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.
In the NE Pacific, Ulvaria obscura is a common component of "green tide" blooms. It is also the only alga known to produce dopamine, which is released into seawater on sunny days when Ulvaria is emersed and then rehydrated. To better understand the mechanisms associated with dopamine release, we experimentally determined whether light quantity and quality, desiccation, temperature, exudates from conspecifics, and dissolved dopamine caused dopamine release. We also examined the effects of desiccation on Ulvaria's ability to photosynthesize, grow, and survive. Desiccation was the only factor that caused significant amounts of dopamine to be lost from U. obscura tissues. The loss of water from Ulvaria tissues was strongly and positively correlated with the loss of dopamine after rehydration. Only 56% of desiccated algae survived for 1 week, compared to 100% of undesiccated control algae. Desiccated algae lost 77% of their pigmented surface area and grew only 15% as much as undesiccated algae, which remained fully pigmented. The oxygen saturation of water containing Ulvaria that was desiccated and then rehydrated was significantly lower than that of seawater containing undesiccated algae. Thus, desiccation, which is coupled with dopamine release, is associated with the deterioration and death of some, but not all, tissues in Ulvaria. Although dopamine released into seawater can reduce the survival or growth of potential competitors, its release is associated with significant physiological stress and tissue mortality. However, the survival and continued growth of some Ulvaria tissues indicates that a net fitness benefit to release dopamine following desiccation cannot be ruled out.
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