مقاله انگلیسی رایگان در مورد تخریب بی هوازی در خاک زمین های متانوژنیک برنج – الزویر 2018

The anaerobic degradation of organic matter accounts for a significant part of the global carbon cycle and involves the production of the greenhouse gas CH4 (Reeburgh, 2003). While the terminal processes liberating CH4 have been thoroughly investigated using isotopic techniques (Conrad, 2005), the contribution of other pathways during the anaerobic degradation is not well investigated. In this study we aimed to specify the contribution of acetogenesis to the acetate pool in rice paddy soils from two geographical origins (Philippines and Italy). The unifying trait of acetogenic bacteria is that acetate is their major product, which is mainly formed by the acetyl-CoA pathway reducing CO2 to acetate. However, the usage of the acetyl-CoA pathway in acetogens is versatile (Drake et al., 2002; Schuchmann and Muller, 2016): acetogens can compete with primary fermenters for monomeric compounds, e.g., glucose, fructose or xylose; they can act as secondary fermenters using lactate, ethanol or even methoxylated aromatics; many acetogens can grow chemolithotrophically converting H2 and CO2 to acetate; and some acetogens can even grow in syntrophic associations at the expense of acetate running the acetyl-CoA pathways in the reverse direction (Hattori et al., 2000; Hattori, 2008). Even though acetate is the major end product of acetogens, their environmental role is hard to address since many anaerobic bacteria also produce acetate by other biochemical pathways than the acetylCoA pathway. To investigate the function of chemolithotrophic acetogens in rice field soils we studied their contribution to the acetate pool using isotope techniques. This is a reasonable approach since studies of microbial pure cultures have shown that the stable carbon isotope fractionation of most fermentative processes is almost negligible (< 5‰) (Blair et al., 1985; Penning and Conrad, 2006; Botsch and Conrad, 2011), while in contrast the fractionation by chemolithotrophic acetogenesis (4 H2 + 2 CO2→ CH3COOH + 2 H2O) is strong (−38‰ to −68‰) (Gelwicks et al., 1989; Preuss et al., 1989; Blaser et al., 2013). One has to keep in mind that the acetate formed by heterotrophic acetogenesis originates partially from fermentation of organic matter and partially from the reduction of CO2 (Freude and Blaser, 2016). However, while the differentiation of aceticlastically and hydrogenotrophically formed CH4 is rather straight forward (Conrad, 2005; Blaser and Conrad, 2016), the application of such techniques for the investigation of chemolithotrophic acetogenic bacteria is much more complex. In contrast to CH4, which is an end product in most anaerobic sediments, acetate is produced and consumed by different metabolic pathways, which all imprint a signature on the stable carbon isotope composition (13C/12C) of acetate (Fig. 1) (Conrad et al., 2014). Hence, the stable carbon isotopic composition of acetate is basically governed by three factors: (1) the isotopic composition of its precursors, (2) the isotopic fractionations (enrichment factors) associated with its formation and consumption, and (3) the relative rates of all acetate producing and consuming processes which influence its pool size. The knowledge of all three factors is of great importance for the evaluation of 13C values of acetate determined in environmental samples.