Energetic Constraints of Subseafloor Life

Mean per-cell rates of catabolic activity, energy flux, and biomass turnover are orders of magnitude slower in subseafloor sediment than in the surface world. Despite extreme scarcity of electron donors, competing metabolic pathways co-occur for hundreds of meters deep in subseafloor sediment deposited over millions of years. Our study of an example site (ODP Site 1226) indicates that the energy yields of these competing reactions are pinned to a thermodynamic minimum (Wang et al., 2010). The simplest explanation of this long-term co-existence is thermodynamic cooperation, where microorganisms utilize different but co-existing pathways that remove each other’s reaction products.

Our Site 1226 results indicate that the energy flux to subseafloor sedimentary microbes is extremely low. Comparison to biomass turnover rates at other sites suggests that most of this flux may be used for building biomolecules from existing components (e.g., amino acids in the surrounding sediment), rather than for de novo biosynthesis from inorganic chemicals.

Given these discoveries, ocean drilling provides a tremendous opportunity to address several mysteries of microbial survival and natural selection under extreme energy limitation. Some of these mysteries are centered on microbial communities. To what extent do counted cells in subseafloor sediment constitute a deep microbial necrosphere? How do different kinds of microbes interact to sustain their mean activity at low average rates for millions of years? Other mysteries relate to individual cells. How slowly can a cell metabolize? How long can a cell survive at such low rates of activity? What properties allow microbes to be sustained by low fluxes of energy? In what ways do subseafloor organisms balance the benefit(s) of maximizing energy recovery with the need to minimize biochemical cost(s) of energy recovery?

References

Wang, G., et al., 2010. Geochimica et Cosmochimica Acta 74, 3938-3947.