A MIXTURE TOXICITY STUDY EMPLOYING COMBINATIONS OF TRIBUTYLTIN CHLORIDE, DIBUTYLTIN DICHLORIDE, AND TIN CHLORIDE USING THE MARINE BACTERIUMVIBRIO-HARVEYI AS THE TEST ORGANISM

Mixture toxicity studies in dual combinations for three metals, tribut yltin chloride, dibutyltin dichloride, and tin chloride, and one exper iment using all chemicals were performed using the bioluminescent mari ne bacterium Vibrio harveyi as the test organism in a direct toxicity test procedure. Combination toxicity was evaluated using an additive i ndex equation method and two isopleth procedures, isobole plot and iso bologram. Additive index values were determined at both estimated effe ctive median concentration (EC(50)) and one-third EC(50) values for ea ch dual combination and all three chemicals for one-third EC(50) value s. Isopleths employed chemical combinations at 20% intervals of the EC (50) concentrations. Additive index values for various mixtures were e ither additive or antagonistic (less than additive). Isobolograms for all mixtures were descriptively additive or synergistic (greater than additive). Isobole plots were also descriptively additive or synergist ic, although a few measurements were statistically different for syner gism. Statistical evaluation between mixtures and single values, using the z test, were in some cases different at the 5% level. Bioluminesc ent counts were determined to be normally distributed using a statisti cal test for small sample numbers at the 1% level. Evaluation for outl iers, using the Dixon test, was also performed and found one mixture t o have an outlier. This single outlier had no influence on the combine d toxicity results. The use of low-cost and rapid bioluminescent micro bial toxicity tests for mixture studies is discussed. (C) 1996 Academi c Press, Inc.