Diseases of the commercially cultivated brown seaweed Saccharina japonica have been noticed since the 1960s in China. However, the usually opportunistic nature of the pathogenic bacteria of seaweeds is still a main obstacle for isolating and identifying pathogenic bacteria responsible for causing diseases of the cultivated S. japonica. In this study, a Gram-positive bacterium XP-2 was isolated from the abnormal diseased mature sporophytes of S. japonica. Pathogenicity test indicated that XP-2 could cause green-rotten disease in healthy juvenile sporophytes and significant algal ultrastructural changes were observed after S. japonica was infected by XP-2. The cells shrank, and the thylakoids of the chloroplasts became blurred after 12 h of re-infection. At 24 h slight deformation was observed in the structure of the mitochondria, chloroplasts, and nucleus. There also were many vacuoles in the infected cells. After 31 h of re-infection, mitochondrial structure and thylakoids became more indistinct. SEM and TEM observations indicated that XP-2 was rod-shaped with no flagella. Analysis of the full length 16S rRNA gene sequence and physiological and chemical characteristics indicated that XP-2 was closely related to Exiguobacterium oxidotolerans 12280 T . Based on these results, XP-2 was designated Exiguobacterium oxidotolerans XP-2. The pathogenicity of E. oxidotolerans XP-2 was identified by Koch's Postulate under laboratory conditions. These results enrich the growing list of pathogenic bacteria in commercially cultivated seaweeds and also provide a foundation for investigating the virulence mechanisms of pathogenic bacteria of commercially cultivated seaweeds in the near future.

No abstract

Microbiome manipulation is gaining fresh attention as a way to mitigate diseases in aquaculture. The commercially farmed seaweed Saccharina japonica suffers from a bacterial-induced bleaching disease, which has major implications for the reliable supply of healthy sporelings. Here, we identify a beneficial bacterium, Vibrio alginolyticus X-2 that significantly reduces the risk of bleaching disease. By combining infection assays and multi-omic analyses, we provide evidence to suggest that the underlying protective mechanisms of V. alginolyticus X-2 involve maintaining epibacterial communities, increasing the gene expression of S. japonica related to immune and stress protection pathways, and stimulating betaine concentrations in S. japonica holobionts. Thus, V. alginolyticus X-2 can elicit a suite of microbial and host responses to mitigate the bleaching disease. Our study provides insights into disease control in farmed S. japonica through the application of beneficial bacteria. IMPORTANCE Beneficial bacteria can elicit a suite of microbial and host responses to enhance the resistance to bleaching disease.