Abstract
Scientific ocean drilling has greatly advanced the understanding of subseafloor sedimentary life. Studies of Ocean Drilling Program (ODP) and Integrated ODP samples and data show that 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. They have also shown that potentially competing metabolic pathways co-occur for hundreds of meter depth in subseafloor sediment deposited over millions of years.
Our study of an example site (eastern equatorial Pacific ODP Site 1226) indicates that the energy yields of these competing reactions are pinned to a thermodynamic minimum. The simplest explanation of this long-term coexistence is thermodynamic cooperation, where microorganisms utilize different but coexisting 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, scientific ocean drilling provides a tremendous opportunity to address several mysteries of microbial survival and natural selection under extreme energy limitations. 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?
A strong scientific ODP will be critical to address these mysteries.