دانلود رایگان مقاله انگلیسی همکاری بالقوه استوژنسیس برای تخریب بی هوازی در خاک زمین های متانوژنیک برنج - الزویر 2018

ABSTRACT

Acetate is an important intermediate in the anaerobic degradation of organic matter. It is not only produced by fermentation but also by the reduction of CO2 via the acetyl-CoA pathway (acetogenesis). However, the interplay of this process in methanogenic rice field soils is not fully understood. Chemolithotrophic acetogenesis results in a rather strong depletion of the 13C of acetate. Therefore, we measured the δ13C of acetate, CO2 and CH4 that were produced during methanogenic degradation of organic matter in rice paddy soils from two geographical origins (Philippines and Italy) and used three different strategies to estimate the contribution of acetogenesis to acetate formation: (1) Incubation of soil slurries under elevated concentrations of H2/CO2 to specifically activate the H2-dependent communities; (2) incubation at three different temperatures (15, 30, 50 °C) to shift the conditions for H2 consumption; (3) incubation in the presence of inhibitors presumed to inhibit acetogenesis (KCN) or methanogenesis (BES). Only incubations under elevated H2/CO2 resulted in13C-depleted acetate (δ13C of −68 to −65‰) compared to the control (δ13C of −25‰). Temperature and presence of inhibitors also affected the δ13C of acetate, CO2 and/or CH4, but δ13C of acetate was never as low as after addition H2/CO2. A significant 13C enrichment of acetate at 15 °C in presence of BES and KCN indicated that H2-dependent acetogenesis is a favoured process at low temperature. Copy numbers in the Philippine soil of the fhs gene coding for the formyltetrahydrofolate synthetase of the acetyl-CoA pathway were on a similar order of magnitude (106 per gram dry soil) irrespectively of the different incubation conditions. Our results indicate that chemolithotrophic acetogenesis was operative in methanogenic rice soil at 15 °C but was more important at elevated H2/CO2 concentrations.

5. Conclusion

Our study showed that rice field soils have the potential for chemolithotrophic acetogenesis, but only when H2/CO2 are added as external substrates. Only then very low δ13C of acetate was observed for all incubations irrespectively of the incubation temperature. However, the very low δ13C of acetate was transient and reverted to values closer to the δ13C of organic matter as soon as the added H2/CO2 was consumed. Without addition of H2/CO2, chemolithotrophic acetogenesis was insignificant and could not be detected using isotopic values of acetate. Importance of acetate production may be higher at low versus medium temperatures, but the relatively high δ13C of acetate indicated production by fermentation (or maybe by heterotrophic acetogenesis) rather than by chemolithotrophic acetogenesis. It was possible to determine production of acetate together with the isotopic signatures without simultaneous consumption when aceticlastic methanogenesis was inhibited with BES. However, this approach was not possible at high temperatures when acetate was presumably consumed by syntrophic acetate oxidizers which were not inhibited by BES. Addition of KCN as putative inhibitor of acetogens was not helpful. The quantification of fhs, as characteristic gene for acetogens, also turned out to be inconclusive. Hence, discrimination of acetogenically formed acetate using the described techniques is still a problem for future research. Currently, it is only safe to determine the potential for chemolithotrophic acetogenesis through stimulation by H2/CO2 and evaluating the δ13C of acetate.