Thermophilic microbial deconstruction and conversion of natural and transgenic lignocellulose

Abstract

The potential to convert renewable plant biomasses into fuels and chemicals by microbial processes presents an attractive, less environmentally intense alternative to conventional routes based on fossil fuels. This would best be done with microbes that natively deconstruct lignocellulose and concomitantly form industrially relevant products, but these two physiological and metabolic features are rarely and simultaneously observed in nature. Genetic modification of both plant feedstocks and microbes can be used to increase lignocellulose deconstruction capability and generate industrially relevant products. Separate efforts on plants and microbes are ongoing, but these studies lack a focus on optimal, complementary combinations of these disparate biological systems to obtain a convergent technology. Improving genetic tools for plants have given rise to the generation of low-lignin lines that are more readily solubilized by microorganisms. Most focus on the microbiological front has involved thermophilic bacteria from the genera Caldicellulosiruptor and Clostridium, given their capacity to degrade lignocellulose and to form bio-products through metabolic engineering strategies enabled by ever-improving molecular genetics tools. Bioengineering plant properties to better fit the deconstruction capabilities of candidate consolidated bioprocessing microorganisms has potential to achieve the efficient lignocellulose deconstruction needed for industrial relevance.

Figure 1.. Schematic representation of the bioenergy feedstocks evolution.

Compared to non-woody starch-based and non-woody lignocellulosic-based feedstocks, forest trees store significantly higher amounts of carbon, reaching 57% of all carbon biomass on earth. The abundance of woody biomass availability makes short-rotation forest trees an extraordinary feedstock source to meet the world’s demands for sustainable bioenergy (A). The integration of omics (e.g., genomics, transcriptomics, proteomics, and metabolomics) to advanced machine-learning algorithms (B) is a powerful approach to design strategic genotypes of interest when combined with genome-editing technologies such as CRISPR-SpCas9 (C). Superior trees with enhanced wood traits, growth, and feedstocks deconstruction are essential for the cost-effective production of renewable fuels and chemicals (D).

Figure 2.. Microbe-feedstock pairing for consolidated bioprocessing.

(A) Production of valuables from plant biomass by CBP microbes has been demonstrated from native and engineered metabolic pathways (chemicals in red), potential exists to engineer microbes to create other desirables (including chemicals in black). (B) Common substrates for (hemi)cellulolytic microbes include microcrystalline cellulose (Avicel), poplar (Populus trichocarpa pictured), and switchgrass (Panicum virgatum). (C) Example 4-day fermentation of poplar using Caldicellulosiruptor bescii.