A total of 31 bacterial isolates that have potential Alexandrium cyst formation-promoting activity (Alex-CFPB) were isolated from Hiroshima Bay (Japan), which is characterized by seasonal blooms of the toxic dinoflagellate Alexandrium tamarense. The population structure of Alex-CFPB was analyzed by means of restriction fragment length polymorphism analysis of the 16S rRNA genes (16S rDNA). Fourteen ribotypes, A to N, were observed among the 31 isolates of Alex-CFPB by using four restriction enzymes, MboI, HhaI, RsaI and BstUI. Among them, seven isolates, which were obtained from the seawater samples taken during the peak and termination periods of the A. tamarense bloom in 1998, belonged to ribotype A. This result suggests that bacterial strains of ribotype A may be dominant in the Alex-CFPB assemblages during these periods. The partial 16S rDNA-based phylogenetic tree of 10 ribotypes studied showed that nine of them fell into the Rhodobacter group of the ␣ subclass of the Proteobacteria. Eight of nine ribotypes of the Rhodobacter group fell into the lineage of the Roseobacter subgroup, and one fell into the Rhodobacter subgroup. The non-Rhodobacter group type fell into the Marinobacterium-Neptunomonas-Pseudomonas group of the ␥-Proteobacteria. Isolates of Alex-CFPB ribotypes A and C do not have clear growth-promoting activities but have strong cyst formationpromoting activities (CFPAs) under our laboratory conditions. These results show that the Alex-CFPB assemblage may consist of various bacteria that belong mainly to the Roseobacter group and have strong CFPAs. These results suggest that not only the Alexandrium cyst formation-inhibiting bacteria (Alex-CFIB) reported previously but also Alex-CFPB, especially bacteria of ribotype A, may play significant roles in the process of encystment and bloom dynamics of Alexandrium in the natural environment.

ABSTRACT:The relationship between the abundance of the toxic marine dinoflagellate Alexandrium tamarense (Lebour) Balech and Alexandrium-cyst-formation-promoting bacteria (Alex-CFPB) was investigated in the water column of Hiroshima Bay (Japan) from 1997 to 1998. Cell density of A. tamarense increased gradually from February to the middle of April, then peaked at the end of April and blooms declined rapidly in the beginning of May in both years. All seawater fractions collected from 5 m depth, where the density of A . tamarense cells was highest and which also contained the bulk of planktonic bacteria, promoted cyst formation of A. catenella (LVhedon and Kofoid) Balech. This promotion was not caused by effects from nutnent h t a t i o n . The number of Alex-CFPB in seawater samples, analyzed by means of the most probable number (MPN) method, increased from the beginning of the Alexandrium bloom and reached 3.60 X lo3 and 1.00 X 103 cells ml-' at the peak bloom period at the end of April in 1997 and 1998, respectively. As the blooms declined, the number of Alex-CFPB decreased rapidly to less than 10 cells ml-'. Alexandrium-cyst-formation-inhibiting bacteria (Alex-CFIB) were not detected. These results show a clear positive correlation between the abundance of A. tamarense and Alex-CFPB during blooms, which suggests that Alex-CFPB play a significant role in the process of encystment and bloom dynamics of Alexandrium in the field.

Jannaschia cystaugens sp. nov., an Alexandrium (Dinophyceae) cyst formation-promoting bacterium from Hiroshima Bay, Japan Heterotrophic bacteria isolated from water samples taken from Hiroshima Bay, Japan, and referred to as Alexandrium (Dinophyceae) cyst formation-promoting bacteria, were assigned to the Roseobacter-Sulfitobacter-Silicibacter group within the a-Proteobacteria on the basis of nearly complete 16S rRNA gene sequences. Phylogenetic analyses showed that two strains, CFPB-A9 T and CFPB-A5, are closely related to each other and that their closest relative was Jannaschia helgolandensis (95?9 % sequence similarity). These strains were Gram-negative, motile, obligately aerobic rods that required sodium ions and 2-7 % sea salts for growth and did not produce bacteriochlorophyll a. Their optimal growth temperature was 25-30 6C. The strains had Q-10 as the dominant respiratory quinone. Primary cellular fatty acid in both strains was 18 : 1v7c. The DNA G+C contents of strains CFPB-A9 T and CFPB-A5 were 59?1 and 59?2 mol%, respectively. Based on physiological, biological, chemotaxonomic and phylogenetic data, the strains are considered to represent a novel species, Jannaschia cystaugens sp. nov., with type strain CFPB-A9 T (=LMG 22015 T =NBRC 100362 T ).