Rapid decomposition rates and nutrient cycling in pelagic Sargassum litter mediated by microbial communities

Pelagic Sargassum in the tropical North Atlantic has reached unprecedented biomass levels intensifying massive algal tides that severely impact coastal ecosystems. Understanding their growth and decomposition dynamics is crucial for elucidating their ecological effects. Here, we measured growth and decomposition rates of pelagic Sargassum and characterized the bacterial microbiome during aerobic decomposition. We found rapid decomposition with a litter half-life of approximately17 days, comparable to phytoplankton rates, and significant biomass losses during growth experiments. Microbial succession involved chemoheterotrophic and nitrogen-fixing bacteria, facilitating efficient nutrient cycling, particularly effective for phosphorus mineralization. This rapid detritus breakdown enables swift nutrient release, fuelling coastal eutrophication and contributing to oxygen depletion and toxic gas emissions during massive beaching events. These findings highlight the pivotal role of the Sargassum microbiome in organic matter mineralization and nutrient fluxes, offering insights into the environmental impact of Sargassum massive tides on coastal ecosystems and public health.