Next to organophosphates carbamates and metals
Next to organophosphates, carbamates and metals, cholinesterase activity can also be affected by other pesticides, such as newer generation neonicotinoid insecticides (NN). NNs do not inhibit AChE activity via the organophosphate-sensitive binding site of the enzyme, but rather, by acting as a structural and functional homologue of punicalagin synthesis in the amino-terminal ligand-binding domain of the post-synaptic nicotinic acetylcholine receptor (Barata et al., 2004, Bocquené et al., 1997, Osterberg et al., 2012, Tomizawa et al., 2008). Nevertheless, few studies have investigated the effect of NNs on ChE and AChE activity in bivalves, and most studies have focused on imidacloprid, the most widely used NN. Non-target toxicity of imidacloprid has been observed in crustaceans (Malev et al., 2012) as well as mussels (Dondero et al., 2010). Mukadam and Kulkarni (2014) described a decrease of bulk protein content in gills, digestive gland, adductor muscle and gonad of the clam Katelysia opima exposed to imidacloprid (LC0 = 38.5mg/L; LC50 = 86.6mg/L) for 96h, and Radwan and Mohamed (2013) reported a reduction in AChE activity in the gastropod Helix aspersa exposed to different concentrations of imidacloprid (2.5–30mg/mL), showing that NNs are capable of affecting AChE, even though the mechanism by which they do so is currently unclear.
The Pacific cup oyster, genus Saccostrea, has been used in Australia and Asia as a sentinel bivalve for pollution monitoring, particularly of metals (Barua et al., 2011, Thompson et al., 2011, Thompson et al., 2012a, Thompson et al., 2012b, Andrew-Priestley et al., 2012, Taylor et al., 2013, Taylor et al., 2015). Saccostrea sp. is native to the Indopacific, but has recently established itself in the Caribbean Sea, including Panama (Pagenkopp et al., 2015) and Colombia (Moncaleano-Niño et al., 2017). Current evidence indicates that cup oyster Saccostrea sp. co-occurs with the native mangrove oyster Crassostrea rhizophorae, sharing the same ecological niche and habitat, such mangrove roots and submerged maritime structures, making it an attractive species for monitoring programs.
The main objective of this study was to evaluate, under controlled laboratory conditions, the sensitivity of different components of cholinesterase activity (total cholinesterase, T-ChE; eserine sensitive cholinesterase, Es-ChE; and eserine resistant cholinesterase, Er-ChE) in three different tissues (gills, adductor muscle and digestive gland) of Saccostrea sp., exposed to two different types of pesticides (chlorpyrifos and imidacloprid) and two metals (Cd and Cu), to evaluate the specificity of these biochemical markers as a monitoring tool for assessing the environmental health of coastal ecosystems in the Colombian Caribbean.
Results Whole tissue metal concentrations, determined at the end of the 96h exposure period for the Cd and Cu bioassays are shown in Table 1. In the Cd bioassay, oysters bioaccumulated Cd in a significant, dose-dependent manner, resulting in whole tissue Cd concentrations as high as 297μg/g dw after 96h for the highest nominal aqueous concentration (1000µg/L). In contrast, no significant dose-dependent bioaccumulation (linear regression p > 0.05) was apparent for the Cu bioassay, with tissue Cu concentrations ranging between 1473–1936μg/g (control: 1243μg/g). For the other 13 metals (As, Ag, Co, Cr, Hg, Fe, Mn, Ni, Pb, Se, Sn, V, Zn), tissue concentrations were similar among the different bioassays and treatment levels, varying by less than a factor of 2, thus ruling out a significant co-variance contribution. Nevertheless, a 1.5 times higher tissue concentration of Fe for the highest Cd treatment compared to controls is noted. Activities of the three cholinesterase components, total cholinesterase (T-ChE), eserine sensitive cholinesterase (Es-ChE) and eserine-resistant cholinesterase (Er-ChE) varied considerably among toxicant, concentration level and tissue in the four bioassays (Table 2). T-ChE activity was typically lowest in adductor muscle (1–22nmol/min/mg protein, control mean for all four assays: 7.5nmol/min/mg protein) and higher in gills (2–57nmol/min/mg protein, control mean for all four assays: 14.0nmol/min/mg protein) and digestive gland (5–50nmol/min/mg protein, control mean for all four assays: 18.0nmol/min/mg protein, but note high variance for this tissue). Similarly, Es-ChE activity was lowest in muscle (0–9nmol/min/mg protein, control mean for all four assays: 3.32nmol/min/mg protein) and generally higher in gill (0–27nmol/min/mg protein, control mean for all four assays: 7.56nmol/min/mg protein) and digestive gland (0–34nmol/min/mg protein, control mean for all four assays: 8.94nmol/min/mg protein). Er-ChE activity showed a similar pattern to T-ChE and Es-ChE, with lowest activities in adductor muscle, and approximately 2-fold higher activities in gills and digestive gland.