Abstract
Aquatic humic substances (AHSs) from the eutrophic Lake Kasumigaura were isolated using XAD-8 resin. The effects of these isolated AHSs on the growth of Microcystis aeruginosa cultures were evaluated. M. aeruginosa was isolated from algal blooms in Lake Kasumigaura and grown in a chmically defined medium under iron limitation. The production of siderophores, iron-specific complexing agents, by M. aeruginosa was also examined. Under iron limitation, M. aeruginosa was confirmed to produce hydroxamate-type siderophores; however, they were not effective to enable substantial recovery of the growth rate. The fulvic acid (lower molecular-size fraction of AHSs) isolated from Lake Kasumigaura was found to significantly inhibit the growth of M. aeruginosa. The fulvic acid at 2 mg dry weight l-1, the same level as in the lake, significantely inhibited the growth of M. aeruginosa even though siderophores were produced. This inhibition was probably due to a deficiency of iron caused by iron complexation with the fulvic acid. These results suggest that AHSs, through their iron complexation reactions, may play an important role in formation of M. aeruginosa algal blooms.
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