The toxic effects of combined exposure of chlorpyrifos and p, p’-DDE to zebrafish (Danio rerio) and tissue bioaccumulation

Abstract

Pesticides are widely used and frequently detected in the environment. The evaluation on the toxic effects of the co-exposure of two or more pesticides or related metabolites could reflect the real situation of the exposing risks. In this study, zebrafish was used as a model to investigate the potential toxic interactions of chlorpyrifos and 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (p,p'-DDE) on the survival rate, oxidative stress response and neurotoxicity, as well as their bioaccumulation and distribution in tissues. Co-exposure of chlorpyrifos and p,p’-DDE resulted in significant additive acute toxic effects on adult zebrafish with model deviation ratio (MDR) = 1.64. Both 7-day short-term at 1% LC₅₀ and 35-day long-term at 0.5% LC₅₀ co-exposure of chlorpyrifos with p,p’-DDE (50 and 100 µg/L) significantly reduced the survival rate of zebrafish colony to 75 and 82.5%. Co-exposure of chlorpyrifos and p,p’-DDE contributed to increased activity of antioxidant enzyme CAT, SOD and GST and excessive MDA generation, and decreased activity of CarE, CYP450 and AChE, compared with either single exposure of them. In co-exposure, the bioaccumulation of chlorpyrifos and p,p’-DDE was significantly different from the single exposure group. Overall, this study unraveled the potential toxic interaction of chlorpyrifos and p,p’-DDE on zebrafish and provided reference for environmental risk assessment of pesticide mixture.

Introduction

Pesticide use has largely promoted the modern agriculture and protected the public health by controlling disease vectors and insect pests, while the unreasonable use may unintendedly lead to contamination in natural ecosystems (Popp et al., 2013). In agricultural practice, pesticides are commonly used as foliar sprays, soil drenches and seed dressings, and a large part of the active ingredients inevitably entered waterway by leaching and runoff, potentially posing a threat to the aquatic life (Goulson, 2014; Jensen, 2019). Regulatory standards are thus documented to mitigate the unexpected consequences caused by pesticide exposure in nontarget environments (Köhler and Triebskorn, 2013). These standards usually rely on the environment risk assessment which incorporates the environmental fate, ecotoxicity endpoints and certain toxicological impact of pesticides towards specific ecological indicators (Köhler and Triebskorn, 2013; Topping et al., 2020).

Chlorpyrifos (CPF) is a typical organophosphorus pesticide (OP) with half-life ranging between 7 and 140 days in various environmental matrixes (John and Shaike, 2015). Due to the potent efficacy against a wide spectrum of insect pests, it has been recommended for use on arable crops and the annually global commercial yield could reach 28,600 tons in 2018 (Huang et al., 2020). The major mode of action (MOA) of CPF is irreversibly binding to active target of cholinesterase via phosphorylation, leading to reduction in bioactivity, ultimately causing neurotransmission disruption and associated locomotor activity disorders (Burke et al., 2017). Increasing evidence indicates the frequent occurrence of CPF in rainwater, groundwater, streams, rivers as well as oceans, and the maximum environmental detectable concentration could reach 303.8 µg/L (Huang et al., 2020). Thus, numerous studies have been employed to elucidate the ecotoxicity to aquatic organisms, such as algae, fish and crustaceans (Morris et al., 2016; Sumon et al., 2018; Huang et al., 2020). Most of them focused on the neurotoxicity, genotoxicity and oxidative stress response resulted from the exposure of chlorpyrifos alone (Bonifacio et al., 2017; Burke et al., 2017).

Pesticide has been acknowledged as a main driving factor for the biodiversity loss in aquatic environment (Dudley et al., 2017). Frequent co-occurrence of pesticides may cause strongly negative impacts on nontarget organisms, therefore the current environmental risk assessments required more eco-toxicity information of pesticide mixtures rather than a single one (Kortenkamp and Faust, 2018). Especially, chlorpyrifos has been reported to induce additive or synergistic toxic interactions towards some aquatic organisms, when jointly exposed to pyrethroids, organophosphates, neonicotinoids as well as herbicides (Perez et al., 2013; Wang et al., 2017; Zhang et al., 2017; Herbert et al., 2021). 1,1-dichloro-2,2-bis-(p-chlorophenyl)-ethylene (p,p′-DDE), a major metabolite of organochlorine pesticide DDT, was still constantly found in aquatic organisms, diverse water bodies and sediments owing to the persistency, hydrophobic nature and biomagnification, despite the use of the parent molecule has been forbidden for years in many countries (Dong et al., 2020; Li et al., 2020). The environmental detectable concentration of p,p′-DDE in natural water and sediment was up to 0.982 µg/L and 420 µg/kg, respectively (Eddy et al., 2005; Lin et al., 2009; Akoto et al., 2016; Dahshan et al., 2016; Lan et al., 2019). Also, as an environmental persistent organochlorine pollutant, p,p′-DDE may exert severely negative effects on the aquatic lives, including estrogenic disruption, oxidation impairments and negative behavioral responses (Poulsen et al., 2012; Monteiro et al., 2015; Dawson et al., 2017). For example, p,p′-DDE was proved to be the dominant contaminant that accounted for antiandrogenic and neurotoxic effects to marine wildlife out of a wide variety of potential contaminants (Tiedeken and Ramsdell, 2010; Suzuki et al., 2011). Current monitoring studies have determined the simultaneous occurrence of chlorpyrifos and p,p′-DDE in aqueous phase and aquatic species (Akoto et al., 2016; Dahshan et al., 2016; Dominguez et al., 2018). Therefore, considering both of chlorpyrifos and p,p′-DDE are highly toxic contaminants in worldwide aquatic environments, there is a need to investigate their possible toxic interactions.

Hence, zebrafish was chosen as a model of aquatic organism for the acute toxicity, short-term and long-term evaluation of the co-exposure risks of chlorpyrifos and p,p′-DDE. Survival analysis and integrated biomarker response calculation were performed to elucidate the joint toxic effects of the two pesticides on the population fitness, oxidative system. The detoxify enzyme carboxylesterase (CarE), cytochrome P450 (CYP450) and acetyl cholinesterase (AChE) was determined in the long-term trial. Furthermore, the distribution and bioconcentration in zebrafish tissues was detected. These findings may offer new insights into the co-exposure toxic effects of chlorpyrifos and p,p′-DDE and facilitate environmental risk assessment of the pesticide mixture.