The Toxicity Of Heavy Metal Ions To Three Ciliates

As the development of modern industry,aquatic environment is facing more and more threaten from effluent containing various heavy metal ions.The timely monitoring of environmental quality and the accurate evaluation of the ecotoxicology of various heavy metals is therefore of crucial importance for and protecting health.Ecotoxicological bioassays using model organisms are attracting increased attention because they can reflect the toxicity of chemicals more comprehensively than direct measurement.Ciliates have been widely used in the assessment of toxicity of heavy metal ions for their high sensitivity to toxicants.In the present study,three widely distributed ciliate species?Paramecium multimicronucleatum,Euplotes muscicola and Euplotes vannus?were used to investigate their utility as model organisms for assessing the ecotoxicity of six heavy metal ions(Cu²⁺,Zn²⁺,Cr3+,Ni²⁺,Cd²⁺ and Hg²⁺)that are commonly found in aquatic environments.The sensitivity and tolerance of the ciliates were investigated by exposing them to heavy metals,both individually and as mixtures,and determining the percent survival and 24 h LC50 of the ciliates.Results will contribute to the assessment of environmental risk and the determination of water quality in terms of heavy metals.In addition,the transcriptome of Paramecium multimicronucleatum were sequenced using Illumina HiSeqTM and Differently expressed unigenes under Cu²⁺ stress were determined to provide basis for further study.The main results were as follows:1.In the individual toxicity tests,Cu²⁺ and Hg²⁺ exerted higher toxicity compared to other heavy metal ions,whereas Zn²⁺ exerted the lowest toxicity to all three ciliates.After 24 h exposure to different concentrations of heavy metal ions,each species experienced a different degree of mortality.The 24 h LC50 of Cu²⁺,Zn²⁺,Cr3+,Ni²⁺,Cd²⁺ and Hg²⁺ was 0.032 mg·L^-1,59.4 mg·L^-1,42.7 mg·L^-1,2.94 mg·L^-1,0.347 mg·L^-1,0.073 mg·L^-1 to Paramecium multimicronucleatum,0.065 mg·L^-1,70.6 mg·L^-1,48.4 mg·L^-1,3.73 mg·L^-1,23.2 mg·L^-1,0.045 mg·L^-1 to Euplotes muscicola,0.233 mg·L^-1,22.8 mg·L^-1,0.88 mg·L^-1,17.4 mg·L^-1,89.2 mg·L^-1,0.019 mg·L^-1 to Euplotes vannus respectively.2.The joint toxicity tests were composed of C²⁺-Zn²⁺,Cd²⁺-Hg²⁺ and Cu²⁺-Zn²⁺-Cr3+-Ni²⁺ mixtures,and the type of interaction between them was varied according to their concentration ratio in mixtures,resulting either in antagonism or synergism.The joint toxicity of Cu²⁺-Zn²⁺ and Cd²⁺-Hg²⁺ mixtures appeared significant difference between the three ciliate species in concentration 1:1 and toxicity unit 1:1.The predominant interaction of Cu²⁺Zn²⁺-Cr3+-Ni²⁺ mixtures was one of antagonism for all three species,which indicated that their effects on ciliates were lower than the simple addition of individual metal ion toxicity.3.The transcriptome of Paramecium multimicronucleatum was sequenced by Illumina HiSeqTM.A total of 6603305 contigs and 80093 unigenes were obtained after de novo assembling with Trinity software and 40538 unigenes were annotated in database.14307 unigenes were categorized into GO terms,including binding,catalytic activity,metabolic process,cellular process etc.17787 unigenes were blasted to 25 COG families,including signal transduction mechanisms,general function prediction,post-translational modification,protein turnover,chaperones etc.6839 unigenes were mapped to 43 pathways in KEGG,including signal transduction,carbohydrate metabolism,translation etc.4.After 48 h exposure to 0.01 mg L^-1 Cu²⁺,a total of 54 unigenes expressed significant difference in the transcriptome of Paramecium multimicronucleatum,with 10 unigenes up-regulated and 44 unigenes down-regulated.By KEGG pathway enrichment analysis,we found that unigenes in four pathways?basal transcription factors,viral carcinogenesis,protein export and nucleotide excision repair?were down-regulated,which meant these process in Paramecium multimicronucleatum cells were inhibited under Cu²⁺ stress.