Biofilm formation and cell plasticity drive diazotrophy in an anoxygenic phototrophic bacterium.

Non-cyanobacterial diazotrophs (NCDs) are widespread and active in marine waters. The carbon and low-oxygen (O 2 ) conditions required for their N 2 fixation may be encountered on marine particles, while a putative role of light remains uninvestigated. This study explored factors that regulate N 2 fixation in Rhodopseudomonas sp. BAL398-a anoxygenic phototrophic bacterium isolated from low-salinity surface waters. Light (250 µmol photons m -2 s -1 ) and anoxia (0 µM O 2 ) stimulated growth and N 2 fixation; however, diazotrophy in light was dependent on high organic carbon levels (35 mM, glucose:succinate). Immunolabeling revealed that cellular nitrogenase levels increased with light, decreasing inorganic nitrogen (N) and ambient O 2 (250 µM). Light and O 2 stimulated motility and biofilm formation on surfaces, and N 2 fixation rates increased compared to the control treatment. N 2 fixation rates were positively correlated with the formation of rosette-like cellular structures, and an increased concentration of nitrogenase was observed toward the center of these structures, which increased their occurrence 600 times when cultures reached maximum N 2 fixation rates vs when they had low rates. Interestingly, N 2 fixation was not completely inhibited under oxic conditions and was accompanied by increased formation of capsules and cysts. Rosettes, as well as capsules and cysts, may thus serve as protection against O 2 . Our study reveals the physiological adaptations that underlie N 2 fixation in an anoxygenic phototroph, emphasizing the significance of biofilm formation for utilizing light and fixing N 2 under oxic conditions, and underscores the need for deciphering the importance of light for marine NCDs.IMPORTANCEThe contribution of non-cyanobacterial diazotrophs (NCDs) to total N 2 fixation in the marine water column is unknown, but their importance is likely constrained by the limited availability of dissolved organic matter and low O 2 conditions. Light could support N 2 fixation and growth by NCDs, yet no examples from bacterioplankton exist. In this study, we show that the phototrophic NCD, Rhodopseudomonas sp. BAL398, which is a member of the diazotrophic community in the surface waters of the Baltic Sea, can utilize light. Our study highlights the significance of biofilm formation for utilizing light and fixing N 2 under oxic conditions and the role of cell plasticity in regulating these processes. Our findings have implications for the general understanding of the ecology and importance of NCDs in marine waters.