Factors influencing nitrogen fixation in epiphytic diazotroph communities associated with pelagic Sargassum

Nitrogen is one of the most commonly limiting nutrients throughout the world’s oceans and it has long been acknowledged that pelagic Sargassum reliably hosts nitrogen fixing epiphytes, yet very little is known about what controls nitrogen fixation rates within this community. Here, we provide an updated examination of this process by relating rates of nitrogen fixation by the pelagic Sargassum community within the Gulf Stream to a range of biotic and abiotic factors. Nitrogen fixation rates were found to be significantly and positively associated with epiphytic cyanobacteria abundances as well as that of epiphytic toxin-producing dinoflagellates. Canonical correspondence analyses and generalized additive models revealed that the cyanobacterial diazotrophs Dichothrix spp. and Oscillatoriales were significantly and positively related to nitrogen fixation rate and that no physicochemical variable measured had a statistically significant relationship with nitrogen fixation rate. Additionally, we show that rates of nitrogen fixation are consistently higher when bioassays are initiated immediately upon Sargassum collection compared to if bioassay start times are delayed until after transport to the laboratory. This finding not only has important implications for the interpretation of macrophyte studies conducted ex situ but also contextualizes reactive nitrogen input estimates previously reported by this community. Additionally, this study advances understanding of the pelagic Sargassum holobiont by documenting the first report of an epiphytic Trichodesmium sp. bloom associated with Sargassum and by characterizing the microbiomes of select epiphytic dinoflagellate taxa that were associated with Sargassum throughout this study. Knowledge of the controls on nitrogen fixation, and the degree to which they influence rates, help to inform biogeochemical models thereby improving our understanding of the marine nitrogen cycle and how it may be altered in the face of global environmental changes.