Interactive Effects of Ionophore Antibiotic Monensin and Polystyrene Microplastics on the Growth and Physiology of Microcystis aeruginosa

This study was conducted to examine the combined effects of monensin (MS) and 3 µm polystyrene microplastics (PEMPs) on the growth and stress-associated physiological responses of Microcystis aeruginosa under controlled laboratory conditions [temperature: 20 ± 1 °C, lighting: (30 ± 4) µmol m−2 s−1 (12 h:12 h light–dark photoperiod), growth medium: BG-11]. The experiments included MS concentrations of 0, 50, 250, and 500 µg/L and PEMPs concentrations of 0.25, 1.25, and 6 mg/L. Measurements included optical density (OD730), chlorophyll ‘a’, cellular protein content, oxidative stress, and the activities of catalase (CAT) and guaiacol peroxidase (GPX). M. aeruginosa exhibited a significant increase in growth on day 7 at elevated MS concentrations across all PEMP levels. Similarly, MS and PEMP treatments had a significant interactive effect on cellular protein content on day 7. However, their combined effect on chlorophyll ‘a’ production was not significant. Oxidative stress measurements showed a dose-dependent decrease with increasing MS concentrations under PEMP administrations. Enzyme activity assays indicated that CAT activity increased while GPX activity decreased with higher MS concentrations. The results imply that co-contamination of PEMPs and MS has a significant impact on the growth and stress physiology of M. aeruginosa in aquatic ecosystems.