Temperature Thresholds Govern Microbial-Mediated Dissolved Organic Carbon Dynamics in Coastal Ecosystems

Temperature is a key factor influencing coastal carbon pools, yet the effects of warming on dissolved organic carbon (DOC) transformations and associated microbial processes remain poorly understood. Through bioassay experiments across a wide temperature gradient (7.6-35.9 °C), three critical thresholds (15.6, 24.6, and 29.9 °C) are identified that delineate distinct regimes of microbial DOC utilization with contrasting carbon fates. Below 15.6 °C, DOC characterized the most unique molecules, and their transformations are governed by bacteria whose relative abundance decreased with warming; hereafter warming-resistant microbes dominated accompanied by DOC molecular signatures that changed till 24.6 °C. Limited substrate availability constrained microbial remineralization rates in these two stages. Once temperatures surpassed 24.6 °C, thermal-favored microbes prevailed but taxa changed after 29.9 °C, DOC accumulated, and a larger fraction of recalcitrant DOC is retained, thereby enhancing carbon sequestration. Network analysis further revealed complex many-one-many resource-consumer-excretion linkages between bacteria and individual DOC molecules, underscoring the tangled nature of microbial DOC processing. The findings demonstrate that coastal DOC cycling responds to warming in a strongly non-linear, threshold-controlled manner, providing critical insights for predicting the behavior of coastal carbon sinks under ongoing climate change.