PH-DEPENDENT SORPTION, BIOCONCENTRATION AND ALGAL TOXICITY OF SULFONYLUREA HERBICIDES

Uptake and toxicity of four sulfonylurea herbicides (metsulfuron-methy l, chlorsulfuron, triasulfuron and tribenuron-methyl) to the freshwate r microalga Chlorella fusca have been investigated under different pH regimes. Toxicity parameters were inhibition of (a) cell volume growth and (b) reproduction of synchronized cell populations, cultivated und er photoautotrophic conditions. At pH 6.5, EC50 values ranged from 3.2 (metsulfuron-methyl) to 0.3 mumol/l (tribenuron-methyl) for the inhib ition of cell volume growth, and from 2.2 to 0.2 mumol/l for the inhib ition of cell reproduction. By lowering the pH of the growth medium to 5.0, the toxicity of chlorsulfuron was enhanced 25-fold, as indicated by an EC50 value of 44. nmol/l for the inhibition of reproduction. Th e bioconcentration of the four sulfonylureas in Chlorella did not exce ed a factor of 9 at pH 6.0, but was significantly increased at pH 5.0, with a maximum value of 53 for chlorsulfuron. These results suggest t hat the sulfonylureas penetrate the algal cell membranes primarily in their undissociated form and accumulate by an ion trapping mechanism.