Biofuel production: an odyssey from metabolic engineering to fermentation scale-up

Abstract

Metabolic engineering has developed microbial cell factories that can convert renewable carbon sources into biofuels. Current molecular biology tools can efficiently alter enzyme levels to redirect carbon fluxes toward biofuel production, but low product yield and titer in large bioreactors prevent the fulfillment of cheap biofuels. There are three major roadblocks preventing economical biofuel production. First, carbon fluxes from the substrate dissipate into a complex metabolic network. Besides the desired product, microbial hosts direct carbon flux to synthesize biomass, overflow metabolites, and heterologous enzymes. Second, microbial hosts need to oxidize a large portion of the substrate to generate both ATP and NAD(P)H to power biofuel synthesis. High cell maintenance, triggered by the metabolic burdens from genetic modifications, can significantly affect the ATP supply. Thereby, fermentation of advanced biofuels (such as biodiesel and hydrocarbons) often requires aerobic respiration to resolve the ATP shortage. Third, mass transfer limitations in large bioreactors create heterogeneous growth conditions and micro-environmental fluctuations (such as suboptimal O2 level and pH) that induce metabolic stresses and genetic instability. To overcome these limitations, fermentation engineering should merge with systems metabolic engineering. Modern fermentation engineers need to adopt new metabolic flux analysis tools that integrate kinetics, hydrodynamics, and 13C-proteomics, to reveal the dynamic physiologies of the microbial host under large bioreactor conditions. Based on metabolic analyses, fermentation engineers may employ rational pathway modifications, synthetic biology circuits, and bioreactor control algorithms to optimize large-scale biofuel production.

Systems strategies for developing industrial microbial strains

“…Another critical problem is reversion of a high-performance strain back to the low-performance strain through the loss of production capacity and phenotype alteration during the course of industrial fermentation, which can be caused by genetic instability and/or fermentation conditions having different mass transfer rates of nutrients and oxygen, and substrate and product concentration profiles in the industrial-sized fermenter 60 . At the moment, no tools exist that guarantee 100% strain stability, but permanent chromosomal manipulation Figure 4 General scheme of systems metabolic engineering and its case study on the overproduction of l-lysine 16 and bio-nylon 17 using C. glutamicum.…”

Consolidated bioprocessing of lignocellulose for production of glucaric acid by an artificial microbial consortium

“…Even if successful, it is nearly impossible to obtain a super strain simultaneously good at cellulase production and D-glucaric acid production. Microbial cell factory would be faced with serious metabolic dilemma when doing the two things at the same time 35 . Thus, the artificial microbial consortium of T. reesei Rut-C30 (cellulase specialist) and S. cerevisiae LGA-1 (D-glucaric acid specialist) was an excellent team with “chemistry” in CBP.…”

Enhanced production of histamine dehydrogenase by <i>Natrinema gari</i> BCC 24369 in a non-sterile condition

“…gari BCC 24369 under sterile and non-sterile conditions were not significantly different (p > 0.05) in which they were corresponding to 92 and 88% of the product yields at flask scales under sterile and non-sterile conditions, respectively (Table 4). A decrease in product yield upon scaling up the fermentation process is a general observation (Hollinshead et al, 2014;Kim et al, 2002). However, this scale-up strategy presents an easy operation system which provides an alternative design of the bioreactor corresponding to cost-effective enzyme production with low investment and operating costs.…”