The effect of chelating agents on the growth of Anabaenamacrospora, a planktonic blue-green algae, and on its production of geosmin was studied.
When A.macrospora was cultured in CT medium containing EDTA (an artificial chelator), the maximum cell numbers were about 1×106 cells/ml. However, in an EDTA-free CT medium, the maximum cell numbers were a tenth of the normal growth level in the complete medium. In addition the chlorophyll-a content was reduced almost a fourth and the color of the algal cell bodies changed from green to yellow. These results showed that growth of this alga in a chelator-free medium was not healthy. At this time, though the geosmin content was constant and in the same range of 1.0-4.0×10−5 ng/cell as in the complete medium, the ratio of the extracellular geosmin concentration in the medium to the gross concentration was 2:10 to 3:10 and 2 to 3 times its normal level. It was considered that the deterioration of alga's physiological condition increased the release of geosmin to the medium. These results suggested that the extracellular concentration of geosmin produced by A.macrospora was affected by the alga's physiological condition. Finally EDTA can be substituted by humic acid. When this alga was grown in a medium containing humic acid instead of EDTA, the maximum cell numbers and the release ratio of geosmin was of the same level as in the case of the complete medium.