Chlorophyll a gradients shift heterotrophic bacterial assembly from dispersal limitation to biotic filtering of picophytoplankton.

The spatiotemporal dynamics of heterotrophic bacterial communities are critical for regulating elemental cycles. Yet, the processes governing the assembly of these communities across chlorophyll gradients remain unclear, especially regarding biotic interactions. Here, we present a framework that integrates biotic filtering of nanoplankton and picophytoplankton with environmental filtering and dispersal limitation to better elucidate the assembly processes of heterotrophic bacterial communities in a mesoscale marine environment. Microbial communities were collected along a 280 km east-west transect in the southern East China Sea during 13 research cruises conducted in late spring and summer during 2014-2018. Variation partitioning analysis explained 49.8%-78.1% of the variation in heterotrophic bacterial communities across cruises. As concentration of chlorophyll increased, dispersal limitation diminished, while biotic filtering of picophytoplankton intensified. Moreover, the ratio of deterministic to stochastic processes increased with chlorophyll a concentration. These results highlight deterministic processes, particularly biotic filtering of picophytoplankton, in structuring heterotrophic bacterial metacommunities during periods of high chlorophyll a concentration. Our findings emphasize the importance of incorporating biotic interactions into the framework of microbial community assembly to better understand microbial ecosystem responses to varying chlorophyll a conditions.