Organic Metabolite Uptake by Diazotrophs in the North Pacific Ocean

Dinitrogen (N₂) fixation by diazotrophs sustains biological productivity in the ocean. Diazotroph groups include photoautotrophic cyanobacteria, non-cyanobacterial diazotrophs (NCDs) and the recently discovered N₂-fixing haptophyte. NCDs' ecology and metabolism are poorly understood despite being ubiquitous in the ocean and often the dominant group in nifH gene surveys. NCDs are primarily heterotrophic, acquiring carbon and energy from external sources of dissolved organic matter (DOM). Their interactions with DOM still need to be clarified. Strikingly, photoautotrophic diazotrophic cyanobacteria have been shown to use DOM, often allowing them to cope with nutritional and energetic stress. Conventional DOM amendment incubations do not allow discerning how different diazotroph groups use DOM molecules. Here, we amended surface microbial communities from the North Pacific Ocean with ¹³C-labelled DOM derived from phytoplankton cultures and used DNA stable isotope probing and nifH gene amplicon sequencing to identify diazotrophs capable of taking up DOM. We further used a metabolomics approach to characterise the ¹³C-labelled DOM composition. The addition of ¹³C-DOM induced a shift from Crocosphaera and other less abundant cyanobacterial diazotrophs, such as Trichodesmium, to a predominance of NCDs in stations when the N₂-fixing haptophyte show low relative abundance. Our results reveal overlooked osmotrophic behaviour by diverse diazotroph lineages with substantial spatial variability driven by the background biogeochemical environment.