Preliminary Insights Into the Interplay Among Oxygen, Organic Carbon, and Microbial Metabolism in North Atlantic Subseafloor Sediment Communities

Abstract:

Deep marine sediments harbor abundant microbial cells that, if active, are likely to exert a strong influence on element biogeochemical cycling. However, our understanding of the fraction of cells that are active in situ and the metabolic processes that sustain them remain underexplored. Here we describe recent results of our studies aimed at unraveling the links between geochemical heterogeneity, cellular viability and synthesis, and metabolism along a vertical depth profile in sediment from four deep sites (>5 km beneath ocean surface) cored by R/V KNORR Expedition KN-223 in the North Atlantic (2014). These sediment columns exhibit varying levels of organic carbon and different vertical extents of oxygen (O₂) penetration, which we hypothesize is due to variation in the extent of heterotrophic metabolism. We prepared most probable number (MPN) assays with acetate or peptone as electron donor and carbon source, and five different terminal electron acceptors (O₂, NO₃, SO₄, MnO₂, and ferrihydrite) with sediments from 4 to 5 depths in each of the four cores MPNs were similar for acetate- and peptone-amended cultures, regardless of electron acceptor, and generally decreased with depth in the sediment column. MPNs amended with O₂ as electron acceptor were greater than MPNs amended with NO₃, SO₄, MnO₂, and ferrihydrite in samples from all depths. Moreover, MPNs were higher for assays amended with O₂ from cores where the depth of O₂ penetration was shallow when compared to cores where O₂ is predicted to penetrate to basement rock. These results are consistent with aerobic heterotrophs limiting the penetration of O₂ in deep marine sediments, and thereby provide a mechanism to explain the relationship between low O₂ penetrations in sediment cores with elevated organic carbon contents. We will also present results of our ongoing isotopic labeling experiments aimed at determining rates of DNA and protein synthesis as proxies for cell replication and productivity, respectively. Collectively, this research improves our understanding of the microbial activities present in subseafloor sediment of the deep North Atlantic Ocean and provides new insight into the interplay among O₂ and organic carbon and microbial activity, viability, and productivity in deep marine sediments.