Pressurized syngas bioconversion: physiological and microbial characterization

Abstract

Syngas is composed mainly by CO, H2 and CO2 and its fermentation is a promising biological process to produce fuels or commodity chemicals. Experiments under increased initial syngas pressures, up to 5.2×105 Pa, were carried out to evaluate the effects on metabolites production and microbial communities structure. Two strategies were applied: NB non-adapted biomass and SB successively syngas-fed biomass. The rise of syngas pressure from 1.2×105 Pa up to 5.2×105 Pa led to a decrease on CO and H2 consumption rates and CH4 production rate. Moreover, when methanogenesis was partially inhibited, propionate and butyrate were the main metabolites produced from syngas. DGGE profiles showed differences on diversity and on similarity indices (SI) with changes in pressure. Regardless the syngas pressure employed, the archaeal communities had higher SI (above 70%) than bacterial community (48% to 62%). From the Illumina sequencing analysis, it was observed that the relative abundance of bacterial communities tend to decrease (72% to 46%), and archaeal communities increased (25% to 54%) by raising the pressure of syngas from 1.2×105 Pa to 5.2×105 Pa. In the inoculum and biomass incubated at 1.2×105 Pa syngas, 40% of total population were from Proteobacteria phylum and Deltraproteobacteria class and their abundance was reduced 4-fold at 5.2×105 Pa. As a direct effect of high pressures of syngas, organisms belonging to Firmicutes, Synergistetes and Thermotogae phyla increased over 10-fold. The predominant phylotypes at 3×105 Pa and 5.2×105 Pa were related to Methanobacterium genus (archaea) and to Eubacteriaceae, Synergistaceae and Syntrophobacteraceae families (bacteria). These results showed a microbial population enrichment suggesting a high specialization for the substrate.