Pseudoalteromonas piscicida X-8 causes bleaching disease in farmed Saccharina japonica

Saccharina japonica is a commercially farmed seaweed of global importance. However, disease occurrence during different stages of cultivation can result in substantial economic losses. Identification of the causative agents of disease remains a significant bottleneck to the large scale cultivation of S. japonica. In this study, an aerobic heterotrophic, flagellated, rod-shaped Gram-negative bacterial strain X-8 was isolated from the bleaching diseased S. japonica sporelings. Pathogenecity of strain X-8 was tested by re-infection assay. The ultrastructural changes of infected S. japonica cells by strain X-8 indicated that chloroplasts were the first organelle responding to X-8 infection with deformed structure and later followed by fragmented nucleus. However, the ultra-structure of mitochondria and cell wall remained intact during the re-infection. Based on 16S rRNA gene sequence, morphological and biochemical characteristics, strain X-8 was designated as Pseudoalteromonas piscicida X-8. The pathogenicity of P. piscicida X-8 was identified by Koch's Postulate under laboratory conditions. Our results will not only help to establish a stable experimental model between the pathogenic bacteria and the host S. japonica to further elucidate the virulence mechanisms, but will also provide information for disease management to effectively prevent and mitigate the occurrence of bleaching disease of S. japonica at nursery stage.

Automated summary

In this study, a pathogenic bacterium X-8 was isolated from bleaching diseased Saccharina japonica sporelings and identified as Pseudoalteromonas piscicida X-8. The pathogenicity was confirmed through re-infection assay and ultrastructural analysis of infected S. japonica cells. This research provides insights into the virulence mechanisms of the pathogenic bacteria and offers valuable information for disease management strategies in nursery stage of S. japonica.