Studies On The Anaerobic Dechlorination Of Trichloroethene And The Simultaneous Reduction Of Perchlorate And Trichloroethene

Trichloroethene(TCE)is widely used as solvents and feedstock for industrial synthesis reactions,and has contaminated soils and groundwater due to improper disposal.Anaerobic microbes degrade TCE effectively,while Dehalococcoides is the only known microorganism able to reduce TCE to ethene completely.The presence and abundance of Dehalococcoides are essential for TCE reductive dechlorination.Therefore,it is important to enrich efficient dechlorinating consortia containing Dehalococcoides and investigate the reductive mechanism.Based on previous studies,we investigated the effects of different electron donors and chlorinated compounds on TCE dechlorination by an enriched TCE-reducing YH consortium;then,we developed a TCE dechlorinating culture from a perchlorate reducing bacterial(PRB)culture by feeding the PRB culture with TCE for 48 days,further studied its dechlorination pattern and oxygen effect;finally,we explored the interaction of TCE and perchlorate to each other by examining the microbial structure shift.The main results are as following:1)The interaction of dechlorinators and methanogens/acetogens under different electron donors was illustrated.Distinct electron donors including lactate,acetate,methanol,and hydrogen gas(H2)were added to YH consortium to study the effects of methanogens and acetogens on the reductive dechlorination of TCE,and the microbial community structure.The cultutre fed by lactate had a faster dechlorinating rate.Phylotypes of Dehalococcoides were relatively abundant(>9%)for the cultures fed with lactate and methanol,but accounted for only-1-2%for the cultures fed by acetate and hydrogen.A low ratio of gene abundance(Dhc/mcrA)was associated with slow and incomplete reductive dechlorination in cultures with methanol and H2.When lactate and methanol were applied as electron donors,a Dhc/mcrA ratio>6.4 was essential to achieve fast and complete dechlorination of TCE to ethene.When methanogenesis was completely suppressed using 2-bromoethanesulfonate(BES),TCE reduction was inhibited as well,indicating that a certain abundance of methanogens was important to achieve complete dechlorination of TCE to ethene,likely Methanobacterium,for providing Dehalococcoides with cofactors other than vitamin B12.Similarly,the presence of homoacetogens was also important to maintain a high dechlorinating rate,because they provided acetate as Dehalococcoides,%obligatory carbon source and possibly co-factors.2)The effects of chlororganic compounds on TCE dechlorination were tested.A short-term study was performed to evaluate the effects of 1,1,1-trichloroethane(TCA)and triclocarban(TCC)on TCE removal by YH consortium.Both TCA and TCC inhibited TCE reduction,while the TCC had a more pronounced effect compared to TCA.Below 15 ?M,TCA had a little effect on the transformation of TCE to cis-dichloroethene(DCE);however,the reduction of cis-DCE and vinyl chloride(VC)were more sensitive to TCA,and ethene production was completely inhibited when the concentration of TCA was above 15?M.In cultures amended with TCC,the reduction of TCE was severely affected,even at concentrations as low as 0.3 dM;all the cultures stalled at VC,and no ethene was detected.The YH culture,which had never been exposed to TCA before,reductively dechlorinated TCA to 1,1-dichloroethane(DCA).3)The TCE dechlorination potential of a perchlorate reducing consortium was proved.A TCE-dechlorinating consortiurn was enriched from a PRB culture with a very low initial abundance of Dehalococcoides by feeding lactate and TCE,was named as PRTB culture.Batch tests showed that the PRTB consortium reduced up to 0.45 mmol/L of TCE to non-toxic ethene in eight days.It simultaneously reduced up to 0.15 mmol/L of ClO4-to Cl-.The TCE reduction by PRTB was sensitive to oxygen,0.2 mg/L of oxygen lagged the TCE reduction to ethene from 5 to 15 days,while>0.5 mg/L of oxygen significantly inhibited the TCE reduction with a mass accumulation of VC.After the consortium was enriched with 0.084 mmol TCE,the Dhc gene(reflects abundance of Dehalococcoides)copy increased from 1.14x 104 to 3.25x 108 copies/mL.The original PRB culture was dominated by Proteobacteria(78.4%),but the PRTB culture was dominated by Chloroflexi(57.7%).The relative abundance of Dehalococcoides increased from 0.02%to 57.4%.Other presumptive dechlorinatorsDesulfuromonas and Geobacter were enriched as well.Possible acetogens Spirochaetes,and putative fermenters Bacteroidetes,Desulfovibrio were also present in the culture,providing Dehalococcoides with acetate as the carbon source,hydrogen gas as the electron donor,and vitamin 312 as one of the growth factors.4)The interaction of perchlorate and trichloroethene bioreduction in a PRTB cultures was clarified.The interaction of perchlorate and TCE was evaluated.TCE at concentrations up to 0.3 mM did not significantly affect perchlorate reduction;however,perchlorate concentrations higher than 0.1 mM made the reduction of TCE significantly slower.Perchlorate primarily inhibited the reduction of VC to ethene.Mechanistic analysis showed that the inhibition was mainly because perchlorate reduction is thermodynamically more favorable than reduction of TCE and its daughter products,but not because of toxicity due to accumulation of dissolved oxygen produced during perchlorate reduction.As the initial perchlorate concentration increased from 0 to 600 mg/L,the relative abundance of Rhodocyclaceae(a putatively perchlorate-reducing genus)increased from 6.3 to 80.6%,while the relative abundance of Dehalococcoides,the only known genus that is able to reduce TCE all the way to ethene,significantly decreased.Similarly,the relative abundance of Proteobacteria(a phylum to which most known perchlorate-reducing bacteria belong)increased from 22%to almost 80%.