Cast adrift: Physiology and dispersal of benthic Sargassum spinuligerum in surface rafts

Connectivity of populations through the transfer of individuals is one of the key processes for maintaining ecosystem stability and resilience of coastal ecosystems. During reproduction, dislodged seaweeds of the genus Sargassum form large pelagic surface rafts that can persist for several weeks, and potentially act as a dispersal vector. In surface rafts, seaweeds are rapidly exposed to increased light, elevated temperatures, and altered hydrodynamic conditions. Acclimation to surface conditions is necessary for survival during the rafting phase, but there is limited knowledge of seaweed physiology in rafts. To understand the mechanisms for acclimation, we created rafts of floating Sargassum spinuligerum for 2 weeks and compared these experimental fronds to those attached to the seafloor. We measured nutrients, photosynthetic and nonphotosynthetic pigments, and phlorotannins to investigate their role in the persistence of mature Sargassum at the ocean surface as indicated by photosynthetic rates and reproductive status. We also studied potential surface movement of the rafted seaweed over 3-weeks of particle tracking using an existing oceanographic model. Photosynthesis and reproductive status were similar between benthic and rafted seaweeds, indicating no change to overall metabolic processes during rafting. While phlorotannin concentrations and photosynthetic pigments were unchanged, photoprotective xanthophyll pigments were more abundant in rafted individuals, suggesting acclimation to surface light conditions. Our results suggest that, in the short term, S. spinuligerum employ chemical strategies to acclimate and maintain physiological processes in the rafting environment and potentially fix more carbon, allowing these rafts to act as dispersal vectors among populations over tens of kilometers apart.