Interactions with bacteria shape diatom adaptation to carbon concentration changes

Diatoms are key contributors to global primary production, and have developed intricate partnerships with bacteria through long-term co-evolution. Here, we uncover a syntrophic relationship between the model obligate photoautotroph diatom Phaeodactylum tricornutum and the rod-shaped bacterium Loktanella vestfoldensis, which enables the diatom to indirectly utilize glucose. To be specific, growth of the diatom depends on the support of L. vestfoldensis for the supply of necessary carbon source when glucose serves as the sole carbon source, while L. vestfoldensis shows dependence on P. tricornutum when CO₂ is the sole carbon source. Reanalysis of Tara Oceans metagenomic data shows frequent co-occurrence of Loktanella with diatoms including Chaetoceros and Thalassiosira, indicating the ecological relevance of this partnership. Co-culture with L. vestfoldensis supports robust growth of Chaetoceros muelleri and Thalassiosira pseudonana in the presence of glucose as the sole carbon source. Transcriptomic and metabolomic analyses reveal that P. tricornutum maintains a photoautotrophic metabolism in co-culture, as indicated by the up-regulation of genes involved in inorganic carbon concentration and photosynthesis, while the co-cultured bacterium likely supplies CO₂ and growth-stimulating metabolites such as indole-3-acetic acid. Our findings demonstrate that bacterial-algal interactions may shape diatom adaptation to carbon changes and contribute to marine carbon cycling.