Regulation and control of the planktonic microbial respiration in the hypoxic waters of the Bay of Bengal

The Bay of Bengal is considered one of the largest oceanic Oxygen Minimum Zone (OMZ), characterized by oxygen levels that remains persistently near the threshold of anoxia, possibly limiting the widespread nitrogen loss observed in other major OMZs. Planktonic microbial respiration is largely responsible for the formation and maintenance of the hypoxic and anoxic conditions found in the OMZs. Understanding the respiratory kinetics of the planktonic microbial community is therefore essential to predicting the sensitivity of this area to further deoxygenation. During a cruise aboard the R/V Thomson in the summer months of 2025, we investigated the regulation and control of microbial oxygen consumption within the upper 300 m of the water column. We combined high-resolution vertical profiling with experimental rate measurements using high-sensitivity oxygen sensors to characterize the metabolic transition from the upper oxic layer through the oxycline into the nearly anoxic core. Microbial community abundance was quantified via flow cytometry to link biomass density with metabolic activity. Respiratory kinetics were characterized by onboard water incubations with samples subjected to a wide range of oxygen levels. Our results demonstrate a clear vertical stratification in respiratory potential, with the highest rates associated with the upper oxic layer and a progressive decrease as oxygen and chlorophyll levels decreased. However, higher values were also found at intermediate depths within the hypoxic water layers. By fitting oxygen consumption rates to kinetic models, we calculated the apparent half-saturation constant (Km) for the microbial community throughout the water column. These Km values showed a complex distribution, generally reaching their minimum in the oxycline and increasing within the hypoxic zones. This suggests a counterintuitive decrease in oxygen affinity at low oxygen levels, although significant consumption rates were observed even at trace levels of oxygen. This trend may indicate a taxonomic shift in the microbial community or a change in the expression of different types of terminal oxidases, thereby demonstrating adaptation of the microbial community to the episodic oxygen supply characteristic of the interior of the Bay of Bengal.