PNNL

Abstract

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 in for the B12-dependent methionine synthase MetH. However, large number of microbes avoid requirements for B12 by encoding cobalamin-independent enzymes, such as the B12-independent methionine synthase MetE. Ineterestingly, many such microbes retain transporters for exogenous B12, produced by neighboring microbes. We hypothesise 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 photoautorophic 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, alows 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 and growth rate 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, and 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. Importance Cobalamin cross-feeding is recognised as a key factor promoting establishment of complex microbial systems. However, the lack of understanding regarding B12 impacts on photoautotroph metabolism hinders our ability to predict structure-function relationships in phytoplankton communities. Our data suggesting B12’s irrelevance to C. stanieri’s growth rate in the absence of oxygen stress may explain the loss of B12 synthesis genes from its genome. However, B12 impacts on fitness during periodic exposures to elevated pO2 in the diffusion-limited environment of a phototrophic microbial mat suggests a rationale for retention of B12-dependent processes and transport in C. stanieri. Furthermore, this study reveals that B12 availability exerts far-reaching impacts on C. stanieri metabolism, such as in redox homeostasis and oxidative stress response. Finally, understanding the mechanisms underlying the protective effect of cobalamin against oxidative stress may help explain the high robustness of phototrophic microbial communities and suggests strategies for engineering more efficient bioprocesses.