Biochemical, metabolic, and behavioural responses and recovery of Daphnia magna after exposure to an organophosphate

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Abstract

The responses of various suborganismal and organismal endpoints of Daphnia magna to pulse exposure to sublethal levels of the organophosphate paraoxon-methyl were compared. The changes and recovery of biochemical, metabolic, and behavioural variables, as well as physiological responses, were observed. The cholinesterase (ChE), filtration, and swimming activities were all affected in a concentration-dependant manner, and these effects reached significance at concentrations of 1.0, 1.5, and 0.7 μg L−1, respectively. The levels of these variables recovered significantly after detoxification for 24 h in clean medium. ChE and swimming activities were affected significantly by lower concentrations of paraoxon-methyl than filtration activity, which had the same threshold as the physiological responses (15N abundance and body size).

This study showed that among the parameters studied, swimming activity was the most sensitive, whereas changes in filtration activity had the most significant physiological consequences, and were therefore important in terms of effects propagation to the population level.

Introduction

Environmental disturbances, for example exposure to toxicants, affect organisms initially at the suborganismal level and might then affect higher levels of biological organisation, such as the populations or communities.

Effects at the suborganismal level can act as early warning signals of the presence of xenobiotics and provide detailed information on how chemicals interact with target sites. Effects at the level of the individual are indicative of the overall fitness of organisms, and might spread to the population and community levels depending on their severity and the compensatory mechanisms employed by the organisms that have been exposed. The responses at different levels of organisation can provide useful information for identifying and quantifying the effects of toxicants on biological systems (Maltby, 1999). To understand the processes involved, an approach is recommended in which the suborganismal impact of sublethal stress is related to higher-order biological consequences at the individual and population levels (Picado et al., 2007).

Numerous studies on the biological effects of toxicants in aquatic ecosystems have focused on invertebrates. Among them, studies of daphnids are highly relevant because these organisms are important components of freshwater food chains; they consume algae and other small organisms, and are in turn prey for small fish and other carnivorous aquatic animals. They are used commonly to assess the hazard or risk of contaminants (ISO (International Standard Organization), 1989), and are particularly useful for that purpose because of their short lifespan and reproductive capabilities, and also because of their sensitivity to toxic compounds including metal and organic compounds (Wogram and Liess, 2001). For example, Daphnia magna are known to be sensitive to organophosphates (OPs) (Guilhermino et al., 1996; Barata et al., 2001). They are also useful for comparing the effects of toxicants on various endpoints at different levels of biological organisation. This is particularly beneficial because there have been few reports that have compared the effects that occur at the suborganismal level with those at higher levels for invertebrates. The few exceptions include studies that link cellular energy markers (De Coen and Janssen, 2003; Durou et al., 2007), metallothionein and DNA damage (De Coen and Janssen, 2003; Neuparth et al., 2005; Costa et al., 2005), and inhibition of cholinesterases (ChE) (Duquesne, 2006) with effects at the population level. In the latter study, pulse exposure to a moderate level of OP induced a decrease in ChE activity in D. magna. Although the decrease in ChE activity was transient, effects occurred at the population level (for example, in terms of reproductive performance and population growth rate) above a threshold concentration.

In contrast to the above, comparisons of the effects that occur at the suborganismal and organismal levels have been reported for a large number of invertebrate species (Livingstone, 2001; Hyne and Maher, 2003; Galloway et al., 2004); in these reports the endpoint was usually the death of the organism. However, sublethal endpoints, such as changes in reproduction or feeding activity, are much more useful because, potentially, they offer a more sensitive and ecologically relevant alternative to lethality (McWilliam and Baird, 2002). Sublethal disturbances that occur commonly in the environment can lead to long-term changes at the population and community levels. In the case of pesticide stress, the widely used OPs, for example, exert their toxicity at the suborganismal level by inhibiting cholinesterases (ChE) and carboxylesterases. Inhibition of ChE has been observed in many species of aquatic invertebrates (e.g. Day and Scott, 1990; Sturm and Hansen, 1999; Diamantino et al., 2000; Barata et al., 2001, Barata et al., 2004; Guilhermino et al. 1996). The relationship between ChE inhibition and mortality is generally less well distinct in invertebrates than in fish (Fulton and Key, 2001).

A more detailed characterisation of the responses of various parameters to pulse exposure to a sublethal level of OP at the suborganismal and organismal levels would lead to a better understanding of the mechanisms of toxic effects. The capacity to relate such information to different biological levels would improve the ability to predict the effects of OPs on ecosystems, especially if linked to parameters that describe population dynamics. In this study, the biochemical, metabolic, and behavioural responses of D. magna after pulse exposure to a sublethal level of paraoxon-methyl, the active metabolite of parathion-methyl, were assessed. The aims were (i) to record changes in ChE, filtration, and swimming activities, (ii) to compare the sensitivity of these activities and their recovery potential, and (iii) to evaluate the implications of the changes on physiological endpoints, namely, nitrogen metabolism from this study and body size from an earlier study (see Table 1; Duquesne, 2006).

Section snippets

Chemicals

The choice of paraoxon-methyl instead of parathion-methyl was based on various criteria. It avoids to quantify, in the daphnids, the exact doses and rates of metabolization of parathion-methyl and enables to assume a direct correlation between exposure concentrations and effective doses. Furthermore, the metabolite is less lipophilic than the parent compound (log Kow: 0.98–1.33 versus 2.75–2.86), and thus the lower sorption on containers facilitates the comparison of results of different tests.

Cholinesterase activity

Exposure of D. magna to paraoxon-methyl inhibited cholinesterase (ChE) activity in a concentration-dependant manner (Fig. 2). After 24 h of exposure, ChE activity was reduced by 70% or more at concentrations equal to or greater (≥) than 1.0 μg L−1 compared with the control values (data from Duquesne, 2006). New data for the subsequent 24 h recovery phase, which occurred directly after the 24 h pulse exposure, showed that the enzyme activity had recovered significantly at the end of this period.

Responses of various parameters after the exposure and recovery phases

This study investigated the effects of a sublethal pulse of paraoxon-methyl on biochemical, metabolic, and behavioural parameters in D. magna. Although these parameters responded to various pathways of different complexities, they all showed significant and simultaneous responses. These effects were all transient and, after a subsequent 24 h period in clean medium, the values had recovered either partly or completely to control levels.

ChE activity varied in a concentration-dependant manner and

Conclusion

The threshold levels for the effects of exposure to pesticides vary depending on the endpoints considered. Despite a high potential for the affected parameters to recover, the effects of pesticides can propagate through biological systems and possibly induce long-term effects at higher levels of biological organisation. It is important to understand such mechanisms to clarify the significance of effects exerted on various levels. Such knowledge is highly relevant for the assessment and

Acknowledgments

The authors acknowledge the scientific support from M. Liess, S. Reynaldi and K. Jung, as well as the technical support from I. Raenker, S. Trogisch and H.A. Dau and the financial support of the UFZ.

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