Ecological roles of vitamin B_<12> binder produced by microorganisms on the growth of red tide phytoplankton

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Ecological roles of vitamin B_<12> binder produced by bacteria and phytoplankton on the growth of red tide phytoplankton were studied. The results were summarized as follows :1. Red tide phytoplankton, Skeletonema costatum, Gymnodinium mikimotoi, and Heterosigma akashiwo were found to produce vitamin B_<12> binder. H.akashiwo secreted most of the binder produced into the medium, but the others kept ones into their cells.2. About 60 per cent of total heterotrophic bacteria in seawater was able to produce and excrete B_<12> binder.3. The B_<12> binder produced by H.akashiwo was found to have a high affinity to free-B_<12> : the dissociation constant was 52.8 ng/l. The binder inhibited non-specifically the growth of phytoplankton requiring free-B_<12> for their growth.4. In natural eutrophic seawater, B_<12> binders decomposed in parallel with their production. The production of B_<12> binder was due to both bacteria and phytoplankton, while the decomposition was due to bacteria as such.Present results revealed that B_<12> binder produced by microorganisms could inhibit the growth of B_<12>-requiring phytoplankton and also influence the competition between B_<12>-requirer and the others.