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

Holopelagic Sargassum has been causing massive strandings on tropical Atlantic Ocean shorelines. Describing the microbiome associated with Sargassum and how it changes after stranding is important to identify potential microbial introductions to coastal environments, as well as sources of potential biotechnological resources. In this study, stranding simulation exploratory experiments were conducted for S. fluitans III and S. natans VIII on shipboard with minimum external influence. Samples for microbiome identification were collected just after removing healthy Sargassum from the seawater (0 hr) and after 24 and 48 hrs of stranding simulation under environmental conditions. The bacterial community was identified by sequencing 16S rRNA gene V3-V4 hypervariable regions, generating a total of 1,565 Amplicon Sequence Variants (ASVs). Of those, 588 were shared between Sargassum species and only 25 persisted throughout the stranding. Stranding also changed the dominance of Microtrichales and Rhodobacterales orders at 0 hr to Alteromonadales and Vibrionales after 24 hrs of exposure, the latter representing up to 92% of the relative abundance in the bacterial community. The increase in Vibrionales reinforces the need to monitor stranding sites for any potential pathogenic bacteria. At the functional level, phototrophs were the main group at 0 hr, shifting to chemoheterotrophs and fermentation within the first 24 hrs of Sargassum exposure to air conditions. The fermentative groups native to Sargassum use stranded biomass as substrate for growth, and therefore constitute the bacteria with higher biotechnological potential.