In a marine environment generally lacking in bioavailable nitrogen, access to atmospheric nitrogen is becoming a key issue. Nitrogen fixation is reserved for diazotrophs possessing a key enzyme, nitrogenase. This enzyme presents two problems: its high energy cost and its sensitivity to oxygen. Cyanobacteria have been widely studied, but it's only recently that a large diversity of non-cyanobacterial diazotrophs bacteria (NCDs) has been discovered. This work is based on a high-throughput sequencing study combined with a genetic and physiological characterization of an NCD, Vibrio diazotrophicus, as well as on cell-scale quantification of nitrogenase expression, in order to determine the adaptive strategies of this NCD when growing under nitrogen-deficient conditions. We were able to demonstrate that V. diazotrophicus has great metabolic versatility, able of assimilating a wide range of organic and inorganic nitrogen sources. In addition, a quantitative epifluorescence microscopy approach revealed posttranscriptional regulation of nitrogenase expression, as well as modulation by V. diazotrophicus of the proportion of cells expressing nitrogenase and its intensity. This modulation is dependent on ammonium concentration and is regulated by the transcriptional regulator NtrC. Finally, we were able to demonstrate that this phenotypic heterogeneity in nitrogenase expression is found in other marine NCDs, suggesting that it is a conserved trait. This study lifts the veil on the physiology of NCDs and their importance in the biogeochemical cycle of marine nitrogen.

Related publication : Crétin, P., Mahoudeau, L., Joublin-Delavat, A., Paulhan, N., Labrune, E., Verdon, J., ... & Delavat, F. (2025). High metabolic versatility and phenotypic heterogeneity in a marine non-cyanobacterial diazotroph. Current Biology.