Analysis of algicidal ranges of the bacteria killing the marine dinoflagellate Gymnodinium mikimotoi isolated from Tanabe Bay, Wakayama Pref, Japan

Twenty eight strains of the marine bacteria which killed the marine dinoflagellate, Gymnodinium mikimotoi, were isolated during the occurrence of the huge red tide by G. mikimotoi in Tanabe Bay in 1990. Among them 22 strains belonged to the genus Vibrio, three to Flavobacterium, two to Acinetobacter and one to Pseudomonas-Alteromonas. All of them grew in filter-sterilized natural seawater without any nutrient supplements, suggesting that the killing bacteria are ubiquitous in seawater and are related to the occurrence of G. mikimotoi opportunistically. Most of the killing bacteria did not affect the growth of three marine diatoms, Skeletonema costatum, Ditylum brightwellii and Thalassiosira a sp., and some of them did not affect the growth of the marine dinoflagellate, Alexandrium catenella. These findings strongly suggest that the killing bacteria influence the change of microalgal community in a marine environment.