Effects of two copper compounds on Microcystis aeruginosa cell density, membrane integrity, and microcystin release

• Efficacy of copper sulfate and Cu–EA for controlling M. aeruginosa was different.
• Minimum amount of copper required for controlling M. aeruginosa did not cause cell lysis.
• Excessive amount of copper could result in cell lysis and greater microcystin release.
• Total microcystin concentration would not exceed pretreatment value after treatment.
• Degradation of microcystin occurred as copper concentration was 80–1000 µg Cu/L.

Microcystin release following Microcystis aeruginosa cell lysis after copper-based algaecide treatment is often cited as a concern leading to restricted use of algaecide in restoration of natural water resources. To examine this concern, bench-scale experiments were conducted to study responses of M. aeruginosa to 8-day copper exposures as copper sulfate and copper–ethanolamine (Cu–EA). M. aeruginosa UTEX 2385 was cultured in BG11 medium to cell density of 106 cells/mL with total and extracellular microcystin of 93 and 53 μg/L, respectively. Exposures of copper concentration ranged from 40 to 1000 μg Cu/L. Cell membrane integrity was indicated by erythrosine B. In the end of experiment, total microcystin and cell density in untreated control (313 μg/L and 107 cells/mL) was 3.3 and 10 times greater than pretreatment value, respectively. Minimum amount of copper required to reduce M. aeruginosa population within 8 days was 160 μg Cu/L as copper sulfate and 80 μg Cu/L as Cu–EA, where total and extracellular microcystin concentrations (47 and 44 μg/L for copper sulfate; 56 and 44 μg/L for Cu–EA) were degraded with degradation rate coefficient 0.1 day−1 and were less than pretreatment values. Given a copper concentration at 80 µg Cu/L as Cu–EA, M. aeruginosa cells were intact and less microcystin were released compared to treatments at 160–1000 µg Cu/L, where lysed cells and relatively greater microcystin release were observed. Based on the laboratory results, a minimum amount of copper required for reducing M. aeruginosa population could decrease total microcystin concentration and not compromise cells and minimize microcystin release.

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