Temperature-induced pathogen Vibrio fortis NH-2 triggers bleaching and rough-shell disease in Porphyra sensu lato

Abstract

Neoporphyra haitanensis and Neopyropia yezoensis are two critically important cultivated species of Porphyra sensu lato in China. Recently, outbreaks of bleaching and rough-shell diseases in Jiangsu Province have led to the death of shell-borne conchocelis (SBC), severely affecting the healthy supply of SBC. In this study, through culture-dependent methods and infection assays, we confirmed that bacterial strain NH-2 was responsible for inducing disease symptoms identical to those observed in the bleaching and rough-shell-diseased samples from the nurseries. Strain NH-2 was also found to induce cell pyknosis in free-living conchocelis (FLC), and thallus cells of Porphyra, leading to degradation of chloroplasts and eventual cell death. By integrating morphological observations, physiological assessments, biochemical characterization, and whole-genome sequencing, strain NH-2 was identified as Vibrio fortis NH-2. This strain demonstrated a mortality rate exceeding 54.0 % at temperatures above 25 °C (p < 0.01), with disease severity increasing at higher temperatures, establishing V. fortis NH-2 as a temperature-induced pathogen. In conclusion, the findings of this study provided critical insights into the etiology of Porphyra diseases and offered valuable guidance for disease prevention strategies in the aquaculture industry.