Presenter: Yubin Raut (University of Southern California)

Description:
Macroalgae provide important ecological functions (e.g., primary production, nutrient sequestration) in coastal ecosystems worldwide. A large fraction of macroalgal productivity is remineralized and microbes are integral in the processing of this organic matter. We currently lack understanding of the different factors that might influence microbial degradation of macroalgal detritus. Nitrogen (N), for instance, is often overlooked as a potentially limiting nutrient to the microbial community throughout decomposition. Considering relatively high carbon (C) to N content of macroalgae, it is plausible that during decomposition labile N is utilized much more rapidly than C by the microbial community, resulting in further increased N limitation. N₂ fixers or diazotrophs are specialized microbes which possess the nitrogenase enzyme responsible for catalyzing the conversion of N₂ gas into a biologically available form of N. In such strongly N limiting conditions, diazotrophs have an ecological advantage and may serve as a previously unexplored source of N. N₂ fixation rates measured during long-term (15 – 32 days) macroalgal decomposition experiments and literature surveys of diazotrophic activity throughout degradation of other coastal macrophytes show a remarkably similar trend. Based on this, we propose a three-phase theoretical model that predicts nitrogenase activity throughout macrophyte decomposition. Lastly, we used the acetylene reduction assay to measure nitrogenase activity with sediment collected at two sites around Santa Catalina Island, CA and concurrently carried out labile C (e.g., glucose) and macroalgal amendments. The addition of fresh brown macroalgal biomass resulted in (> 100-fold) stimulation of nitrogenase activity similar to 10 mM glucose amendments. These findings support an emerging perspective that macroalgal organic matter provides an important niche for the development of diazotrophic communities in coastal ecosystems.