Bacillus sp. enhances the interspecific competitiveness of its host Cyclotella atomus

Whereas microalgae-associated bacteria are known to affect the growth and metabolism of their hosts, there is limited research on how bacteria change the competitiveness of their hosts against other algae. Fistulifera pelliculosa and Cyclotella atomus are valuable bait diatoms in aquaculture and both play roles in regulating water quality. In this study, we analyzed the bacterial community composition of these two diatoms based on 16S rRNA gene amplicons sequencing, and we isolated six bacterial strains which were highly abundant in the xenic diatom cultures. These dominant bacteria affected the interspecific competition between F. pelliculosa and C. atomus in the mixed culture experiments. In the absence of bacteria, or in the presence of any dominant bacterium except Bacillus sp., C. atomus was competitively inferior to F. pelliculosa. However, the addition of Bacillus sp. enabled C. atomus to achieve higher growth and temporarily establish dominance over F. pelliculosa, with this “boost” becoming more pronounced as bacterial density increased. The reintroduction of Bacillus sp. into the culture allowed C. atomus to maintain its lead for a longer period. Our study clearly showed that host-associated bacteria are involved in algal competition in aquatic environments and can significantly influence algal community succession. This provides new ideas for improving the community structure of pond ecosystems and increasing the economic benefits of algal mixed culture in aquaculture. [Display omitted] •Host-associated bacteria affected competition of bait diatoms.•Bacillus sp. could help C. atomus to become dominant against F. pelliculosa.•The density and the time of addition of Bacillus sp. was key to its influence.