LAUSR

Shifts in the bacterioplankton community composition during phytoplankton blooms have been studied extensively; however, investigations on protozoan blooms are rare. This study first evaluated the impact of perturbations caused by sequential protozoan blooms of the heterotrophic dinoflagellate Noctiluca scintillans and the mixotrophic ciliate Mesodinium rubrum on the structuring of these two bacterial communities. ABSTRACT During a series of blooms of Noctiluca scintillans and Mesodinium rubrum, we applied high-throughput sequencing of the 16S rRNA gene to investigate the population dynamics of free-living (FL) and particle-attached (PA) bacteria in an attempt to evaluate the influence of protozoan bloom-induced disturbances on the structuring of these two communities. Our findings revealed that the FL and PA bacterial community compositions (BCCs) displayed distinct profiles during sequential blooms, and the PA flora responded more dynamically to these pulse perturbations. The dominant bacterial groups (e.g., Flavobacteriaceae, Rhodobacteraceae, Vibrionaceae, and SAR11 subclade I) in these two communities displayed different levels of connectivity with the bloom-causative species and environmental factors. In addition, more FL bacterial groups were associated with M. rubrum, while more PA bacterial groups were related to N. scintillans. Potential endocytic bacteria of N. scintillans, particularly Vibrionaceae and Rickettsiaceae, opportunistically thrived at the peak of the bloom, suggesting that they could be important players influencing the dynamics and biogeochemical cycling of the blooms. Overall, disparities in the substrate preferences and thermal niches of various bacterial taxa as well as the short duration of the blooms (1 to 3 days) contributed to the diverse responses of the FL and PA bacterial communities to these protozoan blooms. Our research provides insight into the responses of FL and PA bacterial communities to blooms caused by protozoa like N. scintillans and M. rubrum and highlights the ecological significance of certain keystone bacterial groups during this kind of cosmopolitan protozoan bloom. IMPORTANCE Shifts in the bacterioplankton community composition during phytoplankton blooms have been studied extensively; however, investigations on protozoan blooms are rare. This study first evaluated the impact of perturbations caused by sequential protozoan blooms of the heterotrophic dinoflagellate Noctiluca scintillans and the mixotrophic ciliate Mesodinium rubrum on the structuring of these two bacterial communities. Our findings shed light on the responses of these two bacterial communities to such cosmopolitan protozoan blooms and highlight the possible ecological significance of certain keystone bacterial groups during these blooms. This research prepares the way for more focused studies that will help in understanding the roles that bacteria play during protozoan blooms and their impact on environmental health.