Insight into metabolic potential of carbon-poor pelagic sediments derived from the abundance and composition of organic carbon

Pelagic marine sediments are often carbon-limited, with oxic sediments bearing exceedingly small concentrations of metabolizable organic carbon (OC) and anoxic sediments lacking electron acceptors to drive heterotrophy. This OC is typically considered recalcitrant and presumed to be of limited bioavailability as much of it is difficult to characterize molecularly. Here, we utilize a combination of spectrometry, spectroscopy, and fluorescent assays to characterize the OC content and composition of sediment cores from the western subtropical North Atlantic collected during R/V Knorr expedition 223 in November 2014. We find that OC concentrations decrease linearly over ~15m burial depth from ~0.15 to 0.075 mol OC/kg sediment, beyond which this lower OC level persists to depths approaching 30m. The ratio of organic carbon to nitrogen (C/N) varies but is consistently close to Redfield values of ~6. Further, protein concentrations within the suboxic sediments are ~1.75 to 4.90 μg protein/mg sediment, values in excess of predicted cell abundance in subsurface sediments. After an initial decrease in concentration between 0-3 meters below core top, protein content increases and stabilizes at ~4 μg protein/mg sediment. RNA is detectable throughout the core and profiles (as μg cDNA amplified/g sediment) generally correlate with the shape of protein profiles. Combined, these results imply a small but active microbial community and the potential for these proteins to fuel heterotrophy at depth. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy finds that amide and carboxyl C functionalities comprise ~25% of total spectral area, with the remainder dominated by aromatic C and O-alkyl-C groups. These findings suggest that sedimentary OC contain identifiable components, including a substantial concentration of intact proteins that may fuel heterotrophic microbial communities.