Abstract
The carboxylate platform is a flexible, cost-effective means of converting lignocellulosic materials into chemicals and liquid fuels. Although the platform's chemistry and engineering are well studied, relatively little is known about the mixed microbial communities underlying its conversion processes. In this study, we examined the metagenomes of two actively fermenting platform communities incubated under contrasting temperature conditions (mesophilic 40°C; thermophilic 55°C), but utilizing the same inoculum and lignocellulosic feedstock. Community composition segregated by temperature. The thermophilic community harbored genes affiliated with Clostridia, Bacilli, and a Thermoanaerobacterium sp, whereas the mesophilic community metagenome was composed of genes affiliated with other Clostridia and Bacilli, Bacteriodia, γ-Proteobacteria, and Actinobacteria. Although both communities were able to metabolize cellulosic materials and shared many core functions, significant differences were detected with respect to the abundances of multiple Pfams, COGs, and enzyme families. The mesophilic metagenome was enriched in genes related to the degradation of arabinose and other hemicellulose-derived oligosaccharides, and the production of valerate and caproate. In contrast, the thermophilic community was enriched in genes related to the uptake of cellobiose and the transfer of genetic material. Functions assigned to taxonomic bins indicated that multiple community members at either temperature had the potential to degrade cellulose, cellobiose, or xylose and produce acetate, ethanol, and propionate. The results of this study suggest that both metabolic flexibility and functional redundancy contribute to the platform's ability to process lignocellulosic substrates and are likely to provide a degree of stability to the platform's fermentation processes.
Highlights
As energy demands place increasing pressure on global fuel reserves, the need to develop stable, renewable alternatives to fossil fuels continues to become more urgent
After 16 days’ incubation, the mesophilic and thermophilic fermentations resulted in similar rates of biomass conversion, selectivity, yield, and productivity (Table S1); the two temperature conditions differed with respect to the abundances of multiple acids within their product spectra (Table 1)
Shotgun sequencing efforts resulted in the production of more than 2.5 million sequence reads per fermentor library, representing 900 and 588 Mbp of sequence data for the thermophilic and mesophilic metagenomes, respectively (Table 2)
Summary
As energy demands place increasing pressure on global fuel reserves, the need to develop stable, renewable alternatives to fossil fuels continues to become more urgent. An alternative to the aseptic fermentation of simple sugars (i.e., ethanol production from sugar or starch) or thermochemical conversion processes, the carboxylate platform operates under non-sterile conditions and uses a mixed community of anaerobic microorganisms to convert lignocellulosic materials into chemicals and liquid fuels [5,6]. These features allow the platform to be flexible in terms of the variety of feedstocks it can accommodate. The spectrum of products produced by the platform is temperature dependent [7,8,9] and can be varied in response to market demands
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