Metabolic effects of vitamin B12 on physiology, stress resistance, growth rate and biomass productivity of Cyanobacterium stanieri planktonic and biofilm cultures

Although synthesized only by bacteria and archaea, cobalamin (vitamin B12) is essential for virtually all living cells. One major function is its role in methionine synthesis, as a co-factor for the B12-dependent methionine synthase MetH. However, a large number of microbes avoid requirements for B12 by encoding cobalamin-independent enzymes, such as the B12-independent methionine synthase MetE. Interestingly, many such microbes retain transporters for exogenous B12, produced by neighboring microbes. We hypothesize that selection for retention of B12 transport suggests preservation of unrevealed but critical roles for cobalamin in photoautotroph fitness. To identify the impacts of B12 on photoautotrophic metabolism, we studied the physiological and transcriptional adaptation of Cyanobacterium stanieri HL-69 to varying irradiance and oxidative stress in the presence and absence of B12. The metabolic flexibility of C. stanieri, which possesses both MetH and MetE, allows comparative analysis of cobalamin impacts on its global metabolism. As anticipated, B12 availability governed transcription of cobalamin transporter btuB, metH and a number of genes involved in the methionine-folate cycle. Surprisingly, however, B12 impacted the cell integrity, growth rate and biomass productivity of C. stanieri under conditions of likely oxidative stress due to biofilm growth or under high partial pressures of O2. Furthermore, C. stanieri response to B12 globally rewired cellular metabolic networks, including nitrogen metabolism, energy metabolism, redox homeostasis and oxidative stress response. These findings demonstrate previously-unappreciated roles for B12 metabolism beyond methionine synthesis and reveal how interactions with cobalamin-producing heterotrophs may affect phytoplankton function and dynamics in natural microbial communities. Further comprehension and mastering of the natural oxidative stress resistance mechanisms modulated by cobalamin could be used for the design and implementation of more robust algal bioprocesses retaining high biomass productivities especially under stress conditions.