Coupled Bacterial and Microeukaryotic Community Turnover Across a Localized Dissolved Oxygen Reversal in the South China Sea

Vertical environmental gradients are important in structuring marine microbial communities, yet microbial responses to localized reversals within vertical environmental profiles remain less well resolved. We investigated bacterial and microeukaryotic plankton communities at ten depths between 5 and 250 m at station G4 in the northern South China Sea (SCS), with intensified sampling between 122 and 163 m around a localized subsurface dissolved oxygen (DO) reversal. Continuous CTD observations confirmed a reversal in the local DO–depth gradient within this interval. Bacterial and picoeukaryotic abundances peaked at the deep chlorophyll a maximum, while both bacterial and microeukaryotic communities exhibited clear depth-related changes in diversity and composition. Multivariate analyses showed that community variation was associated with a broader vertical environmental gradient involving temperature, salinity, density, nutrients, and chlorophyll a, whereas DO did not show a clear independent association after accounting for depth. Bacterial and microeukaryotic communities nevertheless exhibited broadly concordant vertical turnover. Taxonomically, surface waters were characterized by greater contributions from Cyanobacteriia and Dinophyceae, whereas deeper assemblages contained increased proportions of Gammaproteobacteria, Syndiniales, Polycystinea, and other subsurface-associated groups. Together, these observations suggest that the localized DO reversal occurred within a broader vertical environmental transition accompanied by coordinated changes in bacterial and microeukaryotic community structure.