Microbial Communities Performing Hydrogen Solventogenic Metabolism of Volatile Fatty Acids

Abstract

1AbstractBackgroundProduction of alcohols from wastes through biological processes is environmentally and economically interesting, since they can be valorized as drop-in liquid fuels, which have a high market value. Using microbial mixed cultures in such processes is of great interest since it confers more stability, a higher resistance to both toxicity and contamination, and an increased substrate flexibility. However, there is still a lack of fundamental knowledge on such microbial populations used as inoculum in solventogenic processes. This work evaluates the effect of four different physicochemical pretreatments (acidic, thermal, acidic-thermal and thermal-acidic) on an anaerobic inoculum used for alcohols production from volatile fatty acids.ResultsAll experiments were conducted in single batches using acetate and butyrate as substrates, at 30°C and with a pressurized headspace of pure H2 at 2182 mBar. Higher productions of both ethanol and butanol were achieved with both thermal and acidic-thermal pretreatments of the inoculum. The highest concentrations of ethanol and butanol produced were respectively of 122 mg.L−1 and 97 mg.L−1 for the thermal pretreatment (after 710 hours), and of 87 mg.L−1 and 143 mg.L−1 for the acidic-thermal pretreatment (after 210 hours). Butyrate was consumed and acetate was produced in all assays. A mass balance study indicated that the inoculum provided part of the substrate. Thermodynamic data indicated that a high H2 partial pressure favored solventogenic metabolic pathways. Finally, sequencing data showed that both thermal and acidic-thermal pretreatments selected mainly the bacterial genera Pseudomonas, Brevundimonas and Clostridium.ConclusionThe acidic-thermal pretreatment selected a bacterial community more adapted to the conversion of acetate and butyrate into ethanol and butanol, respectively. A higher production of ethanol was achieved with the thermal pretreatment, but at a slower rate. The thermal-acidic pretreatment was unstable, showing a huge variability between replicates. The acidic pretreatment showed the lowest alcohol production, almost negligible as compared to the control assay.

Bioconversion of Lignocellulosic Biomass into Value Added Products under Anaerobic Conditions: Insight into Proteomic Studies

“…In another study, alcohol dehydrogenase (ADH) production was observed in Acinetobacter strains during ethanol conversion to acetate, the primary pathway of reversion of solventogenesis. The ADH has been related to bacterial quorum sensing, and as a key stimulator for alcohol oxidation [121]. Conversely, spore formation has been reported as a survival mechanism under solventogenesis stress.…”