A microscopic partnership between a freshwater alga and a soil-associated bacterium may help determine how nutritious algae become—and how effectively their valuable compounds move through aquatic food webs. In a new study published in Microbial Ecology, researchers report that vitamin B₁₂ availability and interaction with the bacterium Mesorhizobium loti can alter the growth and biochemical composition of the mixotrophic alga Euglena gracilis. The findings suggest that bacteria do more than simply coexist with algae: by supplying essential micronutrients and other diffusible compounds, they may influence the nutritional quality of algal biomass, including its levels of the omega-3 fatty acids eicosapentaenoic acid, or EPA, and docosahexaenoic acid, or DHA. Because E. gracilis can contribute to freshwater blooms and serves as a potential food source for aquatic organisms, these microbial interactions could have consequences far beyond the laboratory.
Vitamin B₁₂ is central to the story because algae cannot synthesize it themselves. The vitamin is produced exclusively by certain prokaryotes, including bacteria and archaea, and is required for key biochemical reactions associated with cellular metabolism. More than 90 percent of the mixotrophic algae examined in previous surveys appear to require B₁₂, making the vitamin an important point of dependence between photosynthetic microorganisms and their bacterial partners. Euglena gracilis is particularly interesting because it is mixotrophic: it can use light to generate energy through photosynthesis while also taking up organic carbon from its surroundings. This flexible metabolism allows the alga to respond to changing environmental conditions, but it also means that its growth and chemical composition may be shaped by several interacting resources at once. The Finnish research team examined how direct B₁₂ supplementation, bacterial vitamin production, nitrogen availability, organic carbon, and physical contact between cells affected the alga.
The experiments showed that a lack of B₁₂ suppressed algal growth and reduced the accumulation of EPA and DHA. Both compounds are long-chain polyunsaturated fatty acids, or PUFAs, with important roles in cell membranes and high nutritional value for animals. EPA and DHA are often associated with marine foods, yet freshwater algae can also produce or contribute these essential fatty acids to aquatic ecosystems. When algae are eaten by zooplankton and other consumers, their biochemical composition can influence the quality of energy transferred to fish and higher trophic levels. A reduction in these fatty acids under B₁₂ deficiency therefore represents more than a change in laboratory measurements. It indicates that a shortage of a vitamin supplied by microorganisms could affect the nutritional value of algal production and potentially alter the flow of essential nutrients through freshwater food webs.
The presence of Mesorhizobium loti produced an especially notable result. Although adding B₁₂ directly to the algal cultures improved access to the vitamin, co-culturing E. gracilis with the bacterium resulted in greater accumulation of EPA and DHA than direct vitamin supplementation alone. This difference suggests that the bacterium’s influence cannot be explained simply by its role as a B₁₂ dispenser. M. loti may release additional metabolites, modify the surrounding chemical environment, or trigger physiological responses in the alga that redirect cellular resources toward fatty-acid synthesis. The precise mechanism remains unresolved, but the result highlights the complexity of microbial partnerships. A bacterial companion may alter algal physiology through a suite of compounds and interactions, rather than through one nutrient operating in isolation.
At the same time, the algae contained less intracellular B₁₂ when grown with M. loti than when the vitamin was added directly. At first glance, this may appear inconsistent with the stronger accumulation of EPA and DHA in the bacterial treatment. However, the researchers interpret the result as evidence that bacterial interaction may change how the alga acquires or uses B₁₂. Direct addition can expose algal cells to a relatively predictable external supply, while bacterial production may provide the vitamin in pulses, in localized microenvironments, or in association with other dissolved molecules. The alga may also use bacterially supplied B₁₂ more efficiently, maintaining growth and fatty-acid production without storing as much of the vitamin inside its cells. These possibilities remain to be tested, but they point toward a more dynamic model of nutrient exchange than a simple one-way transfer from bacterium to alga.
The study also examined whether physical contact between the two organisms was necessary for the interaction. In co-cultures where the algal and bacterial cells could interact directly, algal growth did not differ significantly from cultures in which physical contact was prevented. This finding suggests that the relevant exchange occurs mainly through diffusible metabolites moving through the surrounding medium. Such compounds could include vitamin B₁₂ itself, precursor molecules, signaling chemicals, or other bacterial products capable of changing algal metabolism. The absence of a measurable growth advantage from direct contact does not rule out close biological associations at the microscale, but it indicates that the partnership does not require the cells to remain attached. In natural waters, this kind of chemical exchange could occur across short distances around suspended algal cells, bacterial aggregates, or nutrient-rich particles.
Carbon availability added another layer to the results. When the researchers supplied highly labile dissolved organic carbon, or DOC, algal growth increased directly. Labile DOC consists of organic molecules that microorganisms can readily take up and metabolize, and the response demonstrated that E. gracilis was able to use the added carbon rather than relying exclusively on photosynthesis. This capacity is one of the defining advantages of mixotrophy. In turbid, shaded, or nutrient-variable freshwater environments, an alga capable of combining photosynthetic energy production with organic carbon uptake may continue growing when light or inorganic resources become limiting. Yet the result also emphasizes that carbon supply and vitamin supply are not interchangeable. Additional DOC promoted growth, while B₁₂ availability shaped growth as well as specific nutritional traits, including fatty-acid and protein production.
Protein responses depended strongly on nitrogen status. Under nitrogen-sufficient conditions, increasing the concentration of B₁₂ enhanced the total protein content of E. gracilis. Under nitrogen limitation, however, the vitamin had no detectable effect on protein accumulation. This contrast is consistent with the biochemical constraints imposed by nitrogen availability. Proteins contain substantial amounts of nitrogen, so even a vitamin-supported improvement in metabolic capacity may not increase protein production when the raw nitrogen required for synthesis is scarce. In other words, B₁₂ can help regulate or support cellular processes, but it cannot replace the elemental building blocks needed to construct biomass. The result demonstrates why the nutritional consequences of microbial interactions must be interpreted in the context of multiple resources. A bacterium may improve one aspect of algal quality while another environmental limitation prevents a response in a different biochemical component.
Together, the findings portray vitamin B₁₂-producing bacteria as influential partners in freshwater algal ecosystems. The bacterial contribution may affect not only how rapidly algae grow, but also the types of molecules they accumulate and the nutritional value of the resulting biomass. This matters for ecological productivity, especially in systems where algae form the foundation of aquatic food webs. It may also matter for biotechnology, since Euglena and other microalgae are being investigated as sources of proteins, pigments, lipids, and omega-3 fatty acids. Cultivation systems that include carefully selected bacterial partners could potentially produce biomass with a different biochemical profile than systems supplied with purified nutrients alone. However, the researchers caution implicitly through their results that the outcome will depend on environmental conditions, including nitrogen availability, organic carbon supply, and the specific mode of interaction between the organisms.
The study ultimately challenges the idea that algal nutrition can be understood by measuring isolated nutrients one at a time. Euglena gracilis responded not only to the presence or absence of B₁₂, but also to whether the vitamin came from a bacterium, whether organic carbon was available, and whether nitrogen limited protein formation. The stronger EPA and DHA accumulation observed in bacterial co-culture, despite lower intracellular B₁₂, suggests that microbial partnerships can reprogram or fine-tune algal metabolism in ways that direct supplementation may not reproduce. In freshwater environments, where dissolved nutrients and microbial communities constantly fluctuate, these interactions could help determine which algae thrive and how nutritious they are to their consumers. By revealing that a vitamin-producing bacterium can influence both algal performance and food quality, the research provides a closer look at the invisible partnerships that shape aquatic ecosystems.
Subject of Research: Effects of vitamin B₁₂ availability and interaction with the bacterium Mesorhizobium loti on the growth, metabolism, fatty-acid composition, protein content, and dissolved-organic-carbon use of the mixotrophic freshwater alga Euglena gracilis.
Article Title: Vitamin B₁₂ Availability and Interaction with Bacterium Mesorhizobium loti Affect Growth and Biochemical Composition of Mixotrophic Alga Euglena gracilis
Article References: Ghimire, S., Sohrabi, M. S., Zhou, X., et al. “Vitamin B₁₂ Availability and Interaction with Bacterium Mesorhizobium loti Affect Growth and Biochemical Composition of Mixotrophic Alga Euglena gracilis.” Microbial Ecology, 2026.
Image Credits: AI Generated
DOI: 10.1007/s00248-026-02816-0
Keywords: Microalgae, vitamin B₁₂, Euglena gracilis, Mesorhizobium loti, mixotrophy, PUFAs, EPA, DHA, dissolved organic carbon, proteins, freshwater ecosystems
