Organic carbon in a seepage face of a subterranean estuary: Turnover and microbial interrelations

Subterranean estuaries, the mixing zone between terrestrial groundwater and coastal seawater, are important biogeochemical hotspots. In the present study, organic carbon cycling and related drivers, including the characterization of different organic carbon pools and sediment microbial community, were investigated in a subterranean estuary seepage face. Within the first 20 cm depth seepage face sediments, both production and removal of dissolved organic carbon (DOC) were observed, mainly driven by heterotrophic microbes. From spring to autumn, active DOC production occurred on the seepage face at the 15-20 cm depth, likely via aerobic degradation of sediment organic carbon (SOC) with subsequent release of dissolved fractions into the porewater. During winter, DOC production moved to a shallower depth of the seepage face due to increasing SOC content in the surface layer. DOC production rate depended on heterotrophic microbial biomass (e.g. Proteobacteria) and was enhanced by high microbial activity and porewater advection. DOC removal frequently occurred at the 0-5 cm depth layer except in winter. The seasonal shift in carbon source utilization (SOC to DOC) in this layer likely resulted from the decrease in SOC pool, especially the labile portion of SOC and the increased availability of DOC due to production in the deeper sediment (15-20 cm). Given the similarity in microbial community structure along the sediment profile, this shift suggests SOC as the preferential carbon source for benthic microbes as well as adaptive flexibility in microbial carbon source utilization. DOC removal was also significantly tied to microbial activity and advection rate. Because DOC production rates were higher compared to DOC consumption the seepage face acted as a net source of DOC to the coastal ecosystem.