The influence of vitamin B
12 concentration in the medium on the production of vitamin B
12 binder by
H. akashiwo was evaluated, and some of the chemical properties of the B
12 binder and its effect on the growth of B
12-requiring phytoplankton were determined.
H. akashiwo produced B
12 binder and most of it was secreted into the culture medium. The amount of B
12 binder secreted was dependent on the B
12 concentration in the media for both pre-and test-cultures; the lower B
12 concentration increased the secretion of B
12 binder. The production rate of B
12 binder was highest during the middle exponential growth phase when the alga was incubated at B
12 concentrations of 2 ?? 20 ng/
l; their production rates ranged from 0.11 to 0.21 fg B
12/cell/day. The dissociation constant of bound B
12, the complex of free B
12 and B
12 binder secreted by
H. akashiwo, was 52.8ng/
l (association constant=2.57times;10
10M
-1). Binding activity of B
12 binder was stable at pH 8 ?? 9 and temperatures of 4 ?? 50°C, but labile under ultraviolet irradiation. The B
12 binder inhibited the growth of B
12-requiring phytoplankton including
H. akashiwo itself and the effect of B
12 binder was found to be non-specific.
The present study suggests that the B
12 binder secreted by
H. akashiwo could inhibit the growth of B
12-requiring phytoplankton after blooms of
H. akashiwo in natural seawater.
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