mesophilic bacteria rather than psychrophilic. Shewanella algae ACDC, a gram-negative facultative anaerobe that grows optimally at 20°C–30°C and 2% NaCl, was able to reduce chlorate chlorate, but not perchlorate, by overexpressing a few recently acquired key genes in a transposon associated with chlorate reduction. 55 Transferring 5 of these genes to Shewanella oneidensis resulted in robust growth on chlorate. 56 Azospira suillum strain PS contains an environmentally acquired trait called a perchlorate reduction genomic island containing several genes for perchlorate reduction, including the enzyme perchlorate reductase (PcrAB).57–59 Pseudomonas stutzeri PDA reduces chlorate, and the chlorate reductase genes clrA, clrB, and clrC appear to be critical, and co-culture experiments suggest that these genes may also be involved in perchlorate reduction. 60 Certain fungal species, such as yeast Debaryomyces hansenii and the filamentous fungus Purpureocillium lilacinum, found to be tolerant of sodium perchlorate, may also be candidates for genetic engineering. 61
Genetic modification of plants may be an avenue. To the best of our knowledge, there are no studies on the genetic modification of plant species specifically for accumulating (per)chlorate compounds from soil or water. The research, described in an earlier section, has primarily focused upon plants that already possess traits for (per)chlorate accumulation. Perhaps techniques from research on other environmental pollutants may inform future work on this open question. For example, transgenic alfalfa modified to express the genes for glutathione S-transferase (a detoxification enzyme) and human P450 2E1 (CYP2E1) (an enzyme involved in ethanol metabolism) was able to accumulate mixed mercury (Hg)–trichloroethylene from contaminated soil in a laboratory setting. 62 A review by Berken et al. 63 reviewed the body of research on the key genes involved in the bioengineering of plant species for accumulating selenium from contaminated soil.
Electron Donors and Carbon Sources
If electron donors are available, along with appropriate temperature and pH, halophilic organisms may grow in perchlorate under anaerobic conditions.33,64 On Earth, many of the perchlorate-reducing microorganisms utilize electron donors such as acetate and alcohols. 55 Based on meteorites from Mars, the total organic carbon concentration may be between 10 and 500 ppm.65,66 As a result, the organisms that utilize these donors may not be appropriate for Martian regolith, unless a source of organic carbon was artificially introduced. However, there are cases of perchlorate-reducing bacteria that utilize inorganic compounds such as hydrogen and ferrous iron. 67
Carbon monoxide (CO) is a known constituent of the Martian atmosphere, at 700 ppm, that may be able to serve as a carbon source for perchlorate reduction. 37 Myers and King state 37 that, assuming there is an even distribution of CO carbon in the regolith, CO occurs at 2.8 mol·m−2. They examined the potential of perchlorate-coupled CO oxidation in 2 halophiles: Haloarcula sp. PCN7 and Halobaculum sp. WSA2. This research demonstrated that both halophiles were capable of reducing perchlorate. Their experiment also demonstrated that anaerobic conditions with NaCl brines could support perchlorate-coupled CO oxidation.
THE ECONOMICS OF REGOLITH FARMING
While the cost and profit potential of in situ resource extraction, processing, storage, handling, and/or delivery has been modeled for Mars 68 and lunar projects, 69 no economic models have been conducted so far to compare farming methods, let alone to determine how perchlorate removal from regolith would impact the cost of regolith crop production. Implementing perchlorate removal technology would certainly increase the energy and water requirements of these farming techniques, but it is unknown how these trade-offs differ between proposed alternatives such as hydroponics. To predict the cost of ISR4 farming, and thus profit potential for businesses interested in these endeavors, research is needed to compare the nutrient, water, and energy inputs for potential Martian farming systems. Extending from that, comparisons between perchlorate remediation methods would be needed.
Comparing Farming Systems
Comparing ISRU farming and hydroponics would likely involve quantifying the energy and water consumption for (1) transporting additional fertilizers not available on Mars, (2) equipment and personnel required to construct and operate the farming and remediation systems, (3) the choice of remediation strategy—phytoremediation, microbial remediation, or both, and (4) the farming system itself, which would depend on crop type and food demand. These factors are, of course, additional to the sunk costs (carbon dioxide injection, water and oxygen acquisition, artificial lighting, climate control, ventilation, construction costs, etc.) involved in maintaining pressurized, closed Martian habitats that these farming systems would exist within. Perchlorate itself may be a resource as well as a contaminant; perchlorate reduction has the possible advantage of producing breathable oxygen as a by-product,2,70 which would impact trade-off calculations as well.
Comparing Perchlorate Remediation Techniques
Evaluating the optimal choice for biological perchlorate reduction of regolith is also not straightforward. Possible challenges are enumerated as follows:
A microbial bioreactor designed similarly to the pumped, water-wash method developed by Nair et al.
12
for dissimilatory perchlorate reduction would increase water and energy requirements of crops grown in regolith. Perchlorate reduction using halophilic psychrophilic archaea would require, at minimum, maintenance of healthy cultures under stressed conditions. Maintaining microbial cultures in microgravity and in space flight present numerous challenges.
71
Phytoremediation with terrestrial plants, such as crop production itself, would require successful plant growth in regolith, thus nitrogen—a rate-limiting nutrient in biomass accumulation—would need to be added.
72
Sufficient quantities of zinc, and copper also may be missing from regolith7,65 but are important minerals for plant growth. Research thus far demonstrates that lettuce and Arabidopsis thaliana seedlings are unsuccessful after 5–7 days in nonsupplemented regolith,
7
so it is possible that phytoremediation plants (e.g., noncrops used only for remediation) would have similar difficulties establishing themselves in these low-nutrient conditions. Nitrogen supplementation is further complicated by the fact nitrates are often a preferred electron donor over perchlorate by microbes that perform dissimilatory perchlorate reduction.3,21 which may inhibit rhizodegradation or perchlorate reduction in a bioreactor.
Recommendations for Calculating Cost and Profit Potential of Regolith Farming
On Earth, trade-offs between different farming methods are typically assessed using the life cycle assessment methodology, 73 but this approach may not be as applicable for lunar and Martian farming because raw material acquisition chains will differ drastically on Mars or the Moon in the amount of water, energy, and other inputs required. The economic model by Shishko et al. 68 to estimate cost and profit potential for water resource extraction may be a more useful approach. They incorporate 4 major model components: the architecture of a hypothetical Mars colony, the demand for resource extraction by number of ISRU systems needed and the types of technologies used, the demand for water based on characteristics and number of human settlers, followed by an Economics Integration Model that computes profits, losses, “net present value,” and internal rate of return. Their Economics Integration Model utilizes the Monte Carlo simulations to estimate an average commodity price for water. Since water is likely to be a limiting resource in ISRU farming, a similar economic model could conceivably be built to determine the average commodity price and profit potential for farm-ready regolith, informed by type of perchlorate remediation technology, farming method, and food demand.
CONCLUSIONS
Overall, both microbial and phytoremediation of perchlorate are promising methods of bioremediation for Martian applications. Trees that demonstrate the ability to degrade perchlorate can be utilized in domed environments to bioremediate perchlorate in the immediate vicinity. However, this analysis indicates that rhizodegradation and other forms of microbial reduction are preferred mechanisms of perchlorate phytoremediation as thorough reduction of perchlorate is desired; phytoaccumulation and phytodegradation do not completely neutralize perchlorate. Biostimulation of the rhizosphere with DOC can be beneficial in Martian applications to increase the rate of microbial perchlorate reduction and to limit the uptake and phytoaccumulation by the plant. Bioreactor-based microbial remediation inspired by rhizodegradation pathways offer the potential for removing perchlorate from regolith for agricultural applications as well, under more controlled conditions. Dissimilatory perchlorate-reducing bacteria are widely available in the environment, but biological reactors using them require significant water input, a reliable carbon source, and time. Furthermore, these microorganisms may preferentially reduce nitrate, and thus, the presence of nitrates may inhibit removal of perchlorate.
Only 1 experiment 32 has demonstrated perchlorate removal at concentrations close to those detected by the Phoenix Lander, and none to our knowledge have attempted to remove perchlorate from regolith simulants. Regolith is unique6,7,65 from soil and water in that it contains potential electron acceptors but no organic carbon and thus presents a need for more regolith-specific studies. Regolith will likely have other properties that will influence perchlorate removal efficiency that are not known. Typically mesophilic environmental and rhizosphere microorganisms are neither psychrophilic nor halotolerant, which would be a requirement for perchlorate reduction in the Martian environment unless conducted under controlled conditions such as a pressurized human habitat. We recommend future studies in these areas. We also urge investigators to conduct economic assessments of the energy and water expenditures of ISRU farming methods compared with hydroponics. Advancing biological perchlorate remediation technology for Mars applications would allow astronauts to live safely in domed environments with negligible perchlorate contamination and would enhance the viability of ISRU crop production.
Footnotes
ACKNOWLEDGMENTS
The authors are grateful for comments and feedback provided by our colleagues in the Blue Marble Space Institute of Science “Plants in Space” Research Action Group as well as insights from Dr. Colleen Knight, Associate Professor of Chemistry at the College of Coastal Georgia. We would also like to thank the Blue Marble Space Institute of Science's Young Scientist Program, which provided this research opportunity for author Madeline Garner.
AUTHOR DISCLOSURE STATEMENT
No competing financial interests exist.
FUNDING INFORMATION
No funding was received for this article.