Surface and deep chlorophyll microbial community response to mixing events in the stratified Bay of Bengal (NE Indian Ocean)

The Bay of Bengal (BoB) is characterized by intense stratification driven by monsoonal freshwater flux coupled with high irradiance levels, which limits nutrient supply to the euphotic zone and restricts oxygen ventilation. While the northern part of the Bay remains hypoxic, ubiquitous mesoscale eddies provide a mechanism to break this stratification, pumping nutrients into surface layers and transporting shelf-associated microbial communities to the central bay. The response of these communities to the dynamics of this region remains however poorly understood.

The objective of the present study was to analyse the response of two distinct microbial communities to the environmental dynamics of this region. We conducted a series of onboard incubations using natural microbial communities collected from surface and deep chlorophyll maximum (DCM) waters during a cruise aboard the R/V Thomas G. Thompson in the summer months of 2025. Two distinct treatments were established: a control representing standard stratified conditions (characterized by an abrupt oxygen gradient and low irradiance at depth), and an experimental treatment designed to simulate the nutrient injection and mixing typically induced by cyclonic eddies, under well-oxygenated conditions and the local photoperiod. We monitored physiological parameters as chlorophyll-a concentration, the maximum photosynthetical quantum yield (F_v/F_m) and intracellular nitrate pools.

Our findings indicate that both communities (surface and DCM) exhibited similar response patterns under stratified conditions, with no significant growth and intracellular nitrate levels remaining lower (≈ 0.1 µM) than the freely dissolved pool (0.9-1.2 µM). In contrast, nutrient enrichment from bottom waters resulted in a rapid community response. The surface community exhibited a rapid uptake of nitrate within the first hours of incubation, resulting in an increase in the intracellular pool, which was followed by a gradual consumption over the following days. These results demonstrate the physiological plasticity of the community in response to a highly dynamic environment, with the capacity to utilize episodic nutrient enrichment within this highly variable system. Such plasticity may have significant implications for the nitrogen biogeochemical cycle as well as for the overall microbial community composition in the highly dynamic and increasingly deoxygenated North Indian Ocean.