Microbiota associated with benthic <i>Sargassum</i> (Fucales, Phaeophyceae): From morphological structures to geographically dispersed populations

Seaweed-associated microbiota distribution is influenced by factors such as symbiosis, season, life cycle, environmental conditions, and geographic location. This study investigated how microbial communities vary across different parts of benthic Sargassum thalli from nine locations spanning three regions over 600 km apart along Brazil's coast, with sites in each region within 20 km of each other. Using 16S rDNA gene sequencing of the V4 region, we identified 16,802 amplicon sequence variants (ASVs), with 1169 shared across thallus structures and 1100 shared across regions. Our analysis showed that microbial communities varied both along the thallus and between regions, though communities were similar within regions less than 20 km apart. Among thallus structures, the holdfast had the most distinct microbiota, differing from the phylloid and receptacle. This pattern was consistent across Brazil's coastline and has also been observed in studies from Singapore and Portugal. The holdfast microbiota was marked by an unidentified Alphaproteobacteria, along with sulfur-cycling families Desulfocapsaceae and Desulfosarcinaceae. Phylloids and receptacles were mainly associated with photosynthetic cyanobacteria. We also identified shared taxonomic biomarkers across Sargassum species from Asia, Europe, and South America. These results suggest that the microbiota are more influenced by the thallus structure than by geographic location. These consistent patterns across Sargassum species from different continents-Asia, Europe, and South America-support the hypothesis of microbiota specialization within morphological niches.