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

Abstract

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).

Keywords: Tisochrysis lutea; cobalamin; haptophytes; microbial interactions; phytoplankton; vitamin B12.

Figure 1

Screening of 12 bacterial strains for vitamin B₁₂ content (means of n = 9 technical replicates ± standard deviation of the mean). The genus of the isolates is indicated.

Figure 2

Cultivation experiments of T. lutea with different B₁₂ sources. (A) Growth curves for axenic controls (left panel), with bacterial extracts (center panel), or in co-culture with single live bacterial strains (right panel), with different B₁₂ availability levels (means of n = 3 biological replicates ± standard deviation of the mean). (B) Bacteria/algae ratios for the co-cultures with live bacterial cells with different B₁₂ availability levels, as above (means of n = 3 biological replicates ± standard deviation of the mean).

Figure 3

Co-culture experiment of T. lutea with a complex microbial consortium with different B₁₂ availability levels (means of n = 3 biological replicates ± standard deviation of the mean).

Figure 4

Sankey diagram representing the composition of T. lutea-associated microbial consortium (xenic-B₁₂) estimated by metabarcoding. From left to right are the relative proportions of taxa at the phylum, class and order levels for one of three replicates (see Supplementary Table S2A for details on the other replicates) using the ASVs assigned starting at the phylum level. Actinobacteria_P: phylum Actinobacteria; Actinobacteria_Cl: class Actinobacteria.