Differential inactivation of microalgae and cyanobacteria by UV-C and hydrogen peroxide treatments in natural freshwater communities

Aquatic ecosystems provide water for multiple purposes and serve as venues for recreational activities. However, the phytoplankton community in these ecosystems may be a source of economic and health problems, particularly during episodes of harmful algal and cyanobacterial blooms. These episodes are especially concerning when involving potentially toxic cyanobacteria. The objective of this study is to evaluate the inactivating efficacy by the UV irradiation, applied with either a traditional mercury lamp (254 nm) or an emerging UV-LED (275 nm), as well as the use of H₂O₂, separately or combined, as a mechanism to control the microalgae and cyanobacterial growth in natural water collected from the Guabizhun pond (Ecuador). Different facts involved in the phytoplankton inactivation were examined: changes in the viable organisms concentration, regrowth and health monitoring, and changes in the phytoplankton community after the treatment. The results reported that the UV effect was maximum four days after the UV irradiation, with an efficacy decreasing due to the regrowth, enhancing the predominance of cyanobacteria with respect to eukaryotic microalgae in the middle term. The H₂O₂ at 5 and 10 mg L⁻¹ inhibited effectively the cyanobacterial growth, and enhanced noticeably the growth, health and diversity of eukaryotic microalgae. Combined H₂O₂/UV treatment (10 mg H₂O₂ L⁻¹) achieved equivalent inactivation to H₂O₂ alone; however, irradiation at 275 nm limited the regrowth of eukaryotic microalgae while maintaining the inactivation effect on cyanobacteria in the combined process. Overall, 275 nm showed a more favorable dose-effect performance than 254 nm.