Mechanistic Study and the Influence of Oxygen on the Photosensitized Transformations of Microcystins (Cyanotoxins)

Abstract

Microcystins (MCs) produced by cyanobacteria are strong hepatotoxins and classified as possible carcinogens. MCs pose a considerable threat to consumers of tainted drinking and surface waters, but the photochemical fate of dissolved MCs in the environment has received limited attention. MCs are released into the environment upon cell lysis along with photoactive pigments including phycocyanin and chlorophyll a. The concentrations of MCs and pigments are expected to be greatest during a bloom event. These blooms occur in sunlit surface water and thus MCs can undergo a variety of solar initiated or photosensitized transformations. We report herein the role of oxygen, sensitizer, and light on the photochemical fate of MCs. The phycocyanin photosensitized transformation of MCs is elucidated, and photosensitized isomerization plays an important role in the process. The UV-A portion of sunlight was simulated using 350 nm light and the phototransformations of three MC variants (-LR, -RR, -LF) were investigated. Singlet oxygen leads to photooxidation of phycocyanin, the predominant pigment of cyanobacteria, hence, reducing the phototransformation rate of MCs. The phototransformation rate of MC-LR increases as pH decreases. The pH effect may be the result of MCs association with phycocyanin. Our results indicate photosensitized processes may play a key role in the photochemical transformation of MCs in the natural water.

Cyanobacterial and microcystins dynamics following the application of hydrogen peroxide to waste stabilisation ponds

“…Although the processes occurring in the laboratory were likely also occurring in the field, the full-scale trial would have introduced further potential mechanisms for microcystins degradation, including the presence of UV radiation, which catalyses the production of hydroxyl and hydroperoxyl radicals from H 2 O 2 , resulting in a high degree of microcystins degradation in the hours following H 2 O 2 addition (Glaze et al, 1987). Microcystins can also be destroyed by UV radiation through isomerisation, and photosensitised transformation in the presence of humic substances and pigments (Song et al, 2007;Tsuji et al, 1994;Steinberg, 1999, 2000). In the full-scale trial, there would also have been a significantly higher loading of sediment than in the laboratory trials, where wastewater was collected from the surface of ponds.…”

Tin porphyrin immobilization significantly enhances visible-light-photosensitized degradation of Microcystins: Mechanistic implications

“…The involvement of 1 O 2 was also confirmed based on the significant degradation of MC-LR by visible light-irradiated phthalocyanine as an effective 1 O 2 photosensitizer [23]. In contrast, the role of 1 O 2 in oxidative destruction of MCs was inferred to be minor by the observation that photosensitized degradation of MCs by phycocyanin was neither kinetically retarded in the presence of azide ion as a 1 O 2 quencher nor it was negligibly accelerated in deuterated water where the lifetime of 1 O 2 is extended [24]. The dominant pathway for photosensitized degradation of MCs in natural waters may be via direct energy transfer from the photo-excited organic matter that causes isomerization of the Adda side chain [25].…”

“…Accordingly, it is plausible that the capability to sensitize electron transfer is responsible for the effective decomposition of MCs by SnP/silica (Reaction 4). Energy transfer from the photo-excited triplet states of organic substances may initiate the isomerization of a diene bond in the Adda moiety [24,25,40], resulting in MC decay and formation of the corresponding product. This raises the possibility that 3 SnP * may be involved in this photo-induced isomerization.…”

Microbial biodegradation of microcystin-RR by bacterium Sphingopyxis sp. USTB-05

“…Several pathways have been proposed to elucidate the reduction of MCs, in which photolysis and biodegradation play important roles. Photo-degradation is important in removing the dissolved MCs (Zhang et al, 2003;Song et al, 2007). However, ultraviolet can hardly penetrate even a thin layer of colored water, let alone water containing high dense algae.…”