Isolation and Identification of Pseudoalteromonas agarivorans LJ53, a Pathogenic Bacterium Causing Bleaching Disease in Saccharina japonica

As a major export crop in China, Saccharina japonica cultivation suffers from significant economic losses due to disease outbreaks, with pathogen identification remaining a critical bottleneck for mariculture. In this study, a dominant bacterial strain, LJ53, was isolated from the diseased farmed S. japonica. Artificial challenge assay confirmed that this strain is the direct causative agent of bleaching symptoms on sporophytes. Based on morphological characteristics and 16S rRNA gene-based phylogeny, it was identified as Pseudoalteromonas agarivorans LJ53. Ultrastructural observation revealed that this strain destroyed host cells and caused typical pathological changes such as chloroplast disintegration. Interestingly, metagenomic analysis showed no significant difference in the relative abundance of this pathogen between healthy and diseased S. japonica tissues. However, the co-occurrence network of the disease community exhibited increased connectivity, altered modularity, and features characteristic of microbial dysbiosis. This dysbiosis disrupts the water ecological balance by destabilizing microbial symbiosis and nutrient cycling, which are essential for overall ecosystem resilience. As a result, these imbalances can exacerbate disease transmission and weaken the self-regulating capacity of marine environment, highlighting the need for integrated management strategies to restore equilibrium. These findings provide a theoretical basis for elucidating the mechanisms of bacterial diseases in S. japonica and developing future control strategies.