This is the first report to explore the fine-scale diversity, population genetic structure, and biogeography of a typical planktonic microbe in Japanese and Korean coastal waters and also to try to detect the impact of natural and human-assisted dispersals on the genetic structure and gene flow in a toxic dinoflagellate species. Here we present the genetic analysis of Alexandrium tamarense (Lebour) Balech populations from 10 sites along the Japanese and Korean coasts. We used nine microsatellite loci, which varied widely in number of alleles and gene diversity across populations. The analysis revealed that Nei's genetic distance correlated significantly with geographic distance in pair-wise comparisons, and that there was genetic differentiation in about half of 45 pair-wise populations. These results clearly indicate genetic isolation among populations according to geographic distance and restricted gene flow via natural dispersal through tidal currents among the populations. On the other hand, high P-values in Fisher's combined test were detected in five pair-wise populations, suggesting similar genetic structure and a close genetic relationship between the populations. These findings suggest that the genetic structure of Japanese A. tamarense populations has been disturbed, possibly by human-assisted dispersal, which has resulted in gene flow between geographically separated populations. 1

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Strain MS-02-063, gamma-proteobacterium, isolated from a coast area of Nagasaki, Japan, produced a red pigment which belongs to prodigiosin members. This pigment, PG-L-1, showed potent algicidal activity against various red tide phytoplanktons in a concentration-dependent manner. An understanding of a mechanism of PG-L-1 production by this marine bacterium may yield important new insights and strategies for preventing blooms of harmful flagellate algae in natural marine environments. Therefore, we analyzed the mechanisms of PG-L-1 production. In our previous study, the pigment production by this marine bacterium was completely inhibited at 1.56 microg/ml of erythromycin or 3.13 microg/ml of chloramphenicol, while minimal inhibitory concentrations for cell growth of erythromycin and chloramphenicol against this bacterium were >100 and 25 microg/ml, respectively. It is interesting to note that the ability of the pigment production in erythromycin-treated bacterium recovered by an addition of homoserine lactone. In fact, the pigment production was inhibited by beta-cyclodextrin that inhibits autoinducer activities by a complex with N-acyl homoserine lactones. N-acyl homoserine lactones with autoinducer activities are ubiquitous bacterial signaling molecules that regulate gene expression in a cell density dependent process known as quorum sensing. Therefore, it was suggested that PG-L-1 produced by strain MS-02-063 is controlled by the homoserine lactone quorum sensing. It is speculated that this quorum sensing is involved in the production of algicidal agents of other marine bacteria. This bacterium and other algicidal bacteria might be concerned in regulating the blooms of harmful flagellate algae through the quorum sensing system.

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