Collaborative strategies for using microorganisms to produce value-added molecules from lignocellulosic hydrolysates

Abstract

Climate change issues and the foreseen shortage of petroleum supplies call for a switch on the energetic matrix towards favoring the production of chemicals and fuels from cheap and renewable resources. In this regard, plant biomass (i.e., the lignocellulosic raw material) affords the most sustainable and abundant feedstock available globally1. The lignocellulosic matrix constituting the plant cell walls is composed by cellulose, hemicellulose and lignin polymers. The cellulosic and hemicellulosic fractions are a rich source of C6 (glucose, manose, galactose) and C5 (xylose and arabinose) sugars that can be processed via microbial fermentation into high-value compounds, such as biologically active molecules, pigments, bioplastics, surfactants, biocatalysts, and fuels1. One excellent example is the production of ethanol derived from plant biomass through cutting edge industrial units in Brazil, USA, Canada, Norway, Italy, and Romania2. Particularly in Brazil, the production of first generation (1G) ethanol from sugarcane juices and molasses yields bagasse fibers as waste. This by-product has been used by companies like Raízen and GranBio for production of second-generation (2G) ethanol via biomass processing and fermentation with genetically modified Saccharomyces cerevisiae2. The lignin fraction of plant cell polymers is also an important biomass resource. Lignin is a polymer composed of phenylpropanoid units that upon depolymerization provide aromatic compounds suitable for microbial conversion into target products, or serving as sustainable alternatives to petrochemical-based aromatic feedstocks3. Therefore, a concept of lignocellulosic (bio)refineries is to be envisaged as an important driver of a new bioeconomy coming to replace fossil-based technologies.A critical aspect for implementing bioprocesses using lignocellolusic feedstock is that the biomass must be processed by a physical/chemical pretreatment, followed by hydrolysis of the (hemi)cellulosic polymers to release C5/C6 sugars for microbial biotransformation4. The fermentable liquor resulting from this processing step is called the lignocellulosic hydrolysate (LCH), which, unfortunately, is enriched in lignocellulosic-derived compounds that are toxic to microorganisms. These comprise fermentation inhibitors, such as weak acids, phenols and furan aldehydes, that pose a significant challenge for the efficiency and economical sustainability of industrial fermentation of LCHs4. One possibility to overcome this problem is levering the microbial resilience to tolerate, or to detoxify, the LCH inhibitors (PI Jacobus' approach). Organisms such as S. cerevisiae can be metabolic or evolutionary engineered to withstand the toxic effects during production of ethanol from biomass feedstocks4. An alternative and ingenious approach is the use of LCH inhibitors, such as formic and acetic acids, or phenolic compounds, as chemical substrates for metabolic production of value-added molecules by microorganisms, such as Corynebacterium glutamicum5 and Vibrio natriegens6 (PI Blombach's approach). Thereby, mitigation of LCH toxicity is coupled to target product formation. This project aims at combining the expertise from the Jacobus and Blombach groups to implement new metabolic and evolutionary strategies for microbial conversion of LCHs into value-added molecules. (AU)