Early anaerobic metabolisms

Abstract

Before the advent of oxygenic photosynthesis, the biosphere was driven by anaerobic metabolisms. We catalogue and quantify the source strengths of the most probable electron donors and electron acceptors that would have been available to fuel early-Earth ecosystems. The most active ecosystems were probably driven by the cycling of H 2 and Fe 2C through primary production conducted by anoxygenic phototrophs. Interesting and dynamic ecosystems would have also been driven by the microbial cycling of sulphur and nitrogen species, but their activity levels were probably not so great. Despite the diversity of potential early ecosystems, rates of primary production in the early-Earth anaerobic biosphere were probably well below those rates observed in the marine environment. We shift our attention to the Earth environment at 3.8 Gyr ago, where the earliest marine sediments are preserved. We calculate, consistent with the carbon isotope record and other considerations of the carbon cycle, that marine rates of primary production at this time were probably an order of magnitude (or more) less than today. We conclude that the flux of reduced species to the Earth surface at this time may have been sufficient to drive anaerobic ecosystems of sufficient activity to be consistent with the carbon isotope record. Conversely, an ecosystem based on oxygenic photosynthesis was also possible with complete removal of the oxygen by reaction with reduced species from the mantle.

Fe(II) Oxidation Is an Innate Capability of Nitrate-Reducing Bacteria That Involves Abiotic and Biotic Reactions

“…Previous studies have reported a growth advantage from NDFO by Acidovorax species in both batch and continuous-flow cultures (12,14), in addition to the accumulation of nitrite (NO 2 Ϫ ) and periplasmic mineral encrustations (26). On the basis of our findings with A. ebreus, we postulate that all nitrate-respiring organisms are innately capable of catalyzing NDFO, but the survival and growth benefits obtained are dependent on the ability to overcome Fe(II), NO 2 Ϫ , and NO toxicity. Our findings have implications for understanding the evolution of Fe(II) oxidation as a microbial metabolism, the impact of Fe(II) on nitrate-reducing communities, and the influence of Fe(II) on the product distribution of microbial nitrate reduction.…”

“…3c). These results suggest that Nar directly catalyzes NO 3 Ϫ reduction coupled to Fe(II) oxidation (54) and/or NO 2 Ϫ is produced to further react with Fe(II) through the abiotic and biotic mechanisms previously discussed.…”

“…The precipitate was washed with 10 mM phosphate buffer and kept as a suspension under anoxic conditions. To assess the capacities of NO and NO 2 Ϫ to oxidize and solubilize Fe(II) from synthetic vivianite, anoxic aqueous stock solutions of sodium nitrite or diethylamine (DEA) NONOate (Cayman Chemical) in 0.1 M NaOH were added to BBM containing synthetic vivianite. NONOates, or diazeniumdiolates, are stable at alkaline pH but decompose at neutral pH to release NO (28).…”

“…Acetate is utilized before Fe(II) as an electron donor for nitrate reduction, and excess acetate inhibits NDFO and NO 2 ؊ , NO, and N 2 O accumulation. It was previously noted that acetate is consumed before Fe(II) in NDFO growth cultures (5,60), and this is also true for A. ebreus (see Fig.…”

Experimental Hydrogen Production in Hydrothermal and Fault Systems: Significance for Habitability of Subseafloor H2 Chemoautotroph Microbial Ecosystems

“…Sufficiently high concentrations of H 2 occur in the vent fluids of these systems to sustain an ecosystem based on H 2 -driven chemolithoautotrophic primary producers such as methanogens. Moreover, an H 2 -driven ecosystem is the most probable candidate habitat for the earliest life on Earth (Russell and Hall 1997;Sleep et al 2004;Kelley et al 2005;Takai et al 2006;Canfield et al 2006). On this basis, it has been hypothesized that hydrothermal fluids with H 2 , sufficiently abundant to sustain methanogens, existed in the early Earth (Takai et al 2006).…”