Changes in phycospheric and environmental microbes associated with an outbreak of yellow spot disease on Pyropia yezoensis

The composition and diversity of phycospheric microbes are critical to the health of algae. Pyropia yezoensis is a popular edible seaweed in Asia and yellow spot disease (YSD) is a serious disease during its conchocelis sporeling culture period. We studied the changes of the bacterial flora in the phycosphere and the surrounding seawater of P. yezoensis conchocelis during a YSD outbreak and treatment. Four types of algal conditions were studied 1) healthy (no previous YSD); 2) YSD; 3) YSD being treated with ClO2 and 4) recovery (14 d after treatment). In the YSD samples, the microbial diversity was significantly decreased. The abundance of bacteria that were reported having algal dissolving and degrading functions (Kangiella, Alteromonas, Kordiimona, Tenacibaculum and Winogradskyella) was significantly increased, and some beneficial bacteria (Ruegeria and Bdellovibrio) had significantly decreased populations (P < .05). There were negative correlations between these two groups of bacteria (P < .05). After treatment with ClO2, the algal microbial diversities of the recovery samples were restored to normal levels. However, ClO2 treatment increased the abundance of some potential pathogens (Loktanella and Kangiella), and decreased the abundance of beneficial bacteria such as Ruegeria and Bdellovibrio (P < .05). The effects of chemical treatment on the algal microbial community were disruptive and prolonged. Therefore, chemicals should be used cautiously, and less disruptive methods such as biological therapy are recommended for disease prevention and control in algal production systems.