Microorganisms Metabolizing on Clay Grains in 3-Km-Deep Greenland Basal Ice

We have discovered >10⁸ microbial cells/cm³ attached to clay grains in the bottom 13 m of the GISP2 (Greenland Ice Sheet Project) ice core. Their concentration correlates with huge excesses of CO₂ and CH₄. We show that Fe-reducing bacteria produce most of the excess CO₂ and methanogenic archaea produce the excess CH₄. The number of attached cells per clay grain is proportional to grain perimeter rather than to area, which implies that nutrients are accessed at grain edges. We conclude that Fe-reducing microbes immobilized on clay surfaces metabolize via "shuttle" molecules that transport electrons to grain edges, where they reduce Fe(III) ions at edges to Fe(II) while organic acid ions are oxidized to CO₂. Driven by the concentration gradient, electrons on Fe(II) ions at grain edges "hop" to Fe(III) ions inward in the same edges and oxidize them. The original Fe(III) ions can then attach new electrons from shuttle molecules at the edges. Our mechanism explains how Fe-reducers can reduce essentially all Fe(III) in clay minerals. We estimate that the Fe(III) in clay grains in the GISP2 silty ice can sustain Fe-reducing bacteria at the ambient temperature of –9°C for ∼10⁶ years. F ₄₂₀ autofluorescence imaging shows that >2.4% of the cells are methanogens, which account for the excess methane.