Dynamic shift of the epibacterial communities on commercially cultivated Saccharina japonica from mature sporophytes to sporelings and juvenile sporophytes

The supply of healthy sporelings plays a crucial role in the successful aquaculture of commercially cultivated brown kelp Saccharina japonica. Epibacteria are of critical importance for maintaining seaweed health and development. Thus, this study aimed to investigate the epibacterial community associated with mature sporophytes and 1.5- and 2.0-month-old sporelings, as well as juvenile sporophytes of S. japonica by using 16S rRNA gene amplicon sequencing and predicted the bacterial functional profiles. We found that Proteobacteria, Verrucomicrobia, Bacteroidetes, and Planctomycetes were the main phyla, while Loktanella and Rubritalea and Persicirhabdus were the dominant genera for all four developmental stages. Highly abundant core microbiota included Persicirhabdus, Loktanella, Litorimonas, and Rubritalea. Further analysis of the predicted functions showed that different metabolic functions were involved at different developmental stages. More metabolic functions were found in 2-month-old sporelings and mature sporophytes. Both the diversity, composition of the epiphytic bacterial communities, and metabolic functions changed according to different developmental stages at least in 1.5- and 2-month-old sporelings. Our results not only provide fundamental knowledge for the epibacteria associated with S. japonica but will also help nursery farms in preventing and mitigating the disease outbreaks by monitoring the variations of the epibacterial communities at the nursery stage.