Temporal variation and functional potential of epiphytic bacterial communities associated with the brown alga Sargassum fusiforme.

The brown alga Sargassum fusiforme is an economically and nutritionally important marine macroalga widely cultivated in East Asian coastal waters. Its surface harbors diverse epiphytic bacterial communities, yet their temporal variation and functional characteristics remain insufficiently understood. In this study, we investigated the epiphytic bacterial communities associated with S. fusiforme throughout a seven-month mariculture cycle and compared them with those of eight sympatric macroalgae from three major phyla (Phaeophyta, Rhodophyta, and Chlorophyta) using 16S rRNA gene amplicon sequencing. After quality filtering and optimization, a total of 1,203,660 high-quality sequences (comprising 452,840,609 bases) were retained across all samples. Alpha diversity differed among the sampled macroalgal host species, with the Chao index ranging from 272.1 (Sargassum horneri) to 792.8 (Polysiphonia senticulosa), and temporal variation in S. fusiforme showed the highest species richness in November (Chao = 535.9) and lowest in December (Chao = 254.8). Beta diversity analyses revealed distinct bacterial community patterns associated with both sampling months and macroalgal species. Proteobacteria, Bacteroidota, and Firmicutes were the dominant bacterial groups across all samples, with a core microbiome of 33 OTUs consistently detected across all months. Functional prediction using FAPROTAX indicated that epiphytic bacterial communities potentially participate in carbon, nitrogen, and sulfur cycling, with chemoheterotrophy and related metabolic processes being predominant. Culturable epiphytic bacteria (84 isolates) exhibited diverse extracellular enzyme activities, including alginate, starch, chitin, cellulose, and carrageenan degradation. Additionally, 15.5% of isolates produced indole-3-acetic acid (IAA) and 77.4% displayed acyl-homoserine lactone (AHL)-related signaling activity. Collectively, our findings reveal pronounced temporal variation and host-associated differences in the epiphytic bacterial communities of S. fusiforme across a cultivation cycle and highlight the functional potential of seaweed-associated bacteria as a source of microbial resources for future ecological and biotechnological investigations.