Abstract: We analyzed microbial community composition in phycosphere of red-tide alga Karenia mikimotoi by 16S rRNA high-throughput sequencing, and investigated the inhibitory effect of the phycosphere microorganisms on the growth of K. mikimotoi with in situ nutritive stimulation. The results showed that Proteobacteria was the dominant bacterial population in phycosphere of K. mikimotoi, with an abundance of 48.1% for α-Proteobacteria and 8.6% for γ-Proteobacteria. Sphingobacteria and Cyanobacteria accounted for 22.4% and 10.8%, respectively. Axenic culture of K. mikimotoi was obtained via treatment of the combined antibiotics. Nutrient addition did not affect the growth of K. mikimotoi. When 2216E culture medium, glycerol and sodium citrate in 5% volume ratio were added to cultivation system of K. mikimotoi, their growth was inhibited, with an inhibition rate of 99.6%, 96.0% and 91.5%, respectively. The response of phycosphere microbial community to 2216E stimulation showed that the abundance of αProteobacteria increased to 58.7% (within which Rhodobacteraceae increased from 8.7% to 22.3% and Thalassobaculum increased from 2.7% to 13.3%), and that of γ-Proteobacteria increased to 13.3% (within which Pseudomonas increased from 2.4% to 4.9%). There were diverse microbial populations in phycosphere ofK. mikimotoi, whereas appropriate amount of nutrient addition would provide the in situ alga-inhibiting effect of phycosphere bacteria.

Key words: Karenia mikimoto, ;phycosphere, algicidal bacteria, 16S rRNA, in situ algicide.