Microbial community respiration (MCR) strongly controls the fate of organic carbon in the ocean. The balance between MCR and primary production strongly determines whether the ocean is a net sink or source of CO2 to the atmosphere. Thus, it is necessary to estimate MCR to better understand the role of oceans in the global carbon cycle. Methods based on apparent oxygen utilization (AOU) are predominant while electron transport system (ETS) assay gets increasing attention. Although methods get developed, few studies on MCR have been performed on a seasonal cycle. Because MCR is strongly associated with the temperature which changes along with the succession of seasons, it is urgent to study the MCR on a seasonal cycle. Thus, we measured MCR using in vivo tetrazolium salt 2-(piodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazolium chloride (INT) reduction rates (ETS) and oxygen-optode methods (AOU) simultaneously we measured the MCR based on AOU and ETS methods simultaneously from November 2020 to November 2021 in Aoshan Bay, China. The highest AOU appeared in autumn, followed by summer, spring, and winter, whereas the highest ETS activity appeared in summer, followed by spring, autumn and winter. The seasonal trend of MCR estimated from AOU and ETS were not consistent, and further analysis indicated that oxygen consumption induced by nitrification caused the overestimation of MCR in autumn evaluated from AOU. Microbial groups that were strongly correlated with MCR estimated by ETS had the ability to degrade various substrates and could get energy directly from light. It should be careful to notice the deviation of assumed organic carbon demand based on ETS caused by the alternation of day and night. Furthermore, the pattern of bacterial groups associated with year-round MCR was distinct from season-specific MCR. This study raised a warning for caution when estimating MCR based on AOU and it was better to fully take the photoheterotrophy into account when assuming organic carbon remineralization based on ETS.

microbial community respiration, apparent oxygen utilization, electron transport system assay, photoheterotrophy, coastal ocean