Effects of the sulfonylurea herbicide metsulfuron methyl on growth and macromolecular synthesis in the green alga Selenastrum capricornutum

Abstract

The sulfonylureas (SU), are highly phytotoxic due to the inhibition of the enzyme acetolactate synthase (ALS), the key enzyme in the synthesis of branched-chain amino acids. In order to find a short-term algal test to be used as an indicator of the environmental toxicity of SU, the effects of the sulfonylurea metsulfuron methyl (MSM) in the green alga Selenastrum capricornutum were studied. As a comparison of long-term and short-term effects of MSM in Selenastrum, effects on growth were compared to short-term inhibition of protein synthesis and the synthesis of nucleic acids. The EC:50 for inhibition of growth of S. capricornutum by MSM was determined to be 4.1 μmol/l and the No Effect Concentration (NEC) to be between 0.1 and 0.2 μmol/l. The inhibition of growth could be substantially reduced by the addition of the branched chain amino acids and valine was especially important to restore growth. Protein synthesis measured as incorporation of [ 3H]methionine was, in spite of the shortage of amino acids, not inhibited by MSM in a short-term test. In contrast, the incorporation of [ 3H]adenine into cold-TCA-insoluble macromolecules, was inhibited by MSM, although maximal inhibition was restricted to about 50%. The No Effect Concentration for inhibition of adenine incorporation was 0.3 μmol/l, similar to the NEC value for growth inhibition. If alkaline hydrolysis was used to specifically hydrolyse RNA prior to the precipitation with cold TCA, a greater portion of the adenine incorporation could be inhibited by MSM. Adenine incorporation into nucleic acids seems to be appropriate for a short-term test of effects of SU on natural microbial communities, since adenine incorporation is quickly inhibited at concentrations of SU known to inhibit growth.