Glyphosate Accelerates the Proliferation of Microcystis aeruginosa, a Dominant Species in Cyanobacterial Blooms

Glyphosate is a commonly used herbicide known for its high performance in killing certain plants and grasses; however, its use is regulated due to its harmful effects on the aquatic environment. The present study investigated and compared the toxic mechanisms of glyphosate on Microcystis aeruginosa (a toxin-producing cyanobacterium) under 2 conditions: 0 parts per thousand saline media (experiment I) and 2.5 parts per thousand saline media (experiment II). The results indicated that an appropriate concentration of glyphosate provided a phosphate source for M. aeruginosa, resulting in an increased specific growth rate in both experimental groups compared with the controls. Glyphosate-enhanced alkaline phosphatase (ALP) activity increased by up to 1.37-fold in experiment I and 1.68-fold in experiment II. Moreover, the activities of superoxide dismutase (SOD) and catalase (CAT) decreased at glyphosate concentrations below 1.2 mg L-1 but increased at concentrations greater than 1.2 mg L-1 in experiment I, whereas SOD and CAT activities decreased in experiment II and declined by 64 and 49% in the 30 mg L-1 treatments. Furthermore, the transcript abundances of the pyruvate carboxylase (pcB), microcystin synthetase B (mcyB), and paired-like homeobox (phoX) genes were up-regulated by up to 6.92-, 3.63-, and 2.27-fold in experiment I and 6.74-, 6.55-, and 4.86-fold in experiment II after 96 h of incubation. The addition of glyphosate stimulated the production of dissolved organic matter including tryptophan-like substances, fulvic acid-like substances, (marine) humic acid-like substances, and microcystin-leucine-arginine in the culture. In conclusion, glyphosate stimulates the proliferation of M. aeruginosa and enhances the release of dissolved organic matter in saltwater ecosystems. Environ Toxicol Chem 2020;00:1-10. (c) 2020 SETAC

Automated summary

Glyphosate was found to promote the growth of Microcystis aeruginosa by providing a phosphate source, enhancing enzyme activities, and increasing gene transcription levels. Additionally, glyphosate stimulated the production of dissolved organic matter, enhancing its release in saltwater ecosystems.