Reynaldo Patino Brittanie L. Dabney 2018 <p><span>Glyphosate-based herbicides (GBH) are widely used around the globe. While generally toxic to phototrophs, organic phosphorus in glyphosate can become available to glyphosate-resistant phytoplankton and contribute to algal bloom development. Few studies have examined the effects of GBH on growth of eukaryotic microalgae and information for the toxic bloom-forming haptophyte, <i>Prymnesium parvum</i>, is limited. Using a batch-culture system, this study examined the effects on <i>P. parvum</i> growth of a single application 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 low concentrations [0-1000 μg glyphosate acid equivalent (ae) l</span>^-1<span>]. Roundup formulations differ in the percent of glyphosate as active ingredient (Roundup SC, ∼50%; Roundup RtU, 2%), allowing indirect evaluation of the influence of inactive ingredients. Roundup SC enhanced exponential growth rate at 10-1000 μg glyphosate ae l</span>^-1<span>, and a positive monotonic association was noted between Roundup SC concentration and early (pre-exponential growth) but not maximum cell density. Glyphosate and both Roundup formulations enhanced growth rate at 100 μg glyphosate l</span>^-1<span>, but only Roundup SC and glyphosate significantly stimulated early and maximum density. This observation suggests the higher concentration of inactive ingredients and other compounds in Roundup RtU partially counteracts glyphosate-dependent growth stimulation. When phosphate concentration was varied while maintaining other conditions constant, addition of Roundup SC and glyphosate at 100 μg l</span>^-1<span>&nbsp;influenced growth more strongly than equivalent changes in phosphate-associated phosphorus. It appears, therefore, that low doses of glyphosate stimulate growth by mechanisms unrelated to the associated small increases in total phosphorus. In conclusion, glyphosate and GBH stimulate <i>P. parvum</i> growth at low, environmentally relevant concentrations. This finding raises concerns about the potential contribution to <i>P. parvum</i> blooms by glyphosate-contaminated runoff or by direct application of GBH to aquatic environments.</span></p> application/pdf 10.1016/j.hal.2018.11.004 en Elsevier Low-dose stimulation of growth of the harmful alga, Prymnesium parvum, by glyphosate and glyphosate-based herbicides article