Bacterioplankton assembly and interspecies interactions follow trajectories of Gymnodinium-diatom bloom.

The underlying mechanisms of bacterioplankton community assembly and interspecies interactions during harmful algal blooms remain largely unclear. Using 16S rRNA gene amplicon sequencing, we analyzed the bacterioplankton communities over the continuous course of saxitoxin-producing Gymnodinium catenatum blooms and two diatom (i.e., Skeletonema costatum and Chaetoceros curvisetus) blooms in an anthropogenically controlled and eutrophic bay, East China Sea. The succession of bacterioplankton communities correlated with changes in the dynamics of algal species. Deterministic versus stochastic bacterioplankton community assemblage processes were quantified, demonstrating that stochastic processes increased when algal blooms happened. The occurrence of algal blooms caused weaker bacterioplankton interspecies interactions and higher degrees of cooperative activities, changed keystone taxa and diminished the stability of bacterial communities. These findings consequently have important implications for our understanding of bacterioplankton community ecology during algal blooms.