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Background 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. Results 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. Conclusions 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.

Two moss genera (Sphagnum and Polytrichum) were collected seasonally in two close (~0.45 km distance) but environmentally different locations, an Open bog and a Spruce forest at Hala Izerska (the Izerskie Mts./SW Poland), for the stable isotope analyses of plant in-body sulfur. Simultaneously, surface water was collected in places of moss growth and along the creek discharging the bog, for stable isotope analysis of sulfate sulfur (5 locations/5 times in growing seasons). The δ 34 S value of the analyzed mosses varies from 3.99 to 10.24‰ for Sphagnum and from 4.18 to 6.48‰ for Polytrichum. The δ 34 S value of aqueous sulfate in creek waters, depending on location and season, ranges from 3.72 to 20.26‰. The significant correlation between the plant in-body sulfur concentration and the isotopic composition was observed for Sphagnum as well as loose correlation between δ 34 S of sulfates in surface water and moss in-body sulfur. The fractionation factor, possibly caused by two processes of sulfate assimilation by Sphagnum and sulfate reduction by bacteria, calculated based on Rayleigh's distillation model equals about 4‰. The high correlation and simultaneous increase of δ 34 S(SO 4 2-) and δ 13 C(DIC) values downstream the creek discharging the bog suggest that the lighter isotopes of carbon and sulfur ( 12 C and 32 S) are preferentially removed, probably due to assimilation by plants. The present results imply the original signature of the source of sulfur in the environment is greatly altered by the biological activities in bog water.