Individual and combined effects of ionophore antibiotics monensin and lasalocid on growth, oxidative stress and antioxidant responses of Microcystis aeruginosa

To assess the impact of monensin, lasalocid, and a 1:1 (w/w) mixture of monensin and lasalocid on the formation of Microcystis aeruginosa-based harmful algal blooms (HABs), laboratory experiments were conducted [temperature 20 +/- 1 degrees C, illumination PAR 30 +/- 4 mu mol/m2 <middle dot>s (12-h light/dark cycle), growth medium BG-11] at concentrations of 100, 200, 500, 1,000, and 2,000 mu g/L. Measurements included optical density, cell protein content, chlorophyll 'a' (Chl. a) content, oxidative stress, catalase activity (CAT), and guaiacol peroxidase activity (GPX). Monensin treatment showed a dose-dependent positive effect on M. aeruginosa growth at concentrations <= 500 mu g/L and a gradual dose-dependent growth inhibition at concentrations of 1,000 and 2,000 mu g/L. The results indicated that monensin has a significant positive effect on the formation of HABs by M. aeruginosa. Lasalocid treatment showed a growth reduction of M. aeruginosa at concentrations <= 500 mu g/L and a growth increase at higher concentrations (1,000 and 2,000 mu g/L). The monensin and lasalocid 1:1 (w/w) mixture test showed an intermediate response, as indicated by the individual treatments signifying the potential interactive effects of these antibiotics on the growth of M. aeruginosa. Furthermore, alterations in Chl. 'a', oxidative stress, CAT, and GPX measurements provided evidence of the impact of these antibiotics on the existence of M. aeruginosa.