Marine sediments comprise Earth’s largest organic carbon (OC) pool and a vast number of microorganism species (in the order of 1,029 species) that feed on it. However, the inaccessibility of the seafloor and the long timescales over which many sediment processes occur, has resulted in a limited and extremely patchy understanding of subseafloor biogeochemistry. Moreover, outside of data from specific sediment cores and laboratory experiments, how much energy microbes in marine sediments need to survive is not well known.
James Bradley of Queen Mary University of London, UK, and colleagues, modelled the global degradation of OC in marine sediments deposited during the Quaternary and calculated the amount of energy subseafloor microbes use. Three different microbial metabolisms were considered — aerobic respiration, sulfate reduction and methanogenesis. 0.171 Pg C yr-1 of OC degradation was linked to sulfate reduction (64.5% of global Quaternary OC degraded), with another 0.076 C yr-1 (28.6%) associated with methanogenesis. Although aerobic respiration degraded only 0.018 Pg C yr-1 (6.9% of OC), it provided 54.5% (20.3 GW) of the power (energy flux) to the subseafloor. Sulfate reduction and methanogensis provided 39.1% (14.6 GW) and 6.4% (2.4 GW) of global subseafloor power, respectively. When these values were scaled on an individual cell basis, the average amount of power used per microbe was two orders of magnitude less than the lowest experimentally-based estimates of the minimum power required to sustain life. This result suggests that in situ microbial power requirements are much lower than previously thought.
Source: Ecology - nature.com
