Diatoms, one of the most abundant and diverse groups within the phytoplankton community, constitute primary producers in marine ecosystems and account for approximately 20% of global primary productivity. In surface seawater, despite the high concentration of dissolved inorganic carbon, photosynthesis by phytoplankton leads to rapid depletion of inorganic carbon, limiting the growth of diatoms.

Besides inorganic carbon concentrating mechanisms possessed by almost all marine diatoms, mixotrophic growth driven by algae-bacteria interactions is of great significance for establishing the dominance of marine diatom groups. However, the mechanisms underlying diatom-bacteria interactions under mixotrophic conditions have not been fully understood, which limits the understanding of diatom ecological adaptation and the regulatory mechanisms of the marine carbon cycle.

In a study published in Nature Communications, a research team led by Prof. HU Hanhua from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences revealed a syntrophic relationship between the model obligate photoautotroph diatom Phaeodactylum tricornutum and the rod-shaped bacterium Loktanella vestfoldensis, and its important role in the carbon adaptation mechanism of diatoms.

Based on Tara oceans biodiversity data, researchers conducted co-culture experiments, transcriptomics and metabolomics analyses, and co-occurrence analysis. They found that the growth of diatom depended on the support of Loktanella vestfoldensis for supplying necessary carbon source when glucose served as the sole carbon source, while Loktanella vestfoldensis showed dependence on Phaeodactylum tricornutum when CO₂ was the sole carbon source.

The results of growth changes and metabolite analysis of algae and bacteria under co-culture indicated that mixed carbon source induced competitive dynamics. Phaeodactylum tricornutum preferentially utilized inorganic carbon while partially suppressing Loktanella vestfoldensis via antimicrobial secretion (e.g., p-anisic acid and isonicotinic acid).

Furthermore, the analysis of Tara Oceans metagenomic data showed frequent co-occurrence of Loktanella with diatoms including Chaetoceros and Thalassiosira, indicating the ecological relevance of this partnership. Co-culture with Loktanella vestfoldensis supported the robust growth of Chaetoceros muelleri and Thalassiosira pseudonana in the presence of glucose as the sole carbon source.

Moreover, transcriptomic and metabolomic analyses revealed that Phaeodactylum tricornutum maintained 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.

This study demonstrates that bacterial-algal interactions may shape diatom adaptation to carbon changes and contribute to marine carbon cycling. It lays the foundation for constructing a universal model of marine diatom-bacteria interaction under carbon source changes.