Size-fractioned aggregates within phycosphere define functional bacterial communities related to Microcystis aeruginosa and Euglena sanguinea blooms

Phytoplankton growth and degradation are associated with bacterial communities in phycosphere. We compared the bacterial communities in two algal blooms (Microcystis aeruginosa and Euglena sanguinea). In particular, large particle-associated, small particle-associated, and free-living bacteria were collected in succession using membranes with three different pore sizes (10 µm, 1 µm, and 0.22 µm) in this study. The results showed that Proteobacteria and Bacteroidetes were the dominant phyla in both blooms. Significant differences in the structure, function, and assembly were observed between large particle-associated and free-living bacterial community in both blooms. Specifically, relative abundance of Roseomonas was higher in the large-size (> 10 µm) aggregates, while Candidate_division_TM7 and Candidate_division_SR1 were more abundant in the free-living bacterial community (0.22–1 µm). The large particle-associated bacterial community was mainly driven by heterogeneous selection and dispersal limitation, whereas the small particle-associated and free-living bacterial communities were mostly determined by dispersal limitation. Besides, some specific bacterial subgroups were found between M. aeruginosa and E. sanguinea blooms. Chlorobi, Chloroflexi, and Fusobacteria were abundant in the M. aeruginosa blooms, whereas Deinococcus–Thermus was abundant in the E. sanguinea blooms. Those specific bacteria provide competitive advantages for the dominated algae. Altogether, these findings indicate that the phycosphere of bloom-forming algae provides different ecological niches where different functional bacterial groups can adapt.