Seasonal fluctuations in methane emissions from reservoirs: evidence from the methane metaboliccycle throughout the in situ sediment column profile.
Description
Studying the issue of methane emissions from reservoirs, especially elucidating the mechanism of methane greenhouse gas production, revealing the seasonal flux changes of methane production, and providing scientific theories for China's low-carbon economic development strategy of optimizing energy structure and energy conservation and emission reduction. This paper combines geochemical and biological methods, with seasonal thermal stratification as the key entry point, to study the impact of methane production and oxidation processes in reservoir sediments under different thermal stratification conditions on the spatiotemporal dynamic changes of methane from the perspective of biological transformation. The ecological niche distribution of methane production/oxidation functional microorganisms in sedimentary column profiles is finely studied, revealing the ecological adaptation mechanism and production/oxidation efficiency of methane production/oxidation functional groups. Assuming that the methane dissolution in the sediment column profile of the reservoir has obvious seasonal characteristics, the seasonal thermal stratification of the Hongfeng Lake reservoir may be a key factor in the seasonal methane emission differences. The production and emission of methane from in-situ sedimentary columns may be influenced by other electron acceptors. The distribution data of methane dissolution on sedimentary columns indicate that the methane dissolution in the in-situ sedimentary column profile of the reservoir has obvious seasonal characteristics, and the methane metabolism heat zone in summer sediments shifts upward. The results of indoor cultivation experiments further verified the distribution changes of methane heat zones in sediments, proving that seasonal thermal stratification in the Hongfeng Lake reservoir may be a key factor for the seasonal differences in methane emissions. The enrichment of Methanosaeta, a specialized acetic acid trophic methanogenic archaea, was observed in the sedimentary columns during summer and winter. The fractionation coefficient results showed that the methane production pathway in the sedimentary columns of Hongfeng Lake Reservoir was mainly acetic acid trophic, and the methane production trend in this pathway was greater in summer than in winter. There is a very active methane anaerobic oxidation process in the sediment column of Hongfeng Lake reservoir in summer, and the methane production potential of Hongfeng Lake sediment in summer is lower than in winter, while the methane anaerobic oxidation potential in summer is slightly higher than in winter. Functional microbial abundance data indicate that coupled sulfate reduction AOM occurs in in-situ sedimentary columns during summer, and coupled metal iron reduction AOM occurs in shallow and deep layers during summer. In winter sedimentary columns, the iron reducing bacteria Bacillus, a participant in M-AOM, was significantly enriched in the bottom layer.
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Sediment samples were collected from Houwu Island in the middle of Hongfeng Reservoir, with a water depth of 30 meters in the sampling area. A sampler was used to cut sedimentary columns in layers on site, with a depth of 0-20 cm divided at 1 cm intervals and a bottom layer of 20 cm divided at 2 cm intervals.The vacuum bottle containing pore water sample was subjected to vortex oscillation. After the shaken sample was allowed to stand and equilibrate, 1ml of gas sample was extracted from the vacuum bottle using a medical syringe and injected into a gas chromatograph (GC7900) to measure the concentrations of methane and carbon dioxide.The filtered pore water sample was measured for dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and total nitrogen (TN) using a TC/TN analyzer. Ferrous ions (Fe2+) and ammonium salts (NH4+) were measured using a spectrophotometer at wavelengths of 560 nm and 660 nm, respectively. Measure the concentrations of 7 anions using ion chromatography. The chromatographic column of the detector is IonPac AG19-250/4.0 (Dionex, USA), with a flow rate set to 1.0 ml/min. The concentrations and stable carbon isotopic (δ13C) coefficient of CH4 and CO2 in the porewater were determined by gas chromatography (GC; GC-C/TC III) isotoperatio mass spectrometry (IRMS; Delta V Advantage IRMS) and trace ultra GC (Thermo Finnigan), equipped with chromatographic column as HP-PLOT Q (30 m × 0.32 mm × 20.00 μm). The temperature of the burner was 960°C, and that of the reducing furnace was 600 °C. The precision of the δ13C-CH4 and δ13C-CO2 measurements was ± 0.2‰. The concentration and carbon isotopic compositions of dissolved inorganic carbon (DIC) were determined using GasBench- IRMS (Delta V Advantage, USA). DNA was extracted from 0.5 g sediment samples using the Qiagen DNeasy PowerSoil Kit following the instructions. The purity and concentration of DNA were detected with a super differential photometer (Nanodrop 2000, Thermo Fisher, USA), and the quality of DNA was detected with 1.0% agarose gel electrophoresis. For the amplification of 16S bacteria and archaea, specific primers were used. The primer pairs were 338F (5'- ACTCCTCGGGAGGCAGG-3') and 806R (5'- GACTACCCGGTATCTAAT-3') for amplifying the V3-V4 region of bacterial 16S rDNA. The PCR reaction system had a total volume of 20 μL, including 4 μL for 5x Fast Pfu buffer, 2.5 mmol/L for each dNTP, 2 pmol for each pre and post primer, 0.4 μL of Fast Pfu polymerase, 0.2 of μL BSA plus and 10 ng of template DNA. The PCR cycle followed the following procedure: 95°C for 5 minutes, followed by 94°C for 15 seconds, then annealed at 60°C for 30 seconds, 72°C for 30 seconds, a total of 28 cycles, and finally extended for 8 minutes at 72°C. Sequencings were carried out by Majorbio Bio Pharm Technology Co. Ltd. (Shanghai, China) on the MiSeq Illumina platform.