Biodegradation of Terrigenous Organic Matter in a Stratified Large-Volume Water Column: Implications of the Removal of TerrigenousOrganic Matter in the Coastal Ocean

Large amounts of terrigenous organic matter (TOM) aredelivered to the ocean every year. However, removal processes of TOMin the ocean are still poorly constrained. Here, we report results from a339-day dark incubation experiment with a unique system holding avertically stratified freshwater-seawater column. The quality andquantity of dissolved organic matter (DOM), RNA-based size-fractionmicrobial communities, and environmental factors were high-frequency-monitored. Microbial processes impacted TOM composition, includingan increased DOM photobleaching rate with incubation time. The mixedlayer had changed the bacterial community structure, diversity, andhigher oxygen consumption rate. A two-end member modeling analysissuggested that estimated nutrient concentrations and prokaryoticabundance were lower, and total dissolved organic carbon was higherthan that of the measured values. These results imply that DOMbiodegradation was stimulated during freshwater-seawater mixing. In the bottom layer,fluorescent DOM components increasedwith the incubation time and were significantly positively related to highly unsaturated, oxygenated, and presumably aromaticcompound molecular formulas. These results suggest that surfaced-derived TOM sinking leads to increased DOM transformationand likely results in carbon storage in the bottom water. Overall, these results suggest that microbial transforming TOM plays moreimportant biogeochemical roles in estuaries and coastal oceans than what we know before.