Laboratory controls of precursor and temperature on the kinetics and isotope fractionations of microbial methane for anoxic sediments
Date Issued
2009
Date
2009
Author(s)
Ling, Yu-Chen
Abstract
Methanogenesis is the terminal metabolism during mineralization of organic carbon in subsurface environments. The precursors of methanogenesis include hydrogen (carbon dioxide), acetate, formate, and methyl-group compounds (e.g. methanol and methylamine), all of which could be derived from fermentation of complex organic carbon. Although lots of studies have been investigating the mechanisms responsible for methanogenesis by pure cultures, it still remains obscure with regard to which precursors are predominantly utilized by methanogens in natural settings. Despite the precursors for methanogenesis, one of the other critical factors governing the methane production would be temperature. This is especially true for marine sediments within which the temperature increases with depth in accordance with the local geothermal. Commonly observed temperatures for methanogenesis span from ambient temperature to 90℃, a temperature range for most diagenetic reactions. In order to address how different precursors would be activated for microbially catalytic methane formation upon different temperatures, we incubated the sediments collected from Kuan-Tzu-Ling hot spring at six different temperatures (25, 40, 50, 60, 70, 80℃). Five precursors including acetate, formate, methanol, methylamine, and hydrogen were added with the inocula to stimulate methanogenesis and inhibit the fermentation, and were monitored together with methane production through time. Results indicated that although the presence of all precursors stimulated methanogenesis, each precursor yielded various rates at different temperatures. In the experiment supplied with hydrogen (plus carbon dioxide) and formate, methanogenic rates were rapid at all temperatures. Maximum methane production rates occurred at 40~60℃ for incubations with acetate, and 40~50℃ for those with methanol and 50℃ for those with methylamine. The δ13C values of methane varied either toward greater values, less values or remained invariant through time, suggesting either a predominant methanogenic pathway or complex interactions of multiple pathways occurred during the incubations. The ε values for carbon isotope fractionation ranged from -3.9 to -115.0‰ with acetoclastic methanogenesis possessing the least negative values (-11.9 to -3.9‰). The isotopic patterns observed in incubations amended with acetate were comparable with those in positive controls, suggesting that acetoclastic methanogenesis was predominant over the other pathways in the Kuan-Tzu-Ling area. This when combined with the field observations lead to the conclusive interpretation that acetoclastic methanogenesis would constitute an essential proportion to the total methane inventory in the Kuan-Tzu-Ling area. Identification of microbial end member signature would appear to be pivotal for natural gas exploration in the future.
Subjects
methanogen
SDGs
Type
thesis
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