Promotion of cyst formation in the toxic dinoflagellate Alexandrium (Dinophyceae) by natural bacterial assemblages from Hiroshima Bay, Japan

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. tamar ense 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 (Whedon and Kofoid) Balech. This promotion was not caused by effects from nutrient Limitation. 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 10(3) and 1.00 x 10(3) cells ml(-1) 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(-1). 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.