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Microbial Processing of Sediment‐Derived Dissolved Organic Matter: Implications for Its Subsequent Biogeochemical Cycling in Overlying Seawater

Coastal sediments contain a large amount of dissolved organic matter (DOM), which can be mobilized into the overlying water by natural and anthropogenic activities. The bioavailability and subsequent biogeochemical effects of this sediment‐derived DOM are unclear. To investigate those, we collected a sediment pore‐water DOM (SDOM) sample and its overlying seawater to conduct a bioassay experiment, which allowed tracking of both short‐term and long‐term microbial processes in the context of DOM transformations. Short‐term incubation results show that the SDOM extract supported the growth of specific taxa. The microbial community composition changed dramatically and an approximately 50% of SDOM‐derived carbon was consumed within the first 2 days. Viruses likely played a role in promoting bacterial community succession, further enhancing transformation of this SDOM. Long‐term incubation results show that labile DOM was gradually consumed, while approximately 16% of the initial SDOM appeared to be recalcitrant to microbial utilization and remained at the end (after 110 days) of the incubation experiment. Despite the short‐term drastic changes in microbial community composition, a highly diverse microbial community is similar to the control at the end of the incubation. It is suggested here that resuspension of coastal sediments weakens their role as a net sink of carbon, with most of the mobilized SDOM transformed by successive microbial communities in the overlying seawater and the remaining recalcitrant organic material becoming part of the long‐lived DOM pool. Thus, the bioavailability of the coastal SDOM might influence the carbon budget in coastal oceans.