The toxic effects of a mixotrophic golden alga (Poterioochromonas sp. strain ZX1) and a cyanobacterium Microcystis aeruginosa grazed by Poterioochromonas to a cladoceran were investigated through life history experiments using Daphnia magna. Poterioochromonas cultured in two ways (fed M. aeruginosa in an inorganic medium or grown in an organic medium) both induced starvation-like effects on D. magna, indicating that Poterioochromonas is neither acutely toxic nor a good food for D. magna. Despite a microcystin-LR content of 10(-8)microgcell(-1) in M. aeruginosa, no toxins were accumulated in Poterioochromonas fed the cyanobacterium. The toxic effect of M. aeruginosa to D. magna was significantly reduced in the presence of Poterioochromonas, which may be performed in two ways: decrease M. aeruginosa cells ingestion of D. magna by grazing on M. aeruginosa; and decrease the toxicity of the medium by degrading the toxins released by M. aeruginosa. This study provides new information on the interactions between a cyanobacterium and its grazer under laboratory conditions and may increase our understanding of the ecological significance of such interactions in the aquatic food webs.