Abstract
The microbial production of ethanol from lignocellulosic biomass is a multi-component process that involves biomass hydrolysis, carbohydrate transport and utilization, and finally, the production of ethanol. Strains of the genus Thermoanaerobacter have been studied for decades due to their innate abilities to produce comparatively high ethanol yields from hemicellulose constituent sugars. However, their inability to hydrolyze cellulose, limits their usefulness in lignocellulosic biofuel production. As such, co-culturing Thermoanaerobacter spp. with cellulolytic organisms is a plausible approach to improving lignocellulose conversion efficiencies and yields of biofuels. To evaluate native lignocellulosic ethanol production capacities relative to competing fermentative end-products, comparative genomic analysis of 11 sequenced Thermoanaerobacter strains, including a de novo genome, Thermoanaerobacter thermohydrosulfuricus WC1, was conducted. Analysis was specifically focused on the genomic potential for each strain to address all aspects of ethanol production mentioned through a consolidated bioprocessing approach. Whole genome functional annotation analysis identified three distinct clades within the genus. The genomes of Clade 1 strains encode the fewest extracellular carbohydrate active enzymes and also show the least diversity in terms of lignocellulose relevant carbohydrate utilization pathways. However, these same strains reportedly are capable of directing a higher proportion of their total carbon flux towards ethanol, rather than non-biofuel end-products, than other Thermoanaerobacter strains. Strains in Clade 2 show the greatest diversity in terms of lignocellulose hydrolysis and utilization, but proportionately produce more non-ethanol end-products than Clade 1 strains. Strains in Clade 3, in which T. thermohydrosulfuricus WC1 is included, show mid-range potential for lignocellulose hydrolysis and utilization, but also exhibit extensive divergence from both Clade 1 and Clade 2 strains in terms of cellular energetics. The potential implications regarding strain selection and suitability for industrial ethanol production through a consolidated bioprocessing co-culturing approach are examined throughout the manuscript.
Highlights
Consolidated bioprocessing (CBP), whereby microbial enzyme production, biomass hydrolysis and substrate conversion to ethanol all occurs in a single bioreactor, offers improved economic feasibility and process efficiencies compared with alternative approaches to lignocellulosic ethanol production [1,2,3]
The Thermoanaerobacter tengcongensis MB4 genome [47], corresponding to GenBank accession number: AE008691 was not included based upon the reclassification of the strain into the genus Caldanaerobacter [48]
The present analysis indicates that none of the GH3 coding sequences (CDS) are predicted to be secreted extracellularly is in agreement with the findings for T. thermohydrosulfuricus JW 102 [55]
Summary
Consolidated bioprocessing (CBP), whereby microbial enzyme production, biomass hydrolysis and substrate conversion to ethanol all occurs in a single bioreactor, offers improved economic feasibility and process efficiencies compared with alternative approaches to lignocellulosic ethanol production [1,2,3]. The utilization of designer co-cultures, allowing for potential complementary or synergistic phenotypes between multiple organisms to improve process efficiencies and yields, is an alternative strategy to potentially overcome these limitations (reviewed by Brenner et al [5] and Zuroff & Curtis [6]). The native capacity for many strains to produce lignocellulose hydrolytic enzymes in situ may reduce or eliminate the need for enzymatic pre-treatment of biomass. The ability of multiple strains to ferment a broad range of lignocellulose constituent saccharides into ethanol allows for efficient conversion and utilization of the biomass
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