Distinct interactions driven by DMSP between different microalgae and the phycosphere bacterium Sulfitobacter pseudonitzschiae H46

Microalgae-associated bacteria affect the dynamic succession of microalgal communities. The composition of the bacterial community of a single species of microalgae is conserved, and the same bacterial species can be found in different algal phycospheres. However, it is unknown whether the same bacterial species, widely distributed in the phycosphere, carries out the same interactions with different microalgae. Here, the interactions of Sulfitobacter pseudonitzschiae strain H46 and 4 microalgae, Phaeodactylum tricornutum , Cylindrotheca closterium , Heterosigma akashiwo , and Chattonella marina , were investigated using a co-cultivation method. The results showed that H46 had a significant inhibitory effect on the growth of C . marina but no effect on the other 3 microalgae. Further study revealed that H46 produced heat-resistant algicidal metabolites to lyse C . marina specifically, and secreted indoleacetic acid (IAA), which promoted the growth of the other 3 microalgae. Dimethylsulfoniopropionate (DMSP), produced by P . tricornutum , C. closterium , and H . akashiwo , can be utilized by H46. However, DMSP was not detected in C . marina , suggesting that S . pseudonitzschiae promotes the growth of DMSP-producing microalgae and kills DMSP-deficient microalgae. We speculate that DMSP may be a regulatory substance driving distinct algae-bacteria interactions, which is of great ecological significance for regulating the structure of microalgal communities.