Vitamin B12 Availability and Interaction with Bacterium Mesorhizobium loti Affect Growth and Biochemical Composition of Mixotrophic Alga Euglena Gracilis.

Over 90% of the mixotrophic algae surveyed require vitamin B12 (B12), a micronutrient synthesized exclusively by certain prokaryotes. Bacteria-mediated B12 supply presents an important but poorly understood control on physiology and nutritional quality of freshwater algae. Here, we examined the effects of B12 availability and the B12-synthesizing bacterium, Mesorhizobium loti, on the growth and biochemical composition of the freshwater bloom-forming mixotrophic alga, Euglena gracilis. We also investigated bacteria-alga interactions under highly labile dissolved organic carbon (DOC) supplementation with and without physical contact. Our results indicate that B12 deficiency reduced algal growth and the accumulation of the essential fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Co-culture with M. loti resulted in higher EPA and DHA accumulation than supplementation with exogenous B12 alone. Despite this, E. gracilis accumulated less intracellular B12 in co-culture than under direct B12 addition, suggesting altered B12 acquisition or utilization during bacterial interactions. Increasing B12 concentration enhanced total protein content in E. gracilis under sufficient nitrogen conditions but had no effect under nitrogen limitation. The addition of labile DOC directly promoted algal growth, indicating its direct utilization. No significant difference was observed in algal growth in co-cultures with and without physical contact, indicating that possible B12 sharing from bacteria mainly occurs through diffusible metabolites. These findings demonstrate that B12-synthesizing bacteria influence the growth and nutritional quality (in terms of EPA, DHA and protein content) of E. gracilis, with potential consequences for freshwater ecosystem productivity and trophic transfer.