| The spilled petroleum pollution caused by seaborne oil transportation is one of the major marine environmental problems in the world.Petroleum pollutants contain a large number of low molecular weight alkanes and aromatics,which can induce toxic effects once enter marine organisms.And polycyclic aromatic hydrocarbons(PAHs)are one of the organic compounds with the most significant environmental toxic effects.Bivalves have strong enrichment and tolerance capacity for PAHs in water.They are widely distributed and easy to obtain.Therefore,bivalves are often serving as a kind of model organisms for monitoring and evaluation of offshore marine pollution.The blood clam(Tegillarca granosa),a kind of bivalves with high economic value,is widely distributed in mudflat along the coast of Zhejiang province.The blood clam has the characteristics of benthic life,which makes it more likely to be exposed to petroleum pollutants.At present,there are few studies on the toxic effects of PAHs exposure on blood clam.Herein,we chose a typical PAH-benzo[a] pyrene(BaP)as the contaminant,and investigate the toxic effects of acute BaP exposure on the blood clam from multiple aspects,including histological changes,oxidative stress,neurotoxicity and DNA methylation.The results showed that the gills of blood clam were severely distorted accompanied by swelling,hyperemia,and partial gill filament breakage after a total of 96 h exposure to 10 and 100 μg/L BaP.In the digestive gland of blood clam,prominent haemocyte infiltration and necrosis were observed in some areas,which indicated the presence of inflammation,and the sloughing of digestive cells from the inside wall of digestive tubule led to its atrophy.Increased oxidative stress was indicated by elevated malondialdehyde(MDA)and 8-hydroxy-2’-deoxyguanosine(8-OHd G)content,which in turn leads to damage at the cellular level such as lipid peroxidation and oxidative DNA damage.The activities of antioxidant enzymes including superoxide dismutase(SOD),catalase(CAT),peroxidase(POD),and glutathione-S-transferase(GST)increased significantly after 24 h post exposure.This phenomenon further indicated that BaP exposure induced oxidative stress in the blood clam,and the antioxidant defense system actively participated in the regulation of BaP induced oxidative stress.The activities of two key enzymes of neurotransmitters,acetylcholinesterase(ACh E)and acetylcholine transferase(Ch AT),were significantly reduced,indicating that the stress caused by BaP may induce neurotoxicity in the blood clam.In addition,we analyzed the changes of DNA methylation level as well as the gene expression of antioxidant enzymes in the blood clam under acute BaP exposure,and found that the DNA methylation levels were significantly decreased compared to pre-exposure,while the m RNA expression of SOD,CAT and GST were significantly increased.Correlation analysis showed a clear negative correlation between the DNA methylation and the antioxidant enzymes’ gene expression,implying that the blood clam may active the antioxidant systems to fight against BaP toxicity by reducing the DNA methylation level.In conclusion,the acute BaP exposure exerts a significant toxic effect on the blood clam,which was mainly characterized by histological damage,oxidative stress and neurotoxicity,and the changes in DNA methylation level in the blood clam may be involved in the regulatory process of BaP toxic effects.This study is expected to provide some new ideas for in-depth exploration of the intrinsic regulatory mechanisms of bivalves in response to petroleum pollutant stress,and it will be beneficial for resource conservation of the blood clam under the threat of petroleum pollution. |
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