In situ observations and modelling revealed environmental factors favouring occurrence of Vibrio in microbiome of the pelagic Sargassum responsible for strandings

Abstract

Historically, pelagic Sargassum were only found in the Sargasso Sea. Since 2011, blooms were regularly observed in warmer water, further south. Their developments in Central Atlantic are associated with mass strandings on the coasts, causing important damages and potentially dispersion of new bacteria. Microbiomes associated with pelagic Sargassum were analysed at large scale in Central Atlantic and near Caribbean Islands with a focus on pathogenic bacteria. Vibrio appeared widely distributed among pelagic Sargassum microbiome of our samples with higher occurrence than previously found in Mexico Gulf. Six out the 16 Vibrio-OTUs (Operational Taxonomic Unit), representing 81.2 ± 13.1% of the sequences, felt in cluster containing pathogens. Among the four different microbial profiles of pelagic Sargassum microbiome, Vibrio attained about 2% in two profiles whereas it peaked, in the two others, at 6.5 and 26.8 % respectively, largely above the concentrations found in seawater surrounding raft (0.5%). In addition to sampling and measurements, we performed backward Lagrangian modelling of trajectories of rafts, and rebuilt the sampled rafts environmental history allowing us to estimate Sargassum growth rates along raft displacements. We found that Vibrio was favoured by high Sargassum growth rate and in situ ammonium and nitrite, modelled phosphate and nitrate concentrations, whereas zooplankters, benthic copepods, and calm wind (proxy of raft buoyancy near the sea surface) were less favourable for them. Relations between Vibrio and other main bacterial groups identified a competition with Alteromonas. According to forward Lagrangian tracking, part of rafts containing Vibrio could strand on the Caribbean coasts, however the strong decreases of modelled Sargassum growth rates along this displacement suggest unfavourable environment for Vibrio. For the conditions and areas observed, the sanitary risk seemed in consequence minor, but in other areas or conditions where high Sargassum growth rate occurred near coasts, it could be more important.

A NEMO-based model of <i>Sargassum</i> distribution in the tropical Atlantic: description of the model and sensitivity analysis (NEMO-Sarg1.0)

“…Biological nitrogen fixation by diazotrophic macroalgal association has been shown to be important for some Sargassum species (e.g. Sargassum horneri, Raut et al, 2018) and this could also be the case for the holopelagic Sargassum where epibionts Nfixating bacteria have been observed on these species (Carpenter, 1972;Michotey et al, 2020).…”

“…The temperature dependence in Hanisak and Samuel (1987) for Sargassum natans suggests a broad optimal temperature range of 18-30 • C and indicates no growth at 12 • C. We lack information on the Sargassum fluitans response to the variability of the environmental parameters. Lapointe (1986) highlights a growth mainly limited by phosphate availability, while the presence of nitrifying epiphytes (Carpenter, 1972;Michotey et al, 2020) could be a non-negligible source of nitrogen for Sargassum, as could urea and ammonium excreted by fish (Lapointe et al, 2014). It is also likely that Sargassum are able to store some nutrients in their tissues, as do other brown algae (e.g., Hanisak, 1983).…”

A NEMO-based model of <i>Sargassum</i> distribution in the Tropical Atlantic: description of the model and sensitivity analysis (NEMO-Sarg1.0)

“…Biological nitrogen fixation by diazotrophic macroalgal association has been shown to be important for some Sargassum species (e.g. Sargassum horneri, Raut et al 2018) and this could also be the case for the holopelagic Sargassum where epibionts N-fixating bacteria have been observed on these species (Carpenter 1972, Michotey et al 2020.…”

Dispersion of Surface Drifters in the Tropical Atlantic

“…Knowledge of the surface circulation and transport mechanisms for floating material like Sargassum through the Tropical Atlantic Ocean is an important step in predicting Sargassum inundations (Putman et al, 2018(Putman et al, , 2020Michotey et al, 2020). While the exact depth at which Sargassum floats varies with its physiology, degree of fouling, and the sea state (Johnson and Richardson, 1977), the gas-filled vesicles provide sufficient buoyancy so that the organisms are almost always in the upper meter (Putman et al, 2020).…”