Molecular Analysis Reveals Mechanisms For Heavy Metal Resistance And Polycyclic Aromatic Hydrocarbon Degradation Of Two Native Strains

Combined pollutions are commonly consisted of heavy metals and organic compounds which are not easily remediated by current methods.Bacterial strain LZ-E,isolated from a wastewater discharge site of the petrochemical corporation in Lanzhou,is capable of reducing hexavalent chromium(Cr(?))to trivalent chromate(Cr(?))when using naphthalene as the sole carbon source.16S rRNA sequencing data and Vitek analysis have shown that strain LZ-E is closely related to Pseudomonas sp..It can tolerate Cr(VI)up to 52 mg L-1 and reduce 10mg L-1 Cr(?)to Cr(?)in 132h while degrading 800 mg L-1 naphthalene thoroughly.The Cr(VI)reducing efficiency of strain LZ-E using naphthalene as the sole carbon source was 1.7-fold higher than using glucose.Strain LZ-E bears a native plasmid named pHWL10,which contains three genes encoding Rolling circle replication protein,Type? secretory protein and Zonular occudens toxin protein.Removal of pHWL10 has no effect upon Cr(VI)reduction and PAHs degradation of strain LZ-E.Further investigations reveal that strain LZ-E reduces Cr(?)by Cytochrome c reductases and degrades naphthalene through nah pathway.Strain LZ-E simultaneously remediated 8 batches of 800 mg L-l naphthalene and 10 mg L-1 Cr(?)in a bioreactor system using ceramics as carrier in 16 days.The removal rates of naphthalene and Cr(?)were around 99%and 92%respectively in all batches.In summary,strain LZ-E is the first known single strain which can degrade naphthalene and reduce Cr(?)and a good candidate for combined pollutions remediation.Strain LZ-C,isolated from a petrochemical wastewater discharge site,was found to be resistant to heavy metals and to degrade various aromatic compounds,including naphenol,naphthalene,2-methylnaphthalene and toluene.Data obtained from 16S rRNA gene sequencing showed that this strain was closely related to Delftia lacustris.The 5,889,360 bp genome of strain LZ-C was assembled into 239 contigs and 197 scaffolds containing 5,855 predicted open reading frames(ORFs).Among these predicted ORFs,464 were different from the type strain of Delftia.The minimal inhibitory concentrations were 4 mM,30 ?M,2 mM and 1 mM for Cr(?),Hg(?),Cd(?)and Pb(?),respectively.Both genome sequencing and quantitative real-time PCR data revealed that genes related to Chr,Czc and Mer family genes play important roles in heavy metal resistance in strain LZ-C.In addition,the Na+/H+ antiporter NhaA is important for adaptation to high salinity resistance(2.5 M NaCl).The complete pathways of benzene and benzoate degradation were identified through KEGG analysis.Interestingly,strain LZ-C also degrades naphthalene but lacks the key naphthalene degradation gene NahA.Thus,we propose that strain LZ-C exhibits a novel protein with a function similar to NahA.This study is the first to reveal the mechanisms of heavy metal resistance and salinity tolerance in D.lacustris and to identify a potential 2-methylnaphthalene degradation protein in this strain.Taken together,strain LZ-C might be a good candidate for the bioremediation of heavy metals and polycyclic aromatic hydrocarbons.