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).

A unique symbiosome in an anaerobic single-celled eukaryote

“…Diatoms have been shown to strongly induce vitB12-binding proteins under limiting growth conditions (Bertrand et al 2012) and modeling predicts active vitB12 release from bacteria in co-cultures (Grant et al 2014). How vitB12 is harvested from bacteria by microbial eukaryotes is not clear but directed release and lysis have both been suggested (Grant et al 2014; Nef et al 2022).…”

Chlamydomonas reinhardtii, a Reference Organism to Study Algal–Microbial Interactions: Why Can’t They Be Friends?

“…Therefore, this cofactor may act as a common bacterial currency for mutualism with algae in exchange for algal fixed C, and it may play a key role in microbial community structure and function [ 48 ]. Nevertheless, not all B 12 -producing bacteria and algal B 12 auxotrophs establish mutualistic relationships, indicating a species-specificity that may be due to the amount of B 12 released or the type of C fixed secreted by the alga [ 49 , 50 ]. While some studies have shown that bacterial growth was required to support algal B 12 auxotrophs [ 51 ], others have reported that bacterial cell lysis was sufficient for that purpose [ 49 ].…”

“…Nevertheless, not all B 12 -producing bacteria and algal B 12 auxotrophs establish mutualistic relationships, indicating a species-specificity that may be due to the amount of B 12 released or the type of C fixed secreted by the alga [ 49 , 50 ]. While some studies have shown that bacterial growth was required to support algal B 12 auxotrophs [ 51 ], others have reported that bacterial cell lysis was sufficient for that purpose [ 49 ]. In algae, the enzyme methionine synthase ( METH ) uses vitamin B 12 as a cofactor, but some algae, such as Chlamydomonas , also have a B 12 -independent isoform ( METE ) that allows them to grow without the vitamin, albeit less efficiently [ 52 ].…”

<em>Chlamydomonas Reinhardtii, </em>a Reference Organism to Study Algal-Microbial Interactions. <em>Why Can’t They Be Friends</em>

“…Therefore, this cofactor may act as a common bacterial currency for mutualism with algae in exchange for algal fixed C, and play a key role in microbial community structure and function [48]. Nevertheless, not all B12-producing bacteria and algal B12 auxotrophs establish mutualistic relationships, indicating a species-specificity that may be due to the amount of B12 released or the type of C fixed secreted by the alga [49,50]. While some studies have shown that bacterial growth is required to support algal B12 auxotrophs, others have reported that bacterial cell lysis is enough for that purpose [49,51].…”

“…Nevertheless, not all B12-producing bacteria and algal B12 auxotrophs establish mutualistic relationships, indicating a species-specificity that may be due to the amount of B12 released or the type of C fixed secreted by the alga [49,50]. While some studies have shown that bacterial growth is required to support algal B12 auxotrophs, others have reported that bacterial cell lysis is enough for that purpose [49,51]. In algae, the enzyme methionine synthase (METH) uses vitamin B12 as a cofactor, but some algae, like Chlamydomonas, also have a B12-independent isoform (METE) that allows them to grow without the vitamin, albeit less efficiently [52].…”