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Abstract

The role of phenols as defence mechanisms in various terrestrial plants is well studied. However, in aquatic environments, they are probably involved also in the chemical interactions between species. A periodic shift from dominance by submerged macrophytes to dominance by phytoplankton and vice versa without apparent change in the external nutrient loading is sometimes observed in shallow eutrophic lakes. Due to the fact that nutrient levels are not usually limiting to phytoplankton growth, the release of allelochemicals by macrophytes is suspected to be responsible for poor algal growth.
Myriophyllum spicatum is known to produce several phenolic compounds. Aqueous methanolic extracts of fresh or lyophilized as well as axenic or non-axenic M. spicatum plant material exhibited strong algicidal activity against cyanobacteria. Chlorophytes and diatoms are also affected, but to a lesser extent. Bioassay directed fractionation of the extract lead to the isolation of two strong algicidal compounds. These compounds are constitutively present in the plant and were characterized as galloyl glycosides (500-1000 dalton). To test the ecological importance of these substances, the culture water was screened for exudated phenolic compounds. At least 5-10 different polyphenols could be detected. An algicidal galloyl ester and several derivatives of ellagic acid were among them. The possible release mechanisms and the mode of action of these polyphenols as defence compounds are discussed.