Vibrio alginolyticusis the pathogen of ‘Baotou’ disease causing serious damage toGracilariopsis lemaneiformiscultivation in China

Gracilariopsis lemaneiformis and Saccharina japonica , the two very important cultivated economic seaweeds, are often co-cultured in China due to their growth periods overlapped for about 3 months. These two seaweeds are often plagued with bleaching diseases during their high-density commercial cultivation. In recent years, a disease called ‘Baotou’ has been causing large-scale yield reductions of G. lemaneiformis in China. Interestingly, Vibrio alginolyticus once reported to be beneficial to S. japonica by reducing the bleaching disease risk, was strongly proved to be the pathogen causing ‘Baotou’ disease in this study. Analysis of 16S rRNA gene profiling revealed that V. alginolyticus 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 V. alginolyticus cells were found to be attached to the algal thalli with ‘Baotou’ disease and the rotten thalli acquired by the lab infection treatment. V. alginolyticus could cause the rotten symptoms which were consistent with those of ‘Baotou’ disease. According to Koch’s postulates, V. alginolyticus was identified as the pathogen causing ‘Baotou’ disease. IMPORTANCE A highly contagious disease known as ‘Baotou’ disease has persistently triggered significant yield reductions in G. lemaneiformis throughout China. The pathogen of ‘Baotou’ disease was isolated and identified as V. alginolyticus which was once reported to be beneficial to S. japonica by reducing the bleaching disease risk. The study indicates that V. alginolyticus is closely involved in the competition or co-existence between G. lemaneiformis and S. japonica co-cultured in China, and provides a novel perspective and evidence that bacteria are closely involved in the competition or co-existence among different species of seaweeds that grow together.