An alternative view of the role of heterotrophic microbes in the cycling of organic matter in deep-sea sediments

Abstract

The role of microbial heterotrophs in deep-ocean sediment communities is explored using a simplified model of the cycling of carbon within a generic, size-based food web. Measurements of potential rates of respiration and growth of bacteria appear to be low, given the high concentrations of both microbial biomass and dissolved organic matter (DOM) in pore waters. This enigma can be explained theoretically by assuming that much of the microbial biomass is restricted in function at high pressure and low temperature, that a large fraction of the dissolved organic carbon (DOC) is refractory, and that the small fraction of the DOM that is labile and thus available must be liberated from the particulate pool by extracellular enzyme activity, viral infection or through the feeding processes of the metazoans. Free-living heterotrophic microbes in the sediments thus play a minor role in metazoan food webs, providing only a small fraction of the nourishment of metazoans. On the other hand, microbial heterotrophs appear to be responsible in part for the high levels of refractory DOM observed to accumulate in the deep ocean.