Phaeobacter italicus JN-W1: A pathogenic bacterium causing bleaching disease in Porphyra sensu lato (Bangiales, Rhodophyta)

Abstract

The recent emergence of bleaching disease in the shell-borne conchocelis (SBC) of Porphyra sensu lato has escalated with alarming speed, inflicting severe economic losses on the Porphyra aquaculture industry. Despite the severity of the outbreak, the causative agents of this disease have remained elusive. In this study, we successfully isolated and purified the pathogenic bacterium JN-W1 from SBC affected by bleaching disease. Our infection assays revealed that the bacterium induced the degradation of algal cell organelles and the formation of autophagosomes, ultimately leading to the conversion of dead cells into empty-walled remnants, likely accounting for the bleaching symptoms observed in Porphyra. Additionally, the infection caused severe impairment of the algal photosynthetic apparatus, evidenced by a marked reduction in photosynthetic efficiency as the infection advanced. Through a combination of single-copy orthologous gene analysis and detailed morphological, physiological, and biochemical characterization, strain JN-W1 was definitively classified as Phaeobacter italicus JN-W1. The bacterium demonstrated optimal growth and infection capabilities at 25 °C, pH 7.0, and a salinity of 20. JN-W1's pathogenicity was found to be contact-dependent, mediated by intracellular substances. This study laid a crucial foundation for further exploration of JN-W1's virulence mechanism and the development of effective strategies to prevent and control the outbreaks of Porphyra bleaching disease.