Microbiota changes in a stranding simulation of the holopelagic macroalgae Sargassum natans and Sargassum fluitans

Abstract

Abstract Holopelagic Sargassum has been causing massive strandings on tropical Atlantic Ocean shorelines. After stranding, the algal biomass starts to decompose, releasing nutrients, toxic gases, and potentially introduces exogenous macro and microorganisms. Describing the microbiome associated with Sargassum, and how it changes after stranding is important in identifying potential microbial introductions to coastal environments, as well as sources of potential biotechnological resources. In this study, stranding simulation experiments were done for S. fluitans III and S. natans VIII on shipboard. Samples for microbiome identification were taken at 0 hr, just after removing healthy Sargassum from the seawater, and after 24 and 48 hrs of stranding simulation under environmental conditions. The bacterial community was identified through sequencing of 16S rRNA gene V3-V4 hypervariable regions, generating a total of 2,005 Amplicon Sequence Variants (ASVs). Of those, 628 were shared between Sargassum species. The stranding simulation changed the microbial community and only 30, out of 2,005 ASVs, persisted throughout the experiment. Phototrophs were in the main functional group at 0 hr, shifting to chemoheterotrophs within the first 24 hrs of exposure of Sargassum to air conditions. The most abundant orders Microtrichales and Rhodobacterales at 0 hr, were replaced after 24 hrs of exposure by Alteromonadales and Vibrionales, the latter representing up to 91% of the relative abundance in the bacterial community. These findings suggest that after stranding, the Sargassum microbiome goes through dysbiosis, and its biomass could become a fertile ground for potentially pathogenic bacteria.