Hormetic effects of glyphosate and glyphosate-based herbicides on growth of Prymnesium parvum, a harmful alga

Glyphosate-based herbicides (GBHs) are widely used around the globe. In addition to their toxic activities, the organic phosphorus in glyphosate can become available to glyphosate-resistant primary producers. The impact of GBH on aquatic communities has received considerable attention; most previous studies, however, have used high concentrations rarely seen in the field. Tolerance to GBH is common among cyanobacteria, including harmful algal bloom- (HAB)-forming species. Relatively few studies have examined potential effects of GBH on growth of eukaryotic microalgae, especially at low concentrations and information for the HAB-forming haptophyte, Prymnesium parvum, is limited and inconclusive. This study examined the effects on P. parvum growth of Roundup Weed and Grass Killer Super Concentrate Plus® (Roundup SC), Roundup Weed and Grass Killer Ready-to-Use III® (Roundup RtU), and technical-grade glyphosate at environmentally relevant, low concentrations. The Roundup formulations differ in the percent of glyphosate as active ingredient (Roundup SC = 50%; Roundup RtU = 2%), allowing indirect evaluation of the impacts of the inactive ingredients. Growth responses examined were exponential growth rate, early density (pre-exponential growth), and maximum density and exposures were conducted in nutrient rich and phosphate-deficient culture media. Growth rate was stimulated by Roundup SC at 10-1000 μg acid equivalent (a.e.) glyphosate l-1, and a positive association was observed between Roundup SC concentration and early density but not maximum density. In a separate experiment, Roundup formulations and glyphosate stimulated growth rate at 100 μg a.e. glyphosate l-1, but only Roundup SC and glyphosate significantly stimulated early and maximum density. This observation suggests that the relatively high concentration of inactive ingredients in Roundup RtU may partially counteract the stimulatory activity of glyphosate. In experiments where the concentration of phosphate was varied while maintaining other culture conditions constant, Roundup SC and glyphosate influenced growth more strongly than equivalent changes in phosphate-derived phosphorus concentrations. These observations suggest that at low concentrations, glyphosate stimulates P. parvum growth by mechanisms unrelated to eutrophication caused by its degradation and consequent release of phosphorus. In conclusion, glyphosate and GBH showed hormetic effects on P. parvum growth at environmentally relevant concentrations. These effects, however, seemed to be partially counteracted by ready-to-use formulations with a high proportion of inactive ingredient. Management strategies for P. parvum bloom prevention or control may need to consider the potential contribution to these blooms by glyphosate-contaminated runoff in areas where GBH are being used or by direct applications with the purpose of controlling aquatic nuisance plants.