| As a consequence of their wide usage, monocyclic aromatic hydrocarbons toluene, ethylbenzene, and xylene have been detected frequently in the aquatic environment, which could lead to harmful effects to the aquatic systems and human. In this paper, acute and subchronical toxicity of toluene, ethylbenzene, and xylene on an aquatic gastropod were investigated, and we attempted to tentatively enact the Water Quality Criteria for toluene, ethylbenzene, and xylene for protecting the aquatic organisms in Chinese aquatic ecosystem.As far as we know, the model testing organisms are primarily confined to fish in vertebrates and arthropods in invertebrates. Molluscs also are of great importance in the kingdom of invertebrates, whereas they are absent in the model testing organisms, especially gastropods. So, they are seldom considered in the protect policy of water standard and criteria. Nowadays, China is embarking on the development of its own national water quality system fitting for Chinese aquatic ecosystem, and one of the biggest difficulties is short of the local model testing organisms. Therefore, developing our country own gastropod testing organisms will provide technical support to establish Water Quality Criteria for our country and will also avoid the neglected problem of gastropod in establishing the Water Quality Criteria in the other countries. A local gastropod, Bellamya aeruginosa, was selected as the test organism in the present toxicity study of toluene, ethylbenzene, and xylene, and it is further to discuss the practicability of B. aeruginosa as a potential model testing organism in the present study.In the acute study of toluene, ethylbenzene, and xylene, it was found that no mortality occurred, but behavioral alteration, i.e. distress syndrome, was observed in the exposed snails, thus, here the acute effect of toluene, ethylbenzene, and xylene on the snails was evaluated through the behavioral toxicity, and coupled with biochemical responses, including acetylcholinesterase (AChE) in the whole body, superoxide dismutase (SOD), catalase (CAT), glutathione S-transferases (GST), reduced glutathione (GSH), and lipid peroxidation (malonyldialdehyde, MDA) in the hepatopancreas and kidney. Distress syndrome had clear dose-and time-dependent effects (p<0.05), and96-h EC50values for the distress syndrome of toluene, ethylbenzene, and xylene were37.2,13.3, and8.9mg/L in the tested snails. Taken ethylbenzene as example, AChE activity of the distressed snail was all inhibited more than45%, while the inhibition of AChE activity in the nondistressed snail was all less than30%, and the oxidative biomarkers (SOD, CAT, GST, GSH, and MDA) showed no clear correlation with the distress syndrome. The inhibition in AChE activity is found to be related with behavioral alterations in many studies; therefore, more than45%inhibition in AChE activity induced by ethylbenzene was thought to be connected with the distress syndrome in the tested snails. The findings showed that toluene, ethylbenzene, and xylene caused the inhibition on AChE activity, and eventually led to the behavioral alteration occurred in the snails.The focus of present work was performed to elucidate the sublethal effects of ethylbenzene using the freshwater snail, B. aeruginosa (Reeve). The experiment was divided into two periods:a21-day exposure period followed by a17-day recovery period. AChE, SOD, CAT, GST, GSH, and MDA were used as the biochemical biomarkers to evaluate oxidative stress in hepatopancreas and kidney of snails. In addition, alkaline comet assay was applied to determine the genotoxicity of ethylbenzene in hepatopancreas of snails. The results showed that behavioral alteration, namely retraction response, was also observed during the exposure period, and the proportion of retracted snails increased under each treatment as the exposure time prolonged but there was no linear relationship between the retracted proportion and the exposure dose. These six biomarkers exhibited various responses to ethylbenzene in the tested snails, and the differences in these biochemical biomarkers between the two different behavioral snails were not showed same trend, indicating that the correlation between these biochemical biomarkers and the behavioral alteration (retraction response) was not clear. Alkaline comet assay showed that ethylbenzene could significantly induce DNA damage in hepatopancreas of snails, and there was a good dose-and time-response in DNA damage, indicating potential genotoxicity of ethylbenzene on snails. Moreover, the levels of DNA damage had no difference between the two different responses of snails. At the first five days of the recovery period, the increased DNA damage may be due to the repair of DNA damage, such as base or nucleotide excision repair. At the end of the recovery period, the repair of DNA damage was not yet completed, showing that DNA repair requires more time.Meanwhile, the capability of the freshwater gastropod B. aeruginosa to take up and depurate ethylbenzene was studied in the subchronical study; the snail was subjected to two treatments, a23-day exposure period followed by a17-day depuration period. The results showed that ethylbenzene uptake in unretracted snails was greater than in retracted snails, the bioconcentration factor (BCF) in the unretracted snails ranged from2.24to80, and BCF in the retracted snails ranged from2.05to70. The depuration rates followed the first order kinetics for both different behavior snails, and the average half-lives of ethylbenzene depuration were8.5d and8.0d, indicating that the depuration abilities in the two different responses of snails had no significant difference from each other. There were also no significant differences among the five treatments in terms of depuration rate for each group. Because of the limited capability of snails to detoxify ethylbenzene, the depuration was mainly through a slow excretion process, and ethylbenzene was still present in the tissue of snail after the17-day depuration. So, the snails need more time to depurate ethylbenzene completedly.In order to derive Water Quality Criteria for toluene, ethylbenzene, and xylene that can protect the freshwater ecosystem and biota system in China, all available toxicity data of toluene, ethylbenzene, and xylene to Chinese representative species in freshwater were collected. The toxicity percentile rank method, used for criteria derivation in America was used to derive aquatic life criteria for the three substances. The toxicity percentile rank method was suitable to derive Water Quality Criteria to protect the aquatic organisms. The results from the toxicity percentile rank method showed that, the criteria maximum concentrations of toluene, ethylbenzene and xylene were4.49,0.89, and1.15mg/L, respectively, and the criteria continuous concentrations were0.81,0.25, and0.41mg/L, respectively. |
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