To explore the composition of epiphytic bacterial communities on Saccharina japonica and to isolate as well as to identify both the opportunistic pathogenic and antagonistic bacteria, four modified culture media (1/10 ZoBell 2216E, 1/10 ZoBell 2216E supplemented with alginate, 1/10 ZoBell 2216E supplemented with S. japonica extract solution, and S. japonica extract medium) and one traditional culture medium (ZoBell 2216E) were used to isolate the epiphytic bacteria. Combining the full-length 16S rDNA amplicon sequencing, infection assay and the Burkholderia agar diffusion assay, this study analyzed the composition of epiphytic bacteria of S. japonica and screened for pathogenic and antagonistic bacteria. Analysis of full-length 16S rDNA amplicon sequencing data showed that dominant bacterial species in the epiphytic bacterial community were Mariniblastus fucicola, Granulosicoccus coccoide, Litorimonas cladophorae. A total of 195 bacterial strains were isolated using the four modified culture media and one traditional culture medium, and the dominant species were Cobetia amphilecti and Bacillus altitudinis. Among them, the highest number and variety for isolated bacteria strains were the modified culture medium M2 (1/10 ZoBell 2216E medium). Cobetia was the dominant genus (63.6%) of 11 strains of the opportunistic pathogenic bacteria, which can cause green-rotten diseased symptom in S. japonica within 24 h in lab condition. While, Bacillus was the dominant genus (62.5%) of 8 strains of the antagonistic bacteria, showing antagonistic activity to the pathogenic bacteria of bleaching disease of S. japonica (Pseudoalteromonas piscicida X-8).

The pathogenic bacteria of seaweed diseases are mostly the opportunistic pathogenic bacteria. In this study, an opportunistic pathogenic bacterium MS-3 was isolated and identified from healthy mature sporophytes of Saccharina japonica by combining the traditional culture-dependent method, bacteria-S. japonica infection assay and 16s rRNA gene sequence comparison. Through the infection assay and microscopic observations, it was shown that strain MS-3 can cause green-rotten diseased symptom in S. japonica after infected at 6 h under laboratory condition. The colony of MS-3 was round and bright yellow. The cells of MS-3 were rod shaped with a single polar flagellum. MS-3 had the highest similarity of 99.7% with Vibrio neocaledonicus NC470 by comparing 16s rRNA gene sequences. Combining the physiological and biochemical characteristics of MS-3, we identified it as Vibrio neocaledonicus MS-3. Our results not only enrich the strain resource of opportunistic pathogenic bacteria of S. japonica and lay a foundation for investigating the pathogenic mechanisms of the opportunistic pathogenic bacteria, but also help to develop the diagnostic technology for S. japonica diseases.