Necromass as a Limited Source of Energy for Microorganisms in Marine Sediments

The in situ production of necromass and its role as a power source in sustaining heterotrophic microorganisms in marine sediments has never been quantified. Here we quantify the power availability from necromass oxidation to living microorganisms buried in marine sediments over millions of years, first in the oligotrophic South Pacific Gyre (SPG) and second on a global scale. We calculate that power from autochthonously produced necromass in the upper meter of sediment underlying SPG provides only a small fraction (~0.02%) of the maintenance power demand of the living community (1.9 × 10 −19  W cell −1 ). Power from necromass oxidation diminishes considerably with increasing sediment depth (and thus sediment age). Alternatively, the oxidation of allochthonous organic matter, and of radiolytic H 2 , provides power equivalent to or in excess of the maintenance demands of living microorganisms at SPG. On a global scale, necromass may support the maintenance power demand of 2 to 13% of the microbial community in relatively young sediments (<10,000 years) when it is oxidized with SO 4 2− and O 2 , respectively. However, in older sediments, the power supplied by necromass is negligible (<0.01%). Nevertheless, the oxidation of a single dead cell per year provides sufficient power to support the maintenance demands of dozens to thousands of cells in low‐energy marine sediments. This raises the possibility that the production and oxidation of necromass may provide a mechanism for non‐growing microorganisms to endure unfavorable, low‐energy settings over geological timescales.