Allelopathic Effects on Phytoplankton by Substances Isolated from Aquatic Macrophytes (Charales)

Abstract
Two allelopathic compounds, a dithiolane and a trithiane, were isolated from fresh and brackish water Characean species [Chara globularis and Chara sp.]. Bioassay with synthetic 4-methylthio-1,2-dithiolane and 5-methylthio-1,2,3-trithiane (2) on epiphytic diatoms [Nitzschia palea] and lake and pond phytoplankton showed that the trithiane (2) inhibits photosynthesis in a concentration of 3 .mu.M. Elemental S, often found when 2 is extracted from the plants, has an even stronger influence on photosynthesis than 2. Excretion of these compounds from Chara will reduce growth of epiphytic phytoplankton and explains why Chara populations seldom are found with epiphytes and further suppress phytoplankton growth in natural environments.