Increased Nitrogen Fixation Observed With Degrading Sargassum Communities

Sargassum is an important marine organism that dominates coastal benthic macroalgal communities worldwide and forms vast pelagic communities in the Atlantic Ocean. It is relatively well established that biological nitrogen fixation can be a significant source of nitrogen for pelagic Sargassum communities. While nitrogen fixation hasn’t been found to be as significant a source of nitrogen for benthic Sargassum spp., increased rates of nitrogen fixation have been reported with decomposing benthic species of Sargassum. Recently, there have been reports of massive inundation of pelagic and drifting benthic Sargassum spp. referred to as “golden tides”. In 2018, while onboard a transatlantic cruise sailing from Cape Verde to Puerto Rico, we encountered vast stretches of ocean populated with pelagic Sargassum communities. Using the acetylene reduction method, low nitrogenase activity was detected with freshly sampled, living Sargassum biomass relative to visibly senescing macroalgal tissue at multiple stations throughout the transect. During a long term (~20 days) shipboard decomposition experiment, degrading Sargassum detritus was sub-sampled every few days and exhibited increasing nitrogen fixation rates throughout decomposition. In contrast to aging benthic Sargassum detritus where heterotrophic nitrogen fixation is thought to be dominant, higher nitrogenase activity associated with degrading pelagic Sargassum tissue was observed in light incubations than in the dark. Interestingly, repeat additions of carbon substrates (e.g. glucose, mannitol) stimulated nitrogen fixation rates, suggesting that labile carbon can be limiting to the diazotrophic community associated with decomposing pelagic Sargassum detritus. With increasing global frequencies of golden tides, the results from this study suggest that degrading Sargassum biomass may provide an expanding niche for nitrogen fixing organisms worldwide.