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