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 B 12 (B 12 ), a micronutrient synthesized exclusively by certain prokaryotes. Bacteria-mediated B 12 supply presents an important but poorly understood control on physiology and nutritional quality of freshwater algae. Here, we examined the effects of B 12 availability and the B 12 -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 B 12 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 B 12 alone. Despite this, E. gracilis accumulated less intracellular B 12 in co-culture than under direct B 12 addition, suggesting altered B 12 acquisition or utilization during bacterial interactions. Increasing B 12 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 B 12 sharing from bacteria mainly occurs through diffusible metabolites. These findings demonstrate that B 12 -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.