| Summary: | The distribution of growth-inhibiting bacteria (GIB) against the toxic dinoflagellate <i>Alexandrium catenella</i> (Group I) was investigated targeting seagrass leaves and surface waters at the seagrass bed of Akkeshi-ko Estuary and surface waters of nearshore and offshore points of Akkeshi Bay, Japan. Weekly samplings were conducted from April to June in 2011. GIBs were detected from surface of leaves of the seagrass <i>Zostera marina</i> in Akkeshi-ko Estuary (7.5 × 10<sup>5</sup>–4.7 × 10<sup>6</sup> colony-forming units: CFU g<sup>−1</sup> wet leaf) and seawater at the stations in Akkeshi Bay (6.7 × 10<sup>0</sup>–1.1 × 10<sup>3</sup> CFU mL<sup>−1</sup>). Sequence analyses revealed that the same bacterial strains with the same 16S rRNA sequences were isolated from the surface biofilm of <i>Z. marina</i> and the seawater in the Akkeshi Bay. We therefore strongly suggested that seagrass beds are the source of algicidal and growth-inhibiting bacteria in coastal ecosystems. Cells of <i>A.</i><i>catenella</i> were not detected from seawaters in Akkeshi-ko Estuary and the coastal point of Akkeshi Bay, but frequently detected at the offshore point of Akkeshi Bay. It is suggested that <i>A.</i><i>catenella</i> populations were suppressed by abundant GIBs derived from the seagrass bed, leading to the less toxin contamination of bivalves in Akkeshi-ko Estuary.
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