Marine subseafloor sediments host one of Earth's largest microbial habitats, yet global bioenergetic modeling (Bradley et al., 2020) shows that most of their cells subsist at per-cell power levels (median 3.32×10−203.32\times10^{-20} W; 84% below the proposed basal power limit of 1.9×10−191.9\times10^{-19} W per cell) lower than any power flux previously shown to support life. The question is open: field studies suggest a possible maintenance power near 10−1710^{-17} W per sulfate-reducing cell, but cells apparently operating down to 7×10−227\times10^{-22} W remain unexplained, culture-derived maintenance powers sit five orders of magnitude above the modeled fluxes, and no persistence mechanism - dormancy, minimum maintenance, or necromass recycling - has been quantified for natural subseafloor communities. The open problem asks for the minimum power that sustains viable microbial life in subseafloor sediments and the mechanisms of persistence, together with the model refinements needed to compare modeled energy supply against that limit.

Background

Marine sediments buried beneath the seafloor host one of Earth's largest microbial habitats: microbial cells persist there at depth for thousands to millions of years, sustained by the slow degradation of particulate organic carbon (POC, the remains of organisms settling from the overlying ocean). As sediment is buried, the sequence of available electron acceptors creates distinct geochemical zones: an upper oxic zone where aerobic respiration consumes dissolved oxygen, a sulfate-reducing zone where sulfate (SO42−SO_4^{2-}) oxidizes organic matter, and deeper methanogenic sediments beneath the sulfate-methane transition (the depth where sulfate is exhausted and methane begins to accumulate) where methanogenesis dominates. The energy flux - power, in watts - available to microbes in each zone equals the rate of POC degradation multiplied by the Gibbs energy (the chemical free energy released) of the corresponding catabolic reaction; dividing by the number of microbial cells present gives the cell-specific power, the quantity that must exceed a cell's own requirements for it to maintain itself.

Maintenance power is the energy flux a microorganism requires merely to stay alive - repairing damaged biomolecules, sustaining ion gradients across the cell membrane, and replacing cellular components - without growing. Laboratory cultures and field studies had bracketed this quantity many orders of magnitude above the fluxes typical of deep sediments. LaRowe and Amend (2015) combined the lowest maintenance power reported in the literature (1.9×10−171.9\times10^{-17} W per cell, from low-light-adapted green sulfur bacteria) with the estimate that preventing spontaneous amino-acid racemization damage (racemization being the slow, spontaneous mirror-flip of amino-acid building blocks, which gradually corrupts proteins) requires roughly 200-fold less power, and proposed a basal power limit for natural settings of about 1.9×10−191.9\times10^{-19} W per cell - a 'minimum decay prevention power' below which a population should irreversibly decay. Hoehler and Jørgensen (2013) and Lever et al. (2015) synthesized the laboratory and field evidence for life under such extreme energy limitation, concluding that the minimum power requirement of subsurface life was real but poorly constrained.

Bradley et al. (2020) then built a global reaction-transport model (a numerical model that simulates chemical reactions and solute transport through the sediment column) of Quaternary-age (roughly the last 2.6 million years) marine sediments, coupling modeled organic matter degradation rates, Gibbs energies computed for representative in situ chemical compositions, and a global parameterization of cell abundance, to compute power utilization throughout the subseafloor biosphere. They found a total power use of 37.3 gigawatts - under 0.1% of the power fixed by photosynthesis in the overlying ocean - and a median cell-specific power of 3.32×10−203.32\times10^{-20} W, with 84.0% of subseafloor cells below the proposed 1.9×10−191.9\times10^{-19} W basal limit (at least 65.7% under every tested sensitivity scenario). The modeled communities therefore appear to subsist at energy fluxes below any power level previously shown to support active life, yet cells are demonstrably present in sediments millions of years old. Candidate survival mechanisms - dormancy (a reversible, near-inactive physiological state), maintenance at unprecedentedly low power, and recycling of necromass (dead microbial biomass) - remain unquantified for these communities. The model itself also simplifies the natural system: all POC degradation is assigned to aerobic respiration, sulfate reduction, or methanogenesis in their respective zones; other oxidants (nitrate, metal oxides) and fermentation are treated as minor or proxied by acetate as a representative organic compound; Gibbs energies use representative compositions rather than spatially variable pore-water chemistry; and the fraction of cells actively carrying out each prescribed metabolism is assumed from genetic potential rather than measured.

Problem Statement

Determine the power limit to life in marine subseafloor sediments. Specifically: what is the minimum per-cell power at which microbial life can be sustained in these sediments, and by what physiological mechanisms - for example dormancy, extremely low maintenance states, or necromass recycling - do subseafloor microbial communities persist at the per-cell energy fluxes that global bioenergetic models assign them, namely a median of 3.32×10−203.32\times10^{-20} W with most cells below the proposed basal power limit of 1.9×10−191.9\times10^{-19} W per cell? Two linked strands of evidence are required to settle this single question. First, the cell-specific power estimates themselves must be refined by including the reaction pathways the global model neglected (nitrate and metal-oxide reduction, fermentation, and a more explicit representation of diverse organic substrates), by using spatially variable pore-water chemistry rather than representative compositions for the Gibbs energies, and by replacing the assumed active fraction of biomass with measured distributions of metabolic activity - so that the modeled energy supply is compared against the viability limit on a faithful basis. Second, the physiological state and power requirements of subseafloor microorganisms under in situ, long-term energy limitation must be quantified at representative sites, so that it can be decided whether the observed global cell abundances are reconcilable with the modeled energy supply through viable maintenance rather than slow decay. Two outcomes of the comparison are scientifically possible: establishing the basal power limit for subseafloor life together with the persistence of the observed biomass at the modeled fluxes, or a faithful recalibration in which the modeled cell-specific power exceeds the limit. Comparing modeled power against the limit requires a quantified physiological basis, and neither a recalibration in isolation nor a demonstration of cell revival without quantifying the power at which the cells persisted establishes that the observed biomass is sustained.

References

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