In subsurface sediments (both oxic and anoxic), heterotrophic microorganisms slowly oxidize sedimentary organic carbon. The mechanisms by which heterotrophs access sedimentary organic carbon are poorly understood. In dynamic surface environments (e.g., seawater phytoplankton blooms), heterotrophs use extracellular enzymes to hydrolyze biopolymers and subsequently take up low-molecular-weight hydrolysis products. I will discuss my lab’s recent studies of extracellular enzyme activities in sediments of the White Oak River, NC, and IODP-drilled sediments of the Baltic Sea Basin. Although cell densities decrease substantially throughout the top meter of White Oak River sediments, per-cell potential activities are fairly constant, and are similar to values observed in seawater phytoplankton blooms. The set of extracellular enzymes present in subsurface sediments, however, differs from that in surface sediments: as depth increases, extracellular enzymes are increasingly capable of accessing low concentrations of degraded organic carbon. In the Baltic Sea, active extracellular enzymes were observed at depths of up to 50 meters below seafloor, in sediments that are roughly 7500 years old. No enzyme activity was detected between 50 mbsf and 80 mbsf, despite the presence of apparently active cells in those sediments. It is possibly that other, very slow processes liberate bioavailable compounds from sedimentary organic matter. Such processes might include desorption of polymers or oligomers from mineral surfaces and abiotic production of reactive oxygen species, which would rapidly react with detrital organic matter to liberate CO and volatile fatty acids. Extracellular enzymes therefore are a known mechanism by which heterotrophic microorganisms access degraded organic matter in subsurface sediments, but it seems very likely that other, mostly-unknown mechanisms are at work as well.

How heterotrophic microorganisms access sedimentary organic matter: A few known knowns, and plenty of known unknowns

253rd National Meeting (2017)

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