Cyanotoxin Production Dynamics: a Comprehensive Study of the Growth Stage of Selected Cyanobacteria, H2O2-triggered Apoptosis, and Light Conditions

Understanding the link between cyanobacterial growth and cyanotoxin production is key to controlling their proliferation in freshwater reservoirs. This study examined microcystin production in five cyanobacterial isolates, Microcystis aeruginosa, Fischerella sp., Nostoc sp., Pseudanabaena sp., and Leptolyngbya sp., across growth stages and stress conditions, using batch cultures in BG11 medium. Chlorophyll-a levels and Microcystin content were continuously monitored within cyanobacteria samples every eight days for two months. Under apoptosis-inducing conditions, H2O2 concentrations: 10, 60, 125, 250 mg L–1, and exposure duration: 3, 6, 9, 12 h were applied to assess the cytotoxicity, MC content, and morphological activities. Samples were exposed to two light intensities: 0 and 50 µmol m–2 s–1. Analyses were conducted on total soluble protein content, MC content, and ascorbate peroxidase activity (APX). The research shows cyanotoxin production varies between lag and exponential phases, especially in filamentous cyanobacteria (>150 and 60–150 nm colonies). Pseudanabaena sp. (2.541 ± 0.57 mg L–1) and Fischerella sp. (1.5152 ± 0.14 mg L–1) showed higher microcystin toxicity than unicellular Microcystis aeruginosa. Intracellular toxin concentrations surpassed extracellular levels. The MC-LR microcystin variant showed significant differences in toxin levels compared to other variants (p < 0.05, ANOVA). Specifically, MC-LR levels were about 8.15 times higher than MC-RR and 2.16 times higher than MC-YR in treated cultures. Higher H2O2 levels and longer incubation times affected cyanobacterial toxicity, morphology, and microcystin output. Under intense light, APX activity rose by 25.77%, and microcystin production increased by 35.8%. However, toxin production in these organisms can fluctuate due to various environmental and molecular factors.