Pattern And Characteristic Of Microbial Aerobic Oxidation Of Hydrocarbons In Marine Environment

Marine sediments contain a large number of methane-based hydrocarbons?methane,ethane and propane?,which release into seawater and atmosphere per year by decomposition of gas hydrate and oil-gas leakage.It has a profound impact on the ecological environment and global climate change.The light hydrocarbon gases are widely distributed in Bohai Bay,East China Sea and South China Sea.The key mechanism preventing the hydrocarbon gases released from deep subsurface hydrocarbon reservoirs from being emitted into the atmosphere is the anaerobic/aerobic oxidation by microorganisms.The surface sediments and overlying seawater are the potential areas of aerobic oxidation of hydrocarbons.But there are significant differences in environmental parameters such as temperature,pressure and oxygen content.At present,people are not very clear about the regurity of aerobic oxidation of hydrocarbons as well as the characteritics of microbial community.Meanwhile,the research on aerobic oxidation of hydrocarbons in the marine seepage in China is still scarce.Therefore,the study on microbial aerobic oxidation of hydrocarbons in marine environment will be of great significance for understanding the interception capacity of marine sediments and the potential environmental implications.In this paper,the aerobic microbial oxidation of hydrocarbons with marine sediments under different environmental conditions was investigated in the laboratory combined with conventional culture techniques.A comprehensive analysis was performed during the course of the experiments from the perspectives of isotope geochemistry,molecular microbial ecology and organic geochemistry.The analyses mainly include oxidation rate,fractionation of carbon and hydrogen isotopes,bacteria 16S rRNA and pmoA gene community structures,lipid biomarker composition.The characteristics of aerobic oxidation and microbial community structures under different environmental conditions were revealed systematically in this study.Meanwhile,we conclude the interception capacity and the microbial community that play a dominant role in aerobic oxidation of hydrocarbons in Bohai Bay,East China Sea and South China Sea.The aerobic oxidation characteristics under different environmental conditions were obtained through simulation studies in labtoratory with sediments from Bohai Bay in different conditions,i.e.,temperature,pressure,oxygen concentration,gas component and gas flow rate.It is important to know the aerobic oxidation ability and microbial community characteristics under certain environmental conditions.The results show that the process of aerobic oxidation of hydrocarbons is significantly controlled by the environmental parameters.The oxidation rate is much faster and the stable isotope fractionation is more obvious in the higher temperature during experiments under 4?,10?,15?and 28?.Meanwhile,The oxidation rate and isotope fractionation both show a positive correlation with pressure.Although the gas flow rate has a significant effect on the oxidation rate,the effect on isotope fractionation of hydrocarbons is weak.In addition,there is a perfectly suitable oxygen concentration for the aerobic oxidation,which means that too high or too low oxygen concentration would be detrimental to the oxidation.In this study,the oxidation rate in experiment under 10%O₂ is the fastest,while the isotope fractionation is note affected by oxygen concentration.The results from mixed-hydrocarbon treatments show that CH₄ is consumed much faster than C₂H₆ and C₃H₈.The isotope fractionation factors of CH₄,C₂H₆ and C₃H₈ are also different,which indicates that they are characterized by different isotope enrichments.Compared to microbial hydrocarbon gas,thermogenic gas is characterized by higher?C values,implying greater carbon isotopic fractionation of thermogenic gas than microbial gas during oxidation.Furthermore,the high-throughout sequencing results show that the hydrocarbon-degrading bacteria with pmoA genes represent the main dominant bacteria and its abundance is significantly increased during the oxidation.The community composition and abundance of pmoA genes vary at the OTU level under different environmental conditions.The results of aerobic oxidation with sediments from Coal oil point seep field in Bohai Bay,Okinawa Trough in East China Sea and gas hydrate area in Shenhu of South China Sea are verified that the aerobic oxidation of hydrocarbons exists in these areas.The oxidation rate of methane in sediments from Bohai is 1.29?mol/gdw/d.The consumptions of ethane and propane are similar,whose oxidation rate is 0.15 and 0.16?mol/gdw/d,respectively.A trend toward carbon and hydrogen isotopic enrichment in the three residual hydrocarbon gases is observed in the experiments.The CH₄ carbon isotopic fractionation factor??C?is the highest with an average of–10.9±2.9,whereas that for C₂H₆ and C₃H₈ is–3.2±1.1 and–1.0±0.3,respectively.The C₂H₆ and C₃H₈ show much higher fractionation of hydrogen isotopes than their CH₄ counterpart(?H_CH4:–56.3±8.4),with?H being–135.6±24.4 and–70.3±23.1,respectively.Compared with the sediments from Bohai Bay,the oxidation capacity of hydrocarbons in East China Sea is obviously better with a higher methane oxidation rate,i.e.,3.11?mol/gdw/d.Meanwhile,carbon and hydrogen isotope fractionation is also higher with?C and?H being–20.5 and–80.6,respectively.The oxidation rate of sediments from South China Sea is lower than sediments in other two areas,which associate with the hydrocarbon-degrading bacteria.CH₄,C₂H₆ and C₃H₈ are consumed with an oxidation rate of 0.56,0.005 and 0.003?mol/gdw/d.The abundance of pmoA genes by qPCR analysis is 2.3×10³ copies/g,which is only 1/10 of that of the other two areas.Microbiological high-thoughout sequencing show that the dominant microbial community is Methylobacter during the aerobic oxidation of sediments from Bohai Bay and East China Sea,whereas Methylocaldum and Methylobacter both play a leading role in South China sea.In addition,the lipid biomarkers such as n-alkanes,dominant fatty acids and fatty alcohols definitely prove the strong bacterial action during the hydrocarbon consumption.And the dominant position of type I methanographs in the aerobic oxidation of hydrocarbons in the study areas is further instructed.The research in this paper shows that the aerobic oxidation of hydrocarbons in marine environment is controlled by many environmental parameters.The oxidation rate of hydrocarbons is maily affected by temperature,pressure,gas component,gas flow rate and oxygen concentration,whereas carbon and hydrogen isotope fractionation is only related to temperature and pressure.Meanwhile,the characteristics of microbial community structures are obviously different with environmental conditions.This study systematically discussed the aerobic oxidation of hydrocarbons in the three representative gas-seepage areas for the first time.The results show that the interception capacity of the sediments from East China Sea is obviously high,while that of the sediments from South China Sea is relatively low.The same dominant microbial community during the oxidation in the three study areas is Methylobacter,whereas another dominant community in South China Sea is Methylocaldum.The research in this study can provide important theoretical support for the prediction of interception capacity for hydrocarbons and marine environmental assessment in the study areas.