Long-Term Stability of Bacterial Associations in a Microcosm of (Chlorophyta, Mamiellophyceae).

Phytoplankton-bacteria interactions rule over carbon fixation in the sunlit ocean, yet only a handful of phytoplanktonic-bacteria interactions have been experimentally characterized. In this study, we investigated the effect of three bacterial strains isolated from a long-term microcosm experiment with one strain (Chlorophyta, Mamiellophyceae). We provided evidence that two strains (Alphaproteobacteria) had a beneficial effect on the long-term survival of the microalgae whereas one strain (Flavobacteriia) led to the collapse of the microalga culture. Co-cultivation of the beneficial and the antagonistic strains also led to the loss of the microalga cells. Metagenomic analysis of the microcosm is consistent with vitamin B12 synthesis by the strains and unveiled two additional species affiliated to (Balneolia) and (Flavobacteriia), which represent less than 4% of the reads, whereas and recruit 57 and 39% of the reads, respectively. These results suggest that the low-frequency bacterial species may antagonize the algicidal effect of in the microbiome of and thus stabilize the microalga persistence in the microcosm. Altogether, these results open novel perspectives into long-term stability of phytoplankton cultures.