Effects of iron limitation and aquatic humic substances on the growth of Microcystis aeruginosa

Aquatic humic substances (AHSs) from the eutrophic Lake Kasumigaura, Japan, significantly inhibited the growth of Microcystis aeruginosa at a concentration level similar to that found in the lake. AHSs were isolated and quantified using XAD-8 resin. The effects of these isolated AHSs on the growth of M. aeruginosa cultures were evaluated in a chemically defined medium. The production of siderophores, iron-specific complexing agents, of M. aeruginosa under iron limitation was also examined. Fulvic acid (the lower molecular size fraction of the AHSs) accounted for more than 85% of AHSs in Lake Kasumigaura. Under iron limitation, M. aeruginosa produced hydroxamate-type siderophores; however, they were not effective in enabling substantial recovery of growth. Microcystis aeruginosa growth was significantly inhibited when fulvic acid was 2 mg dry weight.L-1, similar to the concentration in the lake, even when siderophores were produced. Growth inhibition was probably due to a deficiency of iron caused by iron complexation with the fulvic acid. It was suggested that AHSs play an important role in formation and disappearance of M. aeruginosa blooms in eutrophic lakes.