Shifts in microbial metabolism for algal-derived organic matter utilization under dynamic oxygen conditions

The increase prevalence of hypoxic zones worldwide requires a comprehensive understanding of how marine organic carbon cycles under these conditions to assess their impact on global carbon cycles and ecosystem health. This study, conducted within the seasonal hypoxia zone of the Yangtze River Estuary, investigates the effects of deoxygenation on the microbial utilization of algal-derived organic carbon. Culture experiments and molecular analyses revealed remarkable adaptability in the composition and metabolic pathways of heterotrophic bacterial communities in response to hypoxic conditions. Findings demonstrate that while initial rates of dissolved organic carbon (DOC) utilization are reduced under hypoxia, the consumption over time is comparable to conditions of deoxygenation gradually. High-throughput 16S rDNA sequencing highlighted notable shifts in microbial community dynamics, with a consistent increase in the roles of key heterotrophic bacterial groups such as Rhodobacteraceae, Flavobacteriaceae, and Vibrionaceae in the transformation of organic carbon under reduced oxygen conditions. Importantly, hypoxia was found to induce more complex patterns observed in the transformations of dissolved organic sulfur. The depleted DOC molecules were predominantly composed of CHON and CHO. Under hypoxic conditions, unique microbial organic modules emerged, displaying positive correlations with dissolved organic sulfur (DOS) molecules, primarily involving the Shewanellaceae and Bacteroidetes. This research reveals specific microbial responses and metabolic mechanisms involved in the transformation of algal-derived organic carbon under hypoxic conditions. These findings improve our understanding of organic carbon cycling processes in hypoxic waters and offer new insights for future research on microbially driven carbon cycling.