Diversity of the epiphytic bacterial communities associated with commercially cultivated healthy and diseased Saccharina japonica during the harvest season

Disease outbreaks cause severe economic losses in commercially cultivated Saccharina japonica. Epiphytic bacteria are increasingly recognized to play an important role in S. japonica health and may also become opportunistic pathogens when the environment deteriorates. However, there is little knowledge about the epiphytic bacterial communities associated with commercially cultivated S. japonica and how these communities change over the harvest season. We analyzed the diversity, composition, and temporal dynamics of epiphytic bacterial communities associated with both healthy and diseased tissue of S. japonica over a harvest season (from April to June, 2015) at Ailian Bay, Rongcheng, China. Our results showed that Proteobacteria and Bacteroidetes were the main phyla and Halomonas spp. were the most dominant bacterial species. Halomonadaceae, Hyphomonadaceae, Rhodobacteraceae, and Phyllobacteriaceae families were abundant in both healthy and diseased S. japonica, but the relative abundance of these bacteria differed between healthy and diseased groups. Spatial (site 1, site 2, and site 3 in April) and temporal comparisons (site 3 from April to June) revealed that the bacterial community associated with cultivated S. japonica was influenced by sampling time and sampling site independent of the health status. Several OTUs from the Roseobacter group, including members of the genera Sulfitobacter and Loktanella, were enriched in diseased tissues suggesting these taxa could be candidate opportunistic pathogens. In contrast, OTUs belonging to the genera Halomonas were enriched in healthy tissues, indicating a potential beneficial role for group during the cultivation of S. japonica. Our results provide important baseline knowledge of the epiphytic bacterial communities and possibly pathogenic bacteria associated with commercially cultivated S. japonica.