[Elucidation of Phenomena Involving Cyanobacteria in Freshwater Ecosystem by Chemically Ecological Approach].

Lakes Sagami and Tsukui are reservoirs constructed by connecting to the Sagami River. Because of eutrophication of the lakes, cyanobacteria have appeared every year. This review deals with phenomena related to occurrence of cyanobacteria that have been observed for 40 years since 1974 at the lakes. These 40 years of observations raised three interesting issues including the retention of cyanobacteria on their surfaces. These phenomena have been attributed to the usual factors, such as illuminance, nutrition and water temperature, but our research results suggested that they cannot be resolved without the introduction of another factor. We have attempted to elucidate various phenomena involving cyanobacteria in lake ecosystems by chemical ecological methods using volatile organic compounds (VOCs) produced by the cyanobacteria as indicators. One of the VOCs, β-cyclocitral, was significantly involved in the above phenomena, which was considered to be produced by the carotenoid cleavage dioxygenase (CCD) of the cyanobacteria. β-Cyclocitral was not produced in the two known CCDs, but two additional CCDs to Microcystis aeruginosa participated to produce the β-cyclocitral. These CCDs did not directly produce β-cyclocitral, but it was accumulated in cells as their precursors. The released β-cyclocitral underwent a Baeyer-Villiger-like oxidation. It was speculated that Microcystis activated the CCD genes through density stress and produced β-cyclocitral, which acted as an allelopathic substance. As a result, the number of cells of cyanobacteria decreased, and the resulting nitrogen and phosphorus were fed to the living cyanobacteria. It is postulated that this "quorum sensing" was functioning in the above-mentioned issues.