Sharing Vitamin B₁₂ between Bacteria and Microalgae Does Not Systematically Occur: Case Study of the Haptophyte...

Haptophyte microalgae are key contributors to microbial communities in many environments. It has been proposed recently that members of this group would be virtually all dependent on vitamin B₁₂ (cobalamin), an enzymatic cofactor produced only by some bacteria and archaea. Here, we examined the processes of vitamin B₁₂ acquisition by haptophytes. We tested whether co-cultivating the model species Tisochrysis lutea with B₁₂-producing bacteria in vitamin-deprived conditions would allow the microalga to overcome B₁₂ deprivation. While T. lutea can grow by scavenging vitamin B₁₂ from bacterial extracts, co-culture experiments showed that the algae did not receive B₁₂ from its associated bacteria, despite bacteria/algae ratios supposedly being sufficient to allow enough vitamin production. Since other studies reported mutualistic algae-bacteria interactions for cobalamin, these results question the specificity of such associations. Finally, cultivating T. lutea with a complex bacterial consortium in the absence of the vitamin partially rescued its growth, highlighting the importance of microbial interactions and diversity. This work suggests that direct sharing of vitamin B₁₂ is specific to each species pair and that algae in complex natural communities can acquire it indirectly by other mechanisms (e.g., after bacterial lysis).