Temporal dynamics of trace metals and arsenic in holopelagic Sargassum spp. during their retention in coastal waters

Since 2011, massive blooms of holopelagic Sargassum spp. have increasingly impacted tropical Atlantic and Caribbean coastlines, prompting research into biomass valorisation and the environmental risks associated with metal contamination. This study investigates the accumulation dynamics of trace elements-including total arsenic (As), inorganic arsenic (iAs), and various metals-in holopelagic Sargassum during their stagnation in coastal waters. A 21-day in situ caging experiment was conducted in Baie du Robert (Martinique) using the three morphotypes of holopelagic Sargassum (S. natans var. wingei, S. natans var. natans, and S. fluitans var. fluitans) under two biomass density conditions (3 or 9 kg m^-3). Elemental composition, photosynthetic efficiency, and environmental parameters were monitored. For the first time, we demonstrate clear and distinct accumulation patterns: (1) Fe, Al, Pb, V, and Cr showed progressive enrichment, especially under low-density conditions, likely via colloidal adsorption; (2) Mn, Zn, Co, and Cd increased rapidly in both densities, suggesting uptake of dissolved species; (3) both total As and iAs declined significantly (p-value<0.05), particularly in S. fluitans, with iAs reductions up to 75%, showing that prolonged retention of Sargassum in coastal environments can substantially reduce iAs concentrations toward regulatory thresholds, a critical finding for the safe valorisation and use of Sargassum biomass.