Succession of bacterioplankton communities over complete Gymnodinium-diatom bloom cycles.

Shifts in bacterioplankton communities during algal blooms have been widely investigated, but our understanding of their succession over the continuous course of paralytic shellfish poisoning producing Gymnodinium catenatum blooms and diatom (Skeletonema costatum and Chaetoceros curvisetus) blooms in natural bays is highly understudied. Here, we used high-throughput sequencing of bacterioplankton 16S rRNA genes to investigate the composition and successional patterns of bacterioplankton communities during Gymnodinium-diatom bloom cycles. Changes in community compositional patterns were then evaluated in context of environmental and phytoplankton community variation. Bacterioplankton α-diversity significantly decreased during the emergence of the algal blooms, with Flavobacteriaceae, Rhodobacteraceae, Cryomorphaceae, and Saprospiraceae as the dominant bacterial families in waters during the blooms. Bacterioplankton community compositions could be separated into three successive stages according to bloom dynamics, wherein the succession of bacterioplankton communities was correlated with changes in algal species. Environmental variables, and particularly pH, salinity, and nutrient concentrations (e.g., of nitrite, nitrate, and ammonium) were strongly associated with variation in bacterioplankton community structures. Variance partitioning analysis indicated that phytoplankton effects alone could explain more variance than only environmental effects. Moreover, LEfSe analysis was used to identify special bacterioplankton genera as "biomarkers" for bloom stages, such as Tepidisphaera and Pseudarcicella, whose abundances were significantly associated with different stages of the phytoplankton blooms. The phylotype "biomarkers" that were identified hold significant potential as indicators for phytoplankton bloom successional dynamics. Overall, these results may contribute to the understanding of the ecological processes shaping microbial communities during successive Gymnodinium-diatom blooms.