Alphaproteobacteria facilitate Trichodesmium community trimethylamine utilization

Summary In the surface waters of the warm oligotrophic ocean, filaments and aggregated colonies of the nitrogen (N)‐fixing cyanobacterium Trichodesmium create microscale nutrient‐rich oases. These hotspots fuel primary productivity and harbour a diverse consortium of heterotrophs. Interactions with associated microbiota can affect the physiology of Trichodesmium, often in ways that have been predicted to support its growth. Recently, it was found that trimethylamine (TMA), a globally abundant organic N compound, inhibits N2 fixation in cultures of Trichodesmium without impairing growth rate, suggesting that Trichodesmium can use TMA as an alternate N source. In this study, 15N‐TMA DNA stable isotope probing (SIP) of a Trichodesmium enrichment was employed to further investigate TMA metabolism and determine whether TMA‐N is incorporated directly or secondarily via cross‐feeding facilitated by microbial associates. Herein, we identify two members of the marine Roseobacter clade (MRC) of Alphaproteobacteria as the likely metabolizers of TMA and provide genomic evidence that they converted TMA into a more readily available form of N, e.g., ammonium (NH4+), which was subsequently used by Trichodesmium and the rest of the community. The results implicate microbiome‐mediated carbon (C) and N transformations in modulating N2 fixation and thus highlight the involvement of host‐associated heterotrophs in global biogeochemical cycling.