HE Lijun，PENG Shitao，XIONG Hongxia，LIU Xianbin*

1.Tianjin University of Science &Technology,Tianjin 300457；

2.Laboratory of Environmental Protection in Water Transport Engineering,Tianjin Research Institute for Water Transport Engineering,Ministry of Transport,Tianjin 300456

Abstract:With the development of economic activities,marine oil pollution is becoming an increasing serious problem.Instead of dissolved in seawater,petroleum hydrocarbons are more easily enriched in sediments,which poses a serious threat to benthos.In this paper,the research progresses on toxicity of marine petroleum hydrocarbon for polychaetes were reviewed in recent years,including the toxic mechanism of petroleum hydrocarbons,the feeding habits,the effects of petroleum hydrocarbons on the antioxidant defense system,the nervous system damages,the molecular damages of polychaetes under the stress of petroleum hydrocarbon,the accumulation and metabolism of petroleum hydrocarbons in polychaetes.The future research direction were also prospected.

Keywords:petroleum hydrocarbon,toxicity,polychaetes,antioxidant defense system,molecular damages

1 Introduction

The rapid development of modern industry and coastal economy increases the demands for oil.According to statistics,the global annual oil production has exceeded 3.6×109t.Meanwhile,there were about 8×106-10×106t of petroleum pollutants entered the ecological environment caused by normal losses and accidental leakages in the crude oil exploitation stage[1].The oil hydrocarbon pollution risk is rising due to oil leakage,industrial wastewater discharge and urban runoff[2].At present,there are more than 400 oil fields and oil-gas fields in China,covering 25 different provinces and cities.The dependences of economic development on oil makes the problems of oil pollution increasingly serious.In the study of some old oil fields areas,it is found that the crude oil content in sediments exceeds 10 g/kg,and some even reaches 100 g/kg,far exceeding the critical load value of sediment[3].With the development of offshore oil industry,a large number of petroleum hydrocarbons are discharged into the sea due to oil spills caused by oil exploitation,transportation and loading and unloading in offshore areas.These petroleum hydrocarbon pollution had seriously threatened the marine environment,and had a serious impact on the marine environment,marine biosafety,marine biodiversity and marine ecosystem[4-6].Many research showed that there were petroleum hydrocarbons pollution in Bohai Bay,Liaodong Bay,Zhoushan and other sea areas[7-9].Compared with land pollution,marine pollution is more complex,more difficult to control,and meet the technical requirements.The increasingly serious pollution of marine petroleum hydrocarbons not only brings damage to the marine ecological environment and adverse to economic development,but also threatens human health to a certain extent.Therefore,more and more scholars focus on the toxic effects of petroleum hydrocarbon pollution on marine organism[10-12].

When petroleum hydrocarbons enter the ocean,a thick oil film will be formed on the surface of the sea,blocking the normal circulation of O2,CO2and other gases in the ocean.CO2cannot be normally absorbed and transformed into carbonate ions,hindering the adjustment of the pH value of the ocean,which is not conducive to the balanced circulation of water gas exchange in the ocean[13].Petroleum is a complex mixture of aromatic hydrocarbons,alkanes,cycloalkanes and a small amount of non-hydrocarbon compounds.Among them,polycyclic aromatic hydrocarbons and cycloalkanes have strong toxic effects on organisms,and the toxicity is obviously related to the number of benzene rings and the degree of alkylation,with carcinogenic,teratogenic and mutagenic effects.These organic compounds can be absorbed by marine organisms,and then enriched and accumulated inside,resulting in negative effects on normal behavior and activities of marine organisms.They further lead to the abnormal reproduction of marine biological population,and finally enter the human body through the amplification of the food chain,endangering human health[14-17].

Polychaetes are a large group of Annelida,which has abundant species.They located at the bottom of the aquatic and terrestrial ecological food chain,and they are important prey for most marine organisms such as fish,crustaceans and other organisms,and occupy an important position in the marine ecosystem.Because of its poor mobility,fixed habitat and long life cycle,it is vulnerable to external pollutants[18-20].Petroleum hydrocarbon pollution will affect the growth and development of polychaetes,destroy the cells,the transmission of the divine system,and lead to DNA damage,resulting in genotoxicity.At the same time,due to the living characteristics of polychaetes,petroleum hydrocarbons in sediments are easily enriched in polychaetes.These polychaetes,which are rich in petroleum hydrocarbons in marine sediments,can be transmitted to human body through the amplification of food chain,and ultimately endanger human health.ln this paper,the toxic effects of marine petroleum hydrocarbon pollution on polychaetes were reviewed,to provide reference for the research of marine ecosystem restoration,marine environmental damage assessment and marine ecological environment health assessment[21-23].

2 Toxic effects of petroleum hydrocarbons on polychaetes

Petroleum hydrocarbons are a kind of hydrophobic mixture.When they enter the seawater,they are usually attached on the suspended solids,migrated with the action of sea breeze,waves and tides,and finally settled on the seabed and intertidal sediments[24].Polychaetes usually live in the intertidal zone,which is a burrow lifestyle.They are easy to be exposed to various marine pollution[20].At present,the main researches on the effects of petroleum hydrocarbons on polychaetes are the enrichments and metabolism of petroleum hydrocarbons by polychaetes,the effects of petroleum hydrocarbon on the antioxidant defense system of polychaetes,the damages of petroleum hydrocarbon in polychaetes nervous system and the DNA damages of petroleum hydrocarbon in polychaetes.

2.1 The toxic mechanism of petroleum hydrocarbons

Total Petroleum Hydrocarbons(TPHs)are listed as one of the main marine pollutants,which contain a variety of carcinogens that are difficult to be degraded by microorganisms.Their lipophilicity makes them easy to accumulate in the fat of marine organisms,and directly endanger human health through the food chain.Petroleum hydrocarbon is composed of various hydrocarbons,such as alkanes,olefins,aromatics,etc,and a small amount of other organic substances,such as sulfides,nitrides,naphthenic acids[25].Aromatic hydrocarbons and polycyclic aromatic hydrocarbons(PAHs),such as benzo(a)pyrene and dibenzo anthracene have high toxicity.High molecular weight compounds are easy to be settled and have poor water solubility and strong lipophilic abilities.When petroleum hydrocarbons are absorbed by polychaetes,PAHs are metabolized by cytochrome P450 mixed functional oxidase system to form intermediate metabolites,such as reactive oxygen species,which covalently combine with DNA to form adducts,destroy the balance of Pro oxidation and anti-oxidation,and produce oxidative stress.They causes lipid peroxidation,protein and DNA damage finally[26,27].The behavior of marine petroleum hydrocarbon pollution caused by land-based discharge or oil spill accidents can be divided into three types.The first is that weathering process can change the physical and chemical properties of oil products,mainly including evaporation,emulsification,dispersion,dissolution and biodegradation.The second is that the expansion process of oil film due to its own characteristics.The third is that the migration process of spilled oil under the action of wind and waves[28].The effects of petroleum hydrocarbons on polychaetes can be divided into short-term and long-term effects.On the one hand,petroleum hydrocarbon pollution directly cause acute effects on marine organisms and directly cause the death of marine organisms.On the other hand,petroleum hydrocarbon pollution will slowly affect marine organisms.Petroleum has good lipophilic fraction and accumulates in polychaetes.When the organism accumulates to a certain volume and reaches a certain concentration,it do harm to the metabolic system of organisms,and is prone to mutation and pathogenicity,causing deformity to the incomplete individuals[29].Wang et al.found that the LC50of petroleum hydrocarbon toPerinereis aibuhitensiswas 28-119.9 μg/L[30].Zhang et al.found that petroleum hydrocarbons(0.15 and 0.2 mL/L)significantly inhibited the activity of acetylcholinesterase(AChE)ofPerinereis aibuhitensis[31].At the same time,the different cell types in the same species and different species in Polychaeta also had different responses to genotoxic damage.Lewis[32]compared the genotoxic damage ofNerels diversicolor,Neris virensandArenicola marina,and found that under the induction of MMS and B(a)P,the DNA damages ofN.diversicolorandN.virensoilcells are more sensitive than that of deformable cells,and the genotoxic damages of oil cells inA.marinaare more sensitive than that inN.virens[32].In addition to cell and gene level hazards,PAHs were enriched or stored by individual marine organisms,and finally entered the human body through the food chain.

When polychaetes are stressed by petroleum hydrocarbons,on the one hand,petroleum hydrocarbons will produce a large amount of reactive oxygen species(ROS)in the process of metabolism.These ROS induce the response of antioxidant system in polychaetes,maintain the dynamic balance of ROS in vivo,and reduce the degree of cell damage.On the other hand,petroleum hydrocarbons participate in the redox reaction of the body through the metabolism of polychaetes own CYP enzyme cluster,and are transformed into more hydrophilic metabolic intermediates,which covalently combine with DNA molecules to form more stable compounds,causing DNA damage[33-34].However,when the antioxidant enzymes produced by the organism are difficult to remove excessive ROS,the normal physiological function of the organism will be affected,and the oxidative stress caused by excessive ROS will lead to biological DNA damage,especially the single strand DNA fragmentation[35-37].When polychaetes are exposed to the sediment containing petroleum hydrocarbons,PAHs and other macromolecular organic compounds in petroleum hydrocarbons enter the body through feeding and other processes,which makes the phase l metabolic enzyme(cytochrome P450 enzyme)and phase ll metabolic detoxification metabolic enzyme(GST)react with oxidation and reduction to produce ROS,which produces oxidative stress on the body.

2.2 The feeding habits of polychaetes

Polychaetes are generally divided into six feeding types:omnivores,vegetarians,filter feeders,carnivores,surficial sediment feeders and cave dwellers.For example,some Perinereis aibuhitensis are both meat eaters and vegetarians,and the Spionidaes and Oweniidaes are also fed in two different ways.When the sediments are rich in organic matter,omnivores,vegetarians and carnivores will become sediment eaters.At the same time,polychaetes have selectivity in the ingestion of particulate matter,and can respond to the concentration and composition,so as to increase the nutritional demand for important organic compounds.Polychaetes have become important parts of marine ecological food chain due to their varied feeding patterns[38,39].

After the marine sediments were stored up the organic pollutants,they entered into the body through polychaetes feeding,and then accumulated in the organism,and finally entered into the marine ecological food web.Pruell exposed the sediments containing chlorinated organic compounds PCDDs,TCDT and PCBs,etc.for 70 days,N.virens were fed to theAmerican lobsterandHomarus americanusfor 112 days,PCBs were detected in the muscles and hepatopancreas of them[40].At the same time,when the sediments were polluted,the feeding abilities of polychaetes were also enhanced.Tankoua collected wild animals at relatively clean sites of Bourgaiu Bay,France and moderately polluted sites Loire estuary,France,and found that the feeding and digging capacity of polluted sites showed an upward trends[41].

2.3 Effects of petroleum hydrocarbons on antioxidant defense system of polychaetas

When organisms are stressed by pollutants,endogenous reactive oxygen species(ROS),such as superoxide anion(O2-),hydrogen peroxide(H2O2)and hydroxyl radical(OH-),are constantly produced in the body.Excessive production of these endogenous reactive oxygen species disturb the balance of pro-oxidation and anti-oxidation systems in the organism,cause oxidative damage to the organism,and make the organism in a state of oxidative stress[42,43].Antioxidant defense system includes antioxidant enzyme system such as catalase(CAT),superoxide dismutase(SOD)and glutathione peroxidase(GPx),and non-enzyme system such as malondialdehyde(MDA),reduced glutathione(GSH)and glutathione-s transferase(GST).Antioxidant defense system can reflect the effect of polluted marine environment on oxidative stress.In recent years,the antioxidant defense enzymes of polychaetes have been used as biomarkers to reflect organic pollutants.

SOD is one of the main antioxidant enzymes that scavenge reactive oxygen species and protect cells from oxidative damage.It plays an important role in the defense of aging and biomolecular damage[44].Moreira suggested that exposure to PAHs contaminated sediments significantly induced SOD activity[45].Zhang et al.putP.aibuhitensisin the water containing B(a)P,the SOD activity was induced first and then inhibited,and Sun also got the same results when he exposed gasoline to water[44,46].Similar to SOD,the response of enzymes in antioxidant system to single pollution is basically the same.Low concentration or short time exposure induced its activity,while long time or high concentration exposure inhibited its activity.According to Catalano,0.5 mg/L B(a)P significantly inhibited the activity of CAT,glutathione reductase(GR),GST,Ca2+,ATPase,and significantly increased the activity of GPx[47].Jgensen et al.discovered that 10 μg/g pyrene could increase the activity of pyrene hydroxylase in intestinal tissue about 3 times inN.virensafter 3 and 7 days of pyrene treatments[48].The results of Wang study showed that under B(a)P pollution stress,CAT activity inP.aibuhitensisalso gradually decreased with the increase of exposure concentration,and there was an obvious concentration effect relationships in the results of Wang[49].

GSH is an important detoxification enzyme,which usually exists with the form of isoenzyme in animals and plants.lt is an important detoxification enzyme and the most important antioxidant in the defense system.lt plays an important role in the process of scavenging ROS.The content of GSH in the cells can be used to characterize the environmental conditions of organisms.When the GSH content increased significantly,the concentration of pollutants in the environment increased,and the intracellular detoxification system was induced[50].At the same time,it is reduced by GSH-Px,which can eliminate harmful peroxidation metabolites in cells,block the chain reaction of lipid peroxidation,avoid the damage of lipid peroxide to cells,thus protecting the structure and function of cell membrane[51].P.aibuhitensiswere exposed in the seawater of crude oil,their MDA content increased with the increase of crude oil concentration in the seawater.CAT activity was significantly induced with low concentration crude oil exposure,but with the increase of exposure concentration,this induction effect was weakened.GST activity increased with the increase of crude oil concentration in seawater,and the induction effect will be enhanced with the increase of crude oil concentration in the study of Li[52].

When polychaetes are stressed by petroleum hydrocarbons,their antioxidant enzymes and non-enzyme systems reflect the toxicity of petroleum hydrocarbons to a certain extent.Antioxidant defense system widely exists in different tissue types of animal species.Under normal physiological conditions,the reactive oxygen species produced by animal metabolism can be effectively removed by antioxidant system[53].However,when polychaetes are exposed to pollutants,such as PAHs,B(a)P,phenanthrene,which can produce oxidation-reduction for a long time,the reactive oxygen species produced by polychaetes exceed the body's own scavenging capacity,they lead to the imbalance of oxygen free radical metabolism and a series of toxic effects,such as lipid peroxidation,DNA damage,apoptosis and even death[54].

2.4 DNA damage of polychaetes induced by petroleum hydrocarbons

At the beginning of the 20thcentury,Depledge and Fossi[55]proposed that by measuring the body fluid,tissue or the whole organism of biomarkers,we can find the effects of pollutants on them,including the physiology,behavior,growth and development,energy and so on.The changes of organisms can indicate the environmental and biological effects of organisms exposure.Toxic substances can inhibit the electron transfer process in the metabolic process of marine organisms.The excessive reduction of electron transfer chain directly leads to a large number of reactive oxygen species,free radicals and other intermediates.Once it exceeds the antioxidant capacity of organisms,it will cause cell damage and biological death[56].In the early stage of active oxygen generation,a certain amount of free radicals promote the operation of antioxidant defense system in organism and form the protection mechanism.The operation of antioxidant defense system will cause the increase of antioxidant enzyme activity and antioxidant substance content,forming the scavenging of free radicals,the degree of lipid peroxidation and the change of enzyme activity of antioxidant defense system are commonly used biomarkers[57-59].Some scholars have also discussed the changes of biomarkers,tissues or cells of marine organisms caused by petroleum hydrocarbons at the sub individual and individual levels.

With the rise of molecular biology,a large number of molecular biological methods have been applied to toxicology experiments.The research of ecotoxicology has gradually developed from the initial individual level to the cellular and molecular level.The purpose is to reflect the toxicity of pollutants by observing the changes of proteins and other macromolecular substances in the body under the influence of pollutants,and then play a warning role in the change of the environment[60,61].The bioavailability of carcinogenic,teratogenic and mutagenic compounds in the environment were easily detected.In terms of function,molecular biomarkers were divided into yield markers,effect markers and susceptibility markers[62].Some scholars have found that the intermediate metabolites of organic pollutants generated by biotransformation were mostly electrophilic compounds with extremely high reactive oxygen species,which interacted with DNA macromolecules to form various types of DNA damage.DNA damage has been proved to be a biomarker for monitoring environmental pollution,the research on DNA damage caused by petroleum hydrocarbon pollution has become a hot spot[63-66].

DNA maintains the stability of genetic information in the process of biological passage,when organisms are stimulated by external pollutants,DNA molecular structure will be damaged,which is not conducive to maintaining the integrity of biological information.The degree of DNA damage can reflect the pollution in the environment[67].Studies have shown that the dispersion of crude oil caused DNA damage in marine invertebrates.PAHs,anthracene,phenanthrene and other persistent organic pollutants covalently combine with DNA bases in organisms.They resulted epigenetic changes such as DNA methylation and histone modification,affected the normal endocrine of the body and becoming mutagenic or carcinogenic factors.PAHs have been shown to alter DNA structural integrity directly or through their metabolites[68].Palmqvist et al.[69]found that 50 μg/L fluoranthene(Flu)caused significant DNA damage in capitella sp.l,and the degree of DNA damage was consistent with the exposure time.Wang[70]studied the DNA damage induced by B(a)P using Tunel assay and DNA laddering methods,which indicated small amount of DNA damages found in low concentration(2.5 μg/L),and obvious DNA damage was found in high concentration B(a)P(50 μg/L),and DNA damage was mainly induced in the body cavity.Su exposedPerinereis aibuhitensisto 2,2,4,4-tetrabromodiphenyl ether,and found that the degree of DNA damage ofPerinereis aibuhitensishad obvious Time-Effect relationships.With the increase of exposure time,a large number of ROS produced by cells damaged DNA and produced a large number of DNA fragments.These fragments were induced to migrate.With the increase of stress time,the length of comet tail increased significantly,Tail DNA% and OTM also increased,but TL,Tail DNA% and OTM did not decrease during the recovery period.This phenomenon indicates that the DNA damage made its repair function unable to work and resulted in irreversible toxic effects[71].DNA damage can be repaired by the body after the pollution is removed,but the accumulation of excessive DNA damage will further lead to irreversible damage such as cell apoptosis.

In the analysis of genotoxic biomarkers,polychaetes treated with B(a)P had obvious effects,and the frequency of micronucleus test(MN)increased significantly,which showed that DNA damage was caused by B(a)P exposure.Numerous researches have investigated that these polychaetes were highly sensitive to the genotoxic damage caused by polycyclic aromatic hydrocarbons,benzene and dioxins[72,73].These studies showed that polychaetas was good biological indicators of organic pollution.

At present,many researches have studied the DNA damage of polychaetes such asHydroides elegans,Capitella capitateandPerinereis aibuhitensiscaused by heavy metals,but there were few studies on the molecular damage of organic compounds such as petroleum hydrocarbons.Therefore,the comet experiments,SNP and other modern molecular means can be introduced into the study of marine benthic polychaetes,and the toxic mechanism of different growth stages or different parts needs to be further studied.lt will broaden the application of pollution toxicology and lay a foundation for the future use of polychaetes as a biological indicator of pollution monitoring.

2.5 Damage of petroleum hydrocarbon to nervous system of polychaetes

The central nervous system includes a large number of neurons with different shapes and types,and the neurotransmitters they produce and release are also different.Immunohistochemical technique provides an effective means for the study of neurotransmitters,including acetylcholine(ACh),serotonin(5-HT),glycine(Gly),etc.These neurotransmitters regulate the physiological function of the body by transmitting various kinds of information.Acetylcholinesterase(AChE)is a key enzyme in biological nerve conduction,which plays an important role in the information transmission of nervous system,regulating the nerve conduction,muscle movement and digestive function of animals[74,75].Pérez et al.[76]found that the AChE activity in the polluted area was the most sensitive to the pollutants,and the AChE activity in the polluted area was nearly four times lower than that in the unpolluted area.Wang et al.[72]also found that petroleum hydrocarbon had obvious inhibitory effect on polychaetes.When exposed for 10 days,the concentration of petroleum hydrocarbon reached 150 μL/L,and the inhibition rate was more than 90%.In addition,Laasberg et al.[77]also found that ACh was related to the differentiation of germ layer inArenicola brasiliensis,and there were 5-HT and 5-HTR1A positive neurons and nerve fibers in all parts of the nervous system.Neurotransmitters play an important role in regulating maturation,fertilization,cleavage,embryo differentiation and morphogenesis.Therefore,when exposed to petroleum hydrocarbon pollution,it may affect its normal development and even lead to its death.

At present,the researches on the nerve injury of polychaetes by petroleum hydrocarbon is very rare.Only a few studies showed that petroleum hydrocarbon had a certain inhibitory effects on AchE,the others studies only discussed the existence of neurotransmitters in polychaetes.Whether they can be affected by pollutants is unknown,and the specific transmission mechanism is unknown.

3 Enrichment and metabolism of petroleum hydrocarbons by polychaetes

After long-term exposure to polluted environment,the tolerance of polychaetes continues to evolve,and the intake of pollutants from the environment is also gradually increasing.The pollutants absorbed are transmitted in the whole food chain,resulting in biomagnification,which is more toxic and harmful to higher nutritional level consumers[78,79].Therefore,the study of metabolic pathways and related mechanisms in polychaetes has practical significance for maintaining ecosystem balance.

Zhang[20]explored the relationship s between the bioaccumulation of petroleum hydrocarbons inPerinereis aibuhitensisand their toxic effects under single petroleum hydrocarbon stress.lt was found that the relative content of petroleum hydrocarbons in sand silkworm was greatly affected by the exposure concentration,and at low concentrations such as 6.5 and 32.5 μg/L,the relative content increased with the increase of time,while the relative content increased with the increase of concentration such as 65 and 130 μg/L increased first and then decreased.Under the stress of high concentration of petroleum hydrocarbon for a long time,the release rates of petroleum hydrocarbon were higher than the absorption rates.This may be due to the serious damage of the tissues and organs of the sand silkworm,and the accumulation of petroleum hydrocarbons in the body at the early stage of exposure was released with a large amount of tissue ulceration.

There are significant differences in the metabolic capacity of polychaetes to petroleum hydrocarbons,and different polychaetes have different metabolic capacity to petroleum hydrocarbons.There are a lot of biosurfactants in the intestines of sediments feeding polychaetes,which can dissolve the hydrocarbons bound by sediments.Rust et al.compared six species of polychaetes exposed to B(a)P:Spio setosa,Nereis succinea,N.virens,Nephys incise,Cirriformia grandisandClymenella torquata.The results showed that they had different conversion rates of B(a)P,which were 92%,85%,72%,27%,14%and 6% respectively[80].Driscoll et al.studied the metabolism of PAHs byLeitoscoloplos fragilis,N.diversicolorandScolecolepides viridis,and found that the species-specific differences in the metabolism of PAHs by polychaetes were not only reflected in the metabolic abilities of PAHs,but also in the induction of cytochromeP450(CYP450)enzyme activity in metabolic activities[81].CYP450 belongs to phase I detoxification system,which transforms fat soluble heterogeneous substances into water-soluble and easily excreted metabolites through oxidation reaction.lt is the main enzyme system that catalyzes the transformation and detoxification of foreign substances in the body.The biotransformation of PAHs by polychaetes is similar to that of vertebrates,and can be divided into two stages.ln phase l,CYP450 is the main enzyme,which catalyzes the entry of a functional group into PAH,thus slightly increasing the water solubility of PAHs.Subsequently,a covalently linked polar group catalyzed by the enzyme in phase ll greatly increased the water solubility of PAHs,thus promoting the elimination of PAHs[82,83].The

full length of CYPs gene was cloned for the first time by Jrgensen.Based on the important role of CYPs in the transformation of exogenous substances and the universality ofN.virensin estuarine organic matter accumulation,this study is of great significance to explore the molecular mechanism of marine CYPs involved in polychaetes biotransformation[83].Zhao also found that the CYP4 enzyme involved in the metabolism of B(a)P,and had a certain deto xification effect on phenanthrene[84].However,with the extension of time,phenanthrene stress may be beyond the tolerance range,cytotoxicity was resulted by these stresses,CYP4 cannot play its normal function.Rewitz et al.showed that the mRNA level of CYP342A1 gene of N.virens was significantly induced by benzo(a)anthracene,petroleum,butyl chlorobenzene and 1-hydroxybenzene,while CYP4BB was only induced by 1-hydroxybenzene;the CYP3AT1 and CYP331A1 gene expression of microcephalus was significantly induced by PAHs[85].Chen et al.used real-time fluorescent quantitative PCR to prove that the change of CYP4 gene expression in vivo had obvious dose-induced effect with the concentration of petroleum hydrocarbon pollution[86].Wong et al.also showed that CYPs gene had a good dose effect on PAHs concentration.The CYP family of polychaetes plays an important role in the oxidative degradation of petroleum hydrocarbons,which can provide early warning for environmental degradation caused by petroleum hydrocarbon pollution[87].But at present,the research on CYP4 mainly focuses on the gene expression,and less on the protein.

In addition to phase I detoxification enzyme,the toxic response of phase II detoxification enzyme GST of polychaetes to PAHs has also been reported.The GST activity and gene expression level of polychaetes are good indicators of PAHs exposure.Catalano et al.[47]studied the changes of GST activity under 0.1 mg/L and 0.5 mg/L B(a)P exposure,and found that different concentrations of B(a)P could increase GST activity.Won et al.[87]found that the expression of GST omega and GST sigma genes were significantly up-regulated after exposure to crude oils.Therefore,the detoxification mechanism of polychaetes is highly sensitive to PAHs.The activities of CYP450,GST and the molecular expression level of genes in polychaetes have been used as important biological indicators for monitoring PAHs in marine environment.

4 Conclusion

Polychaetes are often used as indicators of marine environmental pollution.However,the toxicity of petroleum hydrocarbons to polychaetes is still concentrated on a few polychaetes species,and the pollutants are mostly concentrated in PAHs,B(a)P and a few single pollutants.The organic mixed pollutants need further study.In addition,the research shows that organic pollutants can induce some genes overexpression in polychaetes,but the mechanism of different expression regulation of organic pollutants may be different.The specific aspects of genetic toxicity damage caused by these pollutants need further study.

The oil hydrocarbon stress in sediment can cause oxidative stress to the polychaetes,which will cause lipid peroxidation of their cells,and the excessive production of freeradicals will cause DNA damage and cell withering.In addition to the main organic compounds such as PAHs,petroleum hydrocarbons also include a small number of other organic compounds such as sulfide,nitrogen compounds and naphthenic acids.The toxicity of these organic compounds to polychaetes needs further study,which will be helpful for the better application of polychaetes in marine environment monitoring.The special living habits of hairy animals,such as poor mobility,fixed habitat environment and long life cycle,make them easily access to marine sediments through direct contact and active feeding.Chemicals in sediments,such as some components of total organic carbon,may also cause damage to hairy animals,which need further study.The study of petroleum hydrocarbon pollution on polychaetes can be systematically studied from cell individual molecular development genetic system,which can fully grasp the toxic mechanism of petroleum hydrocarbon pollution to polychaetes,and provide scientific basis for marine pollution restoration,marine environmental damage assessment and marine ecosystem health assessment in the future.

Acknowledgement

This work was supported by the National Natural Science Foundation of China(No.21677065).Thanks for Professor Liu Xianbin from Tianjin University of Science and Technology and Engineer Peng Shitao from Tianjin Research lnstitute for Water Transport Engineering,M.O.T.who have provided help for the study.