Indices of triazine toxicity in Chlamydomonas geitleri Ettl

Abstract

The toxicity of three triazine herbicides (atrazine, simazine and terbutryn) was examined in unialgal batch cultures of Chlamydomonas geitleri Ettl. Changes in growth and chlorophyll accumulation were used as indices of toxicity. Herbicide-induced chlorophyll fluorescence was also used to describe the primary effect of the three triazines. The subsequent effect on inhibition of CO 2 assimilation was also examined. Some cultures were pre-treated with 2.46 μM atrazine, and subsequently treated with varying concentrations of atrazine in an attempt to induce increased tolerance. Terbutryn was the greatest inhibitor of growth and CO 2 fixation, with toxicity being 2 orders of magnitude greater than for atrazine and simazine. Its half saturation constant ( K FRI) for chlorophyll fluorescence was the lowest, suggesting its affinity for binding at the active site was 32 and 50 times greater than that for atrazine and simazine, respectively. Atrazine and simazine were not significantly different in their inhibition of CO 2 fixation or at inducing chlorophyll fluorescence. However, atrazine inhibited growth 2 to 3.5 times more effectively than did simazine. Inhibition of chlorophyll synthesis was most obvious with exposure to atrazine and terbutryn, with only a slight stimulation in chlorophyll occurring at the lowest concentrations of these herbicides. A strong stimulation in chlorophyll synthesis, which was observed with simazine exposure, was interpreted as being a tolerance mechanism. The obvious stimulation in growth and CO 2 fixation observed with simazine was perhaps due to the stimulatory effect of the herbicide on protein synthesis. Pre-conditioning of cultures with 2.46 μM atrazine did not enhance tolerance as evidenced by unchanged growth, CO 2 fixation, and chlorophyll fluorescence response upon subsequent treatment with atrazine.