Potentials And Activities Of Hadal Trench Microorganisms To Degrade Halogenated Organic Compounds

The hadal zone typically refers to oceanic regions with the depth exceeding 6000 meters,which was mainly composed of deep trenches.Due to its unique "V" shape and geographical location,the hadal zone serves as a deposit center for organic matter in the deep ocean.Recent studies have shown that the microbial carbon turnover rates in hadal trench sediments were much higher than those in other deep-sea habitats e.g.,abyssal plain,highlighting the significant role of hadal trench ecosystems in deep-sea carbon cycling.However,current understanding on metabolism and activity of hadal microbes is limited,and the main microbial processes that are driving the organic carbon degradation in the hadal trenches are still not known.Previous studies have shown that the genomes of hadal microbes harbored a large number of genes related with degradation of halogenated organic compounds.As one of the most dominant bacterial groups in Mariana Trench sediments,Chloroflexi have been found to harbor complete pathways for degradation of halogenated organic matter,e.g.,organochlorine pesticides(OCPs)and polychlorinated biphenyls(PCBs).Meanwhile,halogenated organic matter such as OPCs and PCBs have been detected in the sediments,water,and crustacean species of the Mariana Trench,suggesting that the degradation of halogenated organic compounds may be an important metabolic process for hadal trench microorganisms.However,current researches on organohalides in hadal trenches were primarily focused on detecting specific substances(OCPs,PCBs),and the overall contents of organohalides and their proportions in organic carbon pool in the hadal trenches are not known.Furthermore,research on microorganisms capable of degrading halogenated organic matter(dehalogenation microorganisms)has been primarily focused in terrestrial or shallowwater marine environments,and little is known about the dehalogenation microbes in the deep ocean.Current understanding on dehalogenation microbes the hadal trenches is based on bioinformatic analysis,and further researches are needed to investigate whether hadal trench microorganisms can degrade organohalides under extreme environmental conditions of the hadal zone,and what are their activities in degrading different halogenated organic matters.In this thesis,detection methods were firstly developed to determine the content of total organic halogenated compounds and representative anthropogenic organic halogenated compounds,including γ-hexachlorocyclohexane(γ-HCH)and 4-chloro-1,1’-biphenyl(4-PCB),in hadal trench sediments.The total organic halogenated compounds(TOX)and adsorbable organic halogenated compounds(AOX)were detected using combustion methods,while the representative anthropogenic organic halogenated compounds were detected using Soxhlet extraction-chromatography/mass spectrometry.For the detection of organic halogenated compounds in hadal trench sediments,the working curve of TOX detection had an R~2 value of 0.9962 and a recovery rate of91.15%～110.87%,while that of AOX detection had an R~2 value of 0.9893 and a recovery rate of 97.48%～122.95%.For the detection of representative anthropogenic organic halogenated compounds,the standard curves for 4-PCB and γ-HCH had R~2 values of0.9991 and 0.9998,recovery rates of 81.80%-85.44% and 91.59-94.63%,and RSDs of1.51-2.57% and 2.02-4.74%,respectively.The detection limits of the methods were 0.2μg/m L and 0.02 μg/m L for 4-PCB and γ-HCH,respectively.The quality control results suggest that the established methods were stable,accurate,and suitable for subsequent analysis of actual samples.After the establishment of detection methods,this thesis further studied the potentials,activities and microbial diversities of hadal microorganisms to degrade halogenated organic matter,by co-culture of representative synthetic organohalogen compounds(γ-HCH and 4-PCB)with hadal sediments from the Mariana Trench under simulated in situ high pressure and low temperature conditions.Sediment samples were collected at 0,30,90,and 270 days,and the concentrations of representative synthetic organohalogen compounds in the sediments were measured to reveal the potential and activity of abyssal microorganisms in degrading halogenated organic compounds.Meanwhile,total DNA and RNA of microorganisms in sediments were extracted,and the changes in diversity of 16 S r RNA and 16 S r RNA gene amplicons were investigated to explore the succession of the major functional bacterial communities during the degradation process.By testing the changes of content for the halogenated organic substrates in cocultivation systems,it was found that microorganisms in hadal trench sediments have significant degradation capabilities for both of the selected organic halogenated compounds.The most rapid degradation occurred within the first 90 days of cultivation,with the contents of γ-HCH and 4-PCB decreased to 16% and 14% of the initial amounts,respectively.However,the degradation of γ-HCH slowed down after 90 days,while 4-PCB continued to be degraded until the end of the experiment(270 days).This may be due to the relatively complex structure of γ-HCH,which cannot be completely degraded.It may also be due to the firm binding of some substrates to particles in the sediment,reducing their bioavailability and limiting further degradation.High pressure is a typical extreme environmental factor in the deep sea,posing great challenges to microbial survival.To reveal the effect of high pressure on microbial metabolism of halogenated organic compounds,a comparative cultivation system was set up with high pressure and normal pressure.The results showed that the halogenated organic compounds were rapidly degraded under both high pressure and normal pressure conditions,and there was no significant difference in the degradation curves between the two pressure conditions.Analysis of microbial community structures in different co-cultivation systems reveals that microbial species enrichment occurs in all of the incubation systems,but the enriched bacterial taxa differed greatly.Nevertheless,25 OTUs belonging to the Gammaproteobacteria,Alphaproteobacteria,Dehalococcoidia,Anaerolinea and Acidobacteriae were found to be shared by all of the co-cultivation systems.Most of these OTUs are new species that have never been identified before,and these microbial taxa were speculated to serve as core taxa playing important roles in the degradation of halogenated organic compounds.Biostatistical analysis(e.g.,ANOSIM,NMDS)revealed that the community structures of the substrate-added systems were differed significantly from those of the control group,and the community structures of the high-pressure systems were also differed significantly from those under normal pressure,indicating that the addition of substrates and cultivation pressure have significant effects on the microbial communities involved in the degradation of halogenated organic compounds.However,different types of substrates had little effect on the bacterial community structure changes,suggesting that the functional bacterial groups and metabolic pathways involved in the degradation of these two types of substrates may be similar.This paper reveals for the first time the active degradation of halogenated organic compounds by microorganisms in hadal trench sediments,and the degradation activities exists under both high and normal pressure conditions.These results provide an important foundation for further understanding the internal mechanisms and ecological contributions of degradation of halogenated organic compounds by hadal microorganisms,and have significant implications for understanding the biogeochemical cycling of carbon and halogens in deep-sea environments.Additionally,these findings have potential application value in the further development of hadal trench sourced dehalogenation microbial resources and promoting the environmental remediation of halogenated organic pollutants.