A RAPID BIOASSAY FOR SELECTIVE ALGICIDES

Cyanobacteria (blue-green algae) are undesirable In ponds used to rais e fish for human food. Management of cyanobacterial communities in aqu aculture ponds has been hindered by the small number of herbicides app roved for use in food-fish ponds and by the lack of selectivity toward cyanobacteria for those herbicides that are approved for that use. To facilitate development of additional management options, a rapid bioa ssay utilizing 96-well cell culture plates was developed for screening herbicides and other phytotoxins for selective toxicity toward cyanob acteria, Oscillatoria cf. chalybea and Selenastrum capricornutum were chosen as representatives of cyanobacteria (Cyanophyta) and green alga e (Chlorophyta), respectively, In the bioassay, wells of the cell cult ure plates were inoculated with cyanobacterial or unialgal culture, On e of five herbicides (atrazine, diuron, endothall, fluridone, or paraq uat) was then added to the wells at various concentrations, and absorb ance (650 nm) was measured at 24-h intervals. Growth of treated cultur es relative to untreated cultures was used to determine relative toxic ity of the herbicide to the two organisms, Paraquat was the most selec tive of the herbicides tested and was over 10-fold more toxic to O. cf . chalybea than to S. capricornutum. This method was demonstrated to b e a rapid, effective, and highly reproducible-bioassay to identify sel ective algicides.