The impact of the herbicide paraquat on the interaction between a macrophyte (Lemna minor) and two species of cyanobacteria (Microcystis aeruginosa and Microcystis flos-aquae)

Abstract

Macrophytes like Lemna minor and cyanobacteria species (for example, Microcystis aeruginosa and M. flos-aquae) co-exist naturally in water bodies, competing for nutrients, light and space, which influence their growth. The introduction of external factors such as herbicides into water bodies can influence the interaction between these aquatic organisms. This study examined the impact of paraquat, a widely used herbicide, on the interaction between L. minor and M. aeruginosa and M. flos-aquae. This was done by co-culturing L. minor (12 viable colonies) with M. aeruginosa and M. flos-aquae (both at 3.0 × 10⁵ cells mL⁻¹) separately, and with 10 µg L⁻¹ of paraquat in BG-11 medium for 5 days. Monocultures of L. minor, M. aeruginosa, and M. flos-aquae were exposed to different concentrations of paraquat (0, 0.01, 0.1, 1, 10, and 50 µg L⁻¹). An increase in paraquat concentration reduced the growth and biomass of L. minor, M. aeruginosa, and M. flos-aquae in single cultures, while decreasing total microcystin production in M. aeruginosa but increasing it in M. flos-aquae. When co-cultured with either M. aeruginosa or M. flos-aquae, the growth and biomass of L. minor decreased, and its POD activity increased. The presence of M. aeruginosa increased the MDA content and GST activity of L. minor, while the presence of M. flos-aque plus paraquat increased its proteins content and GST activity. On the other hand, the presence of L. minor plus paraquat decreased the growth and biomass of both cyanobacteria (M. aeruginosa and M. flos-aquae) and increased their microcystins production, H₂O₂ contents, POD and GST activities, and total proteins. These results demonstrate that the presence of paraquat amplifies the suppression of growth and increased stress of L. minor caused by both M. aeruginosa and M. flos-aquae, and vice-versa. This situation may exacerbate the competitive dynamics in aquatic environments by potentially altering community structures, a situation that could affect overall ecosystem health.