Biodiversity and community dynamics of pelagic Sargassum: ecological and sustainable use implications of wild vs. cultivated aggregations

The vast holopelagic Sargassum assemblages within the Great Atlantic Sargassum belt (GASB) have been described as an important habitat for marine fauna. While previous studies have investigated the composition of the prokaryotic and eukaryotic communities associated with holopelagic Sargassum, the impact of these organisms on surrounding marine environments remains unclear. Sargassum aquafarming is a novel approach developed to provide companies with a stable biomass supply throughout the seasons. Due to potential impacts of aquafarming on surrounding marine ecosystems, an environmental impact assessment is necessary. Using state-of-the-art genomic methods, we investigated the prokaryotic and eukaryotic communities linked to holopelagic Sargassum mats, and compared the biodiversity of a Sargassum aquafarm off St. Vincent to an unimpacted adjacent bay. We found that water within wild Sargassum mats present a very unique community composition when compared to surrounding open ocean waters, underscoring its ecological importance as a pelagic habitat for a variety of species. We identified Rivularia spp., a nitrogen-fixing cyanobacterium, as a main component within the microbiome, potentially functioning as a nitrogen provider in oligotrophic waters, and a range of epiphytic eukaryotes, such as cnidarians and bryozoans, with E. bellula as one of the main epiphytes. When comparing the aquafarm bay with its adjacent bay, we found slight changes in community composition driven by the presence of Rivularia, but no significant impact on the alpha-biodiversity; this group disappeared in the surrounding marine environment after the trial, indicating its association with the cultivated Sargassum. These insights are relevant for the development of sustainable management practices for wild Sargassum influxes and its aquafarming as a sustainable source of biomass for the blue economy.