During the acetogenic step of anaerobic digestion, the products of acidogenesis are oxidized to substrates for methanogenesis: hydrogen, carbon dioxide and acetate. Acetogenesis and methanogenesis are highly interconnected processes due to the syntrophic associations between acetogenic bacteria and hydrogenotrophic methanogens, allowing the whole process to become thermodynamically favorable. The aim of this study is to determine the influence of the dominant acidic products on the metabolic pathways of methane formation and to find a core microbiome and substrate-specific species in a mixed biogas-producing system. Four methane-producing microbial communities were fed with artificial media having one dominant component, respectively, lactate, butyrate, propionate and acetate, for 896 days in 3.5-L Up-flow Anaerobic Sludge Blanket (UASB) bioreactors. All the microbial communities showed moderately different methane production and utilization of the substrates. Analyses of stable carbon isotope composition of the fermentation gas and the substrates showed differences in average values of δ13C(CH4) and δ13C(CO2) revealing that acetate and lactate strongly favored the acetotrophic pathway, while butyrate and propionate favored the hydrogenotrophic pathway of methane formation. Genome-centric metagenomic analysis recovered 234 Metagenome Assembled Genomes (MAGs), including 31 archaeal and 203 bacterial species, mostly unknown and uncultivable. MAGs accounted for 54%–67% of the entire microbial community (depending on the bioreactor) and evidenced that the microbiome is extremely complex in terms of the number of species. The core microbiome was composed of Methanothrix soehngenii (the most abundant), Methanoculleus sp., unknown Bacteroidales and Spirochaetaceae. Relative abundance analysis of all the samples revealed microbes having substrate preferences. Substrate-specific species were mostly unknown and not predominant in the microbial communities. In this experimental system, the dominant fermentation products subjected to methanogenesis moderately modified the final effect of bioreactor performance. At the molecular level, a different contribution of acetotrophic and hydrogenotrophic pathways for methane production, a very high level of new species recovered, and a moderate variability in microbial composition depending on substrate availability were evidenced. Propionate was not a factor ceasing methane production. All these findings are relevant because lactate, acetate, propionate and butyrate are the universal products of acidogenesis, regardless of feedstock.

中文翻译：

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使用宏基因组学和同位素方法评估产酸产物对沼气生产的影响

在厌氧消化的产乙酸步骤中，产酸产物被氧化成产甲烷的底物：氢气、二氧化碳和乙酸盐。由于产乙酸细菌和氢营养产甲烷菌之间的互养关联，产乙酸和产甲烷是高度相互关联的过程，使得整个过程在热力学上变得有利。本研究的目的是确定主要酸性产物对甲烷形成代谢途径的影响，并在混合沼气生产系统中找到核心微生物组和底物特异性物种。四个产甲烷微生物群落在 3.5 L 上流式厌氧污泥床 (UASB) 生物反应器中使用具有一种主要成分（分别为乳酸盐、丁酸盐、丙酸盐和乙酸盐）的人工培养基，持续 896 天。所有微生物群落的甲烷产量和底物利用率均存在一定差异。对发酵气体和底物的稳定碳同位素组成的分析显示δ13C(CH4)和δ13C(CO2)的平均值存在差异，表明乙酸盐和乳酸极有利于醋营养途径，而丁酸盐和丙酸盐有利于甲烷形成的氢营养途径。以基因组为中心的宏基因组分析恢复了 234 个宏基因组组装基因组 (MAG)，其中包括 31 种古菌和 203 种细菌，其中大部分是未知且无法培养的。MAG 占整个微生物群落的 54%–67%（取决于生物反应器），证明微生物组在物种数量方面极其复杂。核心微生物组由 Methanothrix soehngenii（最丰富）、Methanoculleus sp.、未知拟杆菌目和螺旋毛菌科组成。所有样品的相对丰度分析揭示了微生物具有底物偏好。底物特异性物种大多是未知的，并且在微生物群落中并不占优势。在该实验系统中，进行产甲烷作用的主要发酵产物适度地改变了生物反应器性能的最终效果。在分子水平上，证明了醋营养和氢营养途径对甲烷生产的不同贡献、恢复了非常高水平的新物种以及取决于底物可用性的微生物组成的适度变化。丙酸盐并不是停止甲烷生产的因素。所有这些发现都是相关的，因为无论原料如何，乳酸、乙酸、丙酸和丁酸都是产酸的普遍产物。