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

The contribution of non-cyanobacterial diazotrophs (NCDs) to total N₂ fixation in the marine water column is unknown, but their importance is likely constrained by the limited availability of dissolved organic matter and low O₂ conditions. Light could support N₂ 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 N₂ 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.