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Abstract |
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Three Sphingomonas strains, S5, S6, and 7CY, capable of degrading hepatotoxin microcystins produced by a
blue-green alga,were grown to evaluate the optimum growth condition in LB medium under various conditions as
follows:temperature:25, 30, 35, 40, 45, 50°C;pH:6.0, 7.0, 8.0, 9.0, 10.0;and the density of ingredients of the medium:
1/4-fold,1/2-fold,1-fold. Under these conditions,S5,S6,and 7CY strains showed at 35°C the highest kg in 1/2-fold
LB at pH7.0, in 1/2-fold LB at pH8.0, in LB at pH7.0, respectively.Average generation times were 1.8 in S5, 1.7 in
S6, and 2.4 hours in 7CY under each optimum condition.When microcystin-LR, -LY,-LW,and -LF were added to
S6 cell suspension,S6 cells were able to degrade all these microcystins,and the rate of microcystin-LR degradation
was 0.59μg･ml-1･day-1. S6 strains, furthermore, grew much faster in minimal medium in the presence of
microcystin-LR than in the absence of the toxin. These results indicate that environmental factors such as pH and
temperature which regulate the growth of microcystin-decomposing bacteria are considerably close to those of toxic
blue-green algae. It is,therefore,suggested that water bodies where toxic blue-green algae are dominant should be
convenient for these bacteria as well, and that they should take up microcystins as nutrient after toxic blue-green
algae disappear. It should be noted that the rate of decomposing microcystins by S6 strain is high enough to exhaust
microcystin-LR guideline value (1μg･l-1) recommended by WHO and the usual concentration of microcystins
produced by blue-green algae in eutrophic water bodies.
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