1. A soluble cell fraction lethal to Daphnia magna was isolated by the disruption of net phytoplankton from eutrophic Aculeo Lake. The phytoplankton of this lake is dominated by Microcystis sp.2. The soluble fraction was also lethal to common native zooplankton, including the rotifers Keratella sp., Trichocerca similis, the copepod Boeckella sp., and the cladoceran Chydorus sphaericus. In addition, a partially purified soluble component of the lethal fraction caused the death of the fish Gamhusia afftnis.3. The specific toxicity of the soluble fraction from phytoplankton varied four-to six-fold seasonally with peaks during periods of warm temperature.4. Our results suggest that releases of a toxic peptide from Microcystis sp. may be the cause of massive fish kills at the study site, Aculeo Lake.

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DNA isolated from the protozoan Trypanosoma cruzi has been found to contain 5methylcytosine. Analysis of T. cruzi DNA by both HpaII and MspI restriction endonucleases suggests that the sequence CCGG-is not methylated. Probably T. cruzi DNA also contains N6-methyladenine.This report constitutes the first clear demonstration of the presence of methylated bases in the nuclear DNA from trypanosomes.