Nutrient-driven growth and microbiome shifts in the brown alga Sargassum fluitans III.

Since 2011, holopelagic Sargassum has been accumulating in a region of the tropical Atlantic now referred to as the Great Atlantic Sargassum Belt (GASB). Among the hypothesized contributors to these accumulations are the increased inputs of nitrogen (N) and phosphorus (P) in the tropical Atlantic Ocean. Little is known about the effects of N and P additions on Sargassum physiology and its microbiome. We studied the effects of N, P, and NP additions on the growth, photosynthetic efficiency, and microbiome composition of Sargassum fluitans III in a six-day experiment on the Caribbean Island of Curaçao. Sargassum fluitans III took up most nitrate and phosphate within 3 days with respective uptake rates of 0.343 and 0.0399 μmol · g^-1 DW · h^-1. Fv/Fm decreased in the control after 6 days but remained constant in nutrient treatments. Growth rates did not differ significantly among treatments, but a trend in higher growth rates in the NP treatment was discerned, suggesting a possible NP co-limitation. The relative abundance of epiphytic Cyanobacteria such as Schizothrix and bacteria such as Lentilitoribacter increased under N and P addition, while heterotrophic Rhodobacteraceae decreased in abundance. Microeukaryotic communities responded with varying changes in alpha diversity, possibly steered by increased photosynthesis and growth of S. fluitans III or bacterial interactions. The physiological response to N and P and rapid change of the microbiome demonstrates that the studied S. fluitans III can quickly benefit from increased nutrient concentrations, which might contribute to its growth success in the GASB.