Microbial responses to ocean deoxygenation: Revisiting the impacts on organic carbon cycling.

Ocean deoxygenation is impacting and will also in the future impact fundamental biogeochemical cycles. This review explores the ecological functions of microbes under hypoxic and anoxic conditions, emphasizing their critical roles in carbon source-sink dynamics. We examine microbial ecosystems in both open-ocean oxygen minimum zones and China's coastal hypoxic areas, highlighting the microbial contributions to deoxygenation driven processes. We also explore how organic carbon cycling driven by microbial heterotrophic and autotrophic metabolisms change across oxygen gradients. Furthermore, this review elucidates the interconnected cycling of carbon, nitrogen, sulfur, and phosphorus, which regulate organic matter consumption and/or storage under deoxygenation, and alters the elemental composition of organic matter. Our study highlights the importance of microbial processes in regulating carbon cycle under ocean deoxygenation, emphasizing the dual role of hypoxic zones as transient sources and long-term sinks of organic carbon. Lastly, we highlight current challenges in addressing ocean deoxygenation and provide avenues for future research.