<i>Vibrio alginolyticus</i> is the pathogen of “Baotou” disease causing serious damage to <i>Gracilariopsis lemaneiformis</i> cultivation in China

In recent years, a disease called "Baotou" has been causing large-scale yield reductions of <i>Gracilariopsis lemaneiformis</i> in China. Interestingly, <i>Vibrio alginolyticus</i>, which once reported to be a probiotic or pathogen for multiple marine organisms, was strongly proved to be the pathogen causing "Baotou" disease in this study. Analysis of 16S rRNA gene profiling revealed that <i>V. alginolyticus</i> was the most abundant and dominant bacterium on the algal thalli suffering from "Baotou" disease, whereas its presence was scarcely detected on healthy thalli. Scanning electron microscope (SEM) analysis revealed that a large number of <i>V. alginolyticus</i> cells were found to be attached to the algal thalli with "Baotou" disease and the rotten thalli acquired by the lab infection treatment. <i>V. alginolyticus</i> could cause the rotten symptoms which were consistent with those of "Baotou" disease. According to Koch's postulates, <i>V. alginolyticus</i> was identified as the pathogen causing "Baotou" disease.IMPORTANCEA highly contagious disease known as "Baotou" disease has persistently triggered significant yield reductions in <i>G. lemaneiformis</i> throughout China. The pathogen of "Baotou" disease was isolated and identified as <i>V. alginolyticus</i> in this study. Interestingly, <i>V. alginolyticus</i> was once reported to be a probiotic to <i>Saccharina japonica</i>, and pathogen to <i>Haliotis diversicolor</i>, <i>Paleopneustes cristatus</i>, <i>Ruditapes decussatus,</i> and <i>Litopenaeus vannamei</i>. The study indicates that <i>V. alginolyticus</i> play a significant role in the competition or co-existence between <i>G. lemaneiformis</i>, <i>S. japonica</i>, abalone, sea urchin, bivalve, and shrimp.