In the photolysis of the Paraquat, the pesticide can be degraded by two mechanisms: direct and indirect photolysis. In the case of direct photolysis, the pesticide can absorb energy from UV irradiation, and this energy can transform the molecule into an excited state. Degradation can be promoted by homolysis, heterolysis, and photoionization. In indirect photolysis, highly active species, such as hydroxyl, superoxide, and ozone radicals are produced by the excitement of UV light, which can degrade the pesticide (El-Saeid et al. 2021).
Irradiation with UV light is a potential treatment method for degrading water contaminants, such as natural organic matter, aliphatic and chlorinated volatile organic compounds, aromatic compounds, and herbicides (Choi and Chung 2020). Herein, the ESI mass spectrum of the PQ solution after photolysis with UV irradiation for 10 min showed the peaks at m/z 300, m/z 186, m/z 171, m/z 138, and m/z 93, studies reported similar detection (Florêncio et al. 2004; Marien et al. 2019). However, the ESI mass spectrum of the PQ solution, without irradiation, is mainly characterized by peaks at m/z 186, M+•, the molecule ion; m/z 171, (M+•-CH3)+; m/z 143, (M+•-CH2NCH3)+•; m/z 118, and m/z 93, M2+, the molecular dication.
The ions at m/z 186, 171, and 93 are the most abundant fragments ions in the PQ solution and PQ fragmentation after 10 min UV degradation represents the molecule PQ, monoquat, and 4-methyl-pyridine, respectively.
The exposition of PQ in animal models causes drastic damage to bioenergetic function and mitochondrial metabolism (O'Kane 2011; Müller et al. 2017; Duavy et al. 2019) due to disturbing the intracellular redox balance promoting an intense cycle of metabolic oxidants (Liu et al. 2022), compromising important aspects in the survival of any animal (Silva et al. 2022; Alencar et al. 2022). Our results demonstrated a 15% increase in mortality in animals exposed to fragments generated after 10 min of PQ photodegradation. Furthermore, only 2% of flies fed with PQ (0.474 mM, final concentration) for 10 days died, an effect with less toxicity compared to a recent study, which demonstrated the mortality of 20% of D. melanogaster fed with 10 mM PQ for 15 days (Neves et al. 2022). In addition to its climbing or locomotion capacity, a function that is directly related to neuromuscular activity (Souza et al. 2019) was affected. However, we observed that by-products generated from the photodegradation are more harmful to the health and survival of the animal model, suggesting greater toxicity to D. melanogaster.
PQ may interact with mitochondrial matrix and in the electron transport chain complexes, especially with complexes I and III promoting a redox cycling: divalent PQ cation (PQ2+) is reduced to highly reactive mono cation radical (PQ+), which in turn reacts with the molecular oxygen to generate superoxide radical (O2•−) and subsequently other oxygen and nitrogen reactive species (Blanco-Ayala et al. 2014; Medina-Leendertz et al. 2014). Studies reported that a dose of PQ induces oxidative damage that activates mitochondrial pathways associated with excitotoxicity, autophagy, alteration of dopamine catabolism, and inactivation of tyrosine hydroxylase are positioned as causes for the loss of dopaminergic cells (Cassar et al. 2015; Zhang et al. 2016; Ahmad et al. 2021; Hajam et al. 2022). Our results showed that PQ solution after photolysis with UV irradiation for 10 min disrupted the intracellular redox balance promoting an oxidant status, as evidenced by enhanced carbonyl protein production rates in the heads of D. melanogaster. In addition, flies exposed to PQ and its photodegradation by-products increased the lactate content in the head and thoracic tissues a key product of energy metabolism in the thorax and brain, catalyzed by the reduction of pyruvate and the accumulation in the brain are a valuable biomarker of neuronal and muscle degeneration related to mitochondrial dysfunction (Harris et al. 2016; Silva et al. 2022; Cabral-Costa et al. 2023).
The diminished mitochondrial metabolism in tissues (muscle and brain) of D. melanogaster PQ and its photodegradation by-products supplemented observed here can also be associated with CS activity, an enzyme of Krebs Cycle indicative of mitochondrial content (Hosamani 2013; Chhimpa et al. 2023). Flies fed with byproducts with 10 min of PQ fragmentation showed lower CS activity in the head and thoracic tissues, suggesting damage to mitochondrial content in neurons. Studies of cell culture (Jang et al. 2015; Chowdhury et al. 2020), D. melanogaster (Souza et al. 2019; Carmo et al. 2021; Silva et al. 2022; Alencar et al. 2023), and embryos of zebrafish (Wang et al. 2018) and mice (Li et al. 2022) PQ-induced mitochondrial dysfunction.
The mechanism of muscle contraction is influenced by several factors including the cholinergic system (Jones 2009), with the concentration of ACh being controlled by AChE. PQ and its derivates, such as monoquat were observed as inhibitors of AChE activity (Seto and Shinohara 1988). Our results suggest damage to the cholinergic system in flies, due to a significant reduction in AChE, an enzyme that catalysis favors a greater uptake of calcium for muscle contraction, while the central nervous system maintains a balance between neurotransmitters and ACh (Aldunate et al. 2004), which demonstrate locomotor impairment in D. melanogaster (Souza et al. 2017).
Finally, based on this study, we proposed using D. melanogaster in the initial screening of prospective photodegradation of PQ with UV irradiation. Also, these results are an incentive for the continuation of this research and investigations into the possible treatment methods for organic pollutants in wastewater.