Insights into the impact of increasing temperature, light intensity, and UV-B exposure on the circadian rhythm of microcystin production and release, and the expression of mcy genes in the cyanobacterium Microcystis aeruginosa

Increasing temperature and varying light quality can favor the formation of blooms of toxin-producing cyanobacteria. With respect to light quality, increasing type-B ultraviolet radiation (UV-B) causes cellular damage with cascading effects at different levels of biological systems. This study aimed to evaluate the combined effects of light intensity, UV-B, and increasing temperature on the circadian rhythm of production and release of microcystins (MCs) as well as the expression of mcy genes (mcyA, mcyD, and mcyE) and internal H2O2 formation of Microcystis aeruginosa. Microcystis aeruginosa was exposed to UV-B (0.8 W m−2) in 24-h cycles (12 h light:12 h dark) at 23, 25, and 30 °C combined with 30 or 50 µmol photons m−2 s−1. Samples were collected every 4 h during each 24-h cycle for MC analyses, evaluation of mcy gene expression by RT-qPCR, and internal H2O2 formation. The combination of 23 and 30 °C with UV-B changed the circadian rhythm of MC production and release and reduced the cyanotoxin concentration per cell quota. mcyD gene expression was 10.75-fold higher at 23 °C and 50 µmol photons m−2 s−1 treatment. However, a 7.55-fold decline in the expression of the mcyE gene occurred at 12 h (light period). The combination of 23 °C with UV-B caused the highest intracellular H2O2 formation. The enhanced expression of genes involved in MC synthesis at midday demonstrated that there is a critical period of stress that signals the toxin-production, which has significant implications for the general well-being of aquatic ecosystems.