Acute Toxicity of Organophosphorus Pesticides and Their Degredation By-Products to Daphnia Magna, Lepidium Sativum and Vibrio Fischeri

Abstract

Organophosphorus pesticides (OPPs) attained the growing importance in pests control because of their rapid decomposition and less likely accumulation in environment. They are still of great concern however, for water sources contamination because of their high solubility in water and excessive usage. Their usage amounts were elevated after they were introduced as replacements for the highly persistent organochlorine pesticides. They are classified into two main groups, organophosphates (P=O) and organothiophosphates (P=S) depending on whether oxygen or sulphur forms a double bond with the central phosphorous atom. They were found in environment with enough frequency (Ballesteros and Parrado, 2004) to constitute an ecotoxicological risk. Their concentration in water sources (Barcelo et al., 1990; Konstantinou et al., 2006), in air (Tuduri et al., 2006) and food (Bai et al., 2006; Darko and Akoto, 2008) can vary between a few ppb to ppm levels. The presence of these pesticides can directly affect the health of aquatic and terresterial organisms and may present a threat to humans through contamination of drinking water supplies. OPPs always pose acute toxicity but not chronic toxicity on organisms because of their quick degradation (Ye et al., 2010). OPPs are known to cause inhibition of acetylcholinesterase (AChE) in target tissues which leads to accumulation of acetylcholine. According to its key physiological role in nerve transmission, AChE is the target of various insecticides. AChE is an enzyme vital for normal nerve function and AChE inhibition leads to over stimulation of the central and peripheral nervous systems, resulting in neurotoxic effects in organisms. OPPs also produce oxidative stress in different tissues (Possamai et al., 2007) and shows genotoxic (Bolognesi, 2003; Cakir and Sarikaya, 2005, Arredondo et al., 2008) and immunotoxic (Yeh, et al., 2005; Day et al., 1995) effects. The majority of OPPs give rise to only slight inhibition of AChE by themselves, unless they undergo oxidative activation. This process involves the substitution of the sulfur atom in the P=S bond of the organophosphate pesticide with an oxygen atom resulting with formation of oxon derivatives (OPPs-oxons) (Fig. 1). This substitution is a result of advanced oxidation processes such as O3, O3/UV, H2O2/UV, fenton, photo-fenton, TiO2/UV, etc. in water treatment and natural oxidation processes such as UV radiation and microbial degradation. Combined oxidation systems decreases toxicity effects of by-products via enhancing mineralization. Kim et al. (2006) used Vibrio fischeri and