Removal of Dolichospermum circinale, Microcystis aeruginosa, and their metabolites using hydrogen peroxide and visible light

Highlights: • Treatment of cyanobacterial cultures using H₂O₂ under visible light was studied. • Oxidation resulted in cell lysis and release of intracellular content. • Oxidation treatment impacted on organic matter and true color. • H₂O₂ reduced extracellular geosmin but increased total microcistin concentrations. • Even after low H₂O₂ concentration aplication, M. aeruginosa and D. circinalis were not viable. Frequent cyanobacterial blooms in reservoirs used for human supply increase the risk of noxious secondary metabolites, endangering human health and ecological balance, and requiring constant monitoring by water companies. Although hydrogen peroxide (H₂O₂) has been widely reported as an effective agent for the control of cyanobacteria, being Microcystis aeruginosa one of the most studied species, very limited data is available on its effects over Dolichospermum circinale. Therefore, this study aimed to evaluate the impact of H₂O₂ on D. circinale and comparing it to the effects over the M. aeruginosa. The treatment was performed in cyanobacterial cultures with the application of 2 and 5 mg L⁻¹ of H₂O₂ under visible light. To measure the impact of the treatment, intact cells were counted and cell re-growth monitored. Geosmin and microcystin, cell pigments, color, and organic matter in water were also analyzed during the treatment. The results showed that even the smallest H₂O₂ concentration (2 mg L⁻¹) was able to completely remove D. circinale cells. Although M. aeruginosa could only be completely removed using 5 mg L⁻¹, the few cells remaining after the application of 2 mg L⁻¹ were not viable and did not re-grew after 15 days. Total microcystin concentration increased after M. aeruginosa was exposed to H₂O₂, suggesting that oxidative stress may increase the detection of this metabolite when the cells are lysed. While 2 mg L⁻¹ was able to significantly decrease total geosmin, the addition of 5 mg L⁻¹ did not improve removal. Chlorophyll-a was readily degraded after cell rupture but the same did not happen to phycocyanin, demonstrating its high resilience to this oxidant. Color and organic matter increased for the M. aeruginosa but decreased for the D. circinale suspension, probably because the higher concentration of the M. aeruginosa yielded more extracellular content to the water which was not able to be degraded by the amount of H₂O₂ applied.