Inhibition of cyst formation in the toxic dinoflagellate Alexandrium (Dinophyceae) by bacteria from Hiroshima Bay, Japan

The relationship between the abundance of the toxic marine dinoflagellate Alexandrium tamarense (Lebour) Balech and cyst formation-inhibiting bacteria (Alex-CFIB) was investigated in samples taken from the water column in Hiroshima Bay (Japan) in 1999. The cell density of A. tamarense peaked in the middle of April and blooms declined in May. Alex-CFIB were detected during the bloom period as well as the non-bloom period in 1999 by means of the most probable number (MPN) bioassay as well as the colony counting method. A total of 32 strains that had potential Alexandrium cyst formation-inhibiting activities (CFIB) were isolated from the seawater samples from Hiroshima Bay throughout the year. The population structure and genetic diversity of Alex-CFIB, were analyzed by means of restriction fragment length polymorphism (RFLP) of the 16S ribosomal RNA genes (16S rDNA). Five ribotypes, Ia to Id and II types, were determined among the 32 strains of Alex-CFIB. Most of the strains belonged to ribotype I, suggesting that bacteria of ribotype I may be dominant in the Alex-CFIB assemblages in the field seawater. Almost the entire 16S rDNA-based phylogenetic tree showed that ribotypes I and II fell into the class Proteobacteria gamma-subdivision Alteromonas group and the Vibrio group, respectively. The 6-well microplate approach clarified that Alex-CFIB, obtained in this study do not have growth-inhibiting activities, and Alex-CFIB of ribotype I (Alteromonas group) have strong activities of encystment inhibition among these ribotypes. The existence not only of Alexandrium cyst formation-promoting bacteria (Alex-CFPB) reported previously but also of Alex-CFIB, in Hiroshima Bay throughout the year suggests that Alex-CFPB, as well as Alex-CFIB, especially bacteria of ribotype I, may play significant roles in the process of encystment and bloom dynamics of Alexandrium in the natural environment.