Extremely thermophilic microorganisms as metabolic engineering platforms for production of fuels and industrial chemicals

Abstract

Enzymes from extremely thermophilic microorganisms have been of technological interest for some time because of their ability to catalyze reactions of industrial significance at elevated temperatures. Thermophilic enzymes are now routinely produced in recombinant mesophilic hosts for use as discrete biocatalysts. Genome and metagenome sequence data for extreme thermophiles provide useful information for putative biocatalysts for a wide range of biotransformations, albeit involving at most a few enzymatic steps. However, in the past several years, unprecedented progress has been made in establishing molecular genetics tools for extreme thermophiles to the point that the use of these microorganisms as metabolic engineering platforms has become possible. While in its early days, complex metabolic pathways have been altered or engineered into recombinant extreme thermophiles, such that the production of fuels and chemicals at elevated temperatures has become possible. Not only does this expand the thermal range for industrial biotechnology, it also potentially provides biodiverse options for specific biotransformations unique to these microorganisms. The list of extreme thermophiles growing optimally between 70 and 100°C with genetic toolkits currently available includes archaea and bacteria, aerobes and anaerobes, coming from genera such as Caldicellulosiruptor, Sulfolobus, Thermotoga, Thermococcus, and Pyrococcus. These organisms exhibit unusual and potentially useful native metabolic capabilities, including cellulose degradation, metal solubilization, and RuBisCO-free carbon fixation. Those looking to design a thermal bioprocess now have a host of potential candidates to choose from, each with its own advantages and challenges that will influence its appropriateness for specific applications. Here, the issues and opportunities for extremely thermophilic metabolic engineering platforms are considered with an eye toward potential technological advantages for high temperature industrial biotechnology.

Extreme environments: microbiology leading to specialized metabolites

“…Common thermophilic bacteria include members of the genera Anoxybacillus , Geobacillus , Miobacillus and Thermus whereas their hyperthermophilic counterparts belong to the genera Aquificae and Thermatoga of the families Aquificaceae and Thermatogaceae respectively (Urbieta et al ). The majority of hyperthermophiles are archaea, as exemplified by the genera Desulfurococcus , Pyrodiotum , Pyrococcus , Pyrolobus , Sulfolobus , Thermophylum and Thermoproteus of the Crenarchaeota (de Miguel Bouzas et al ; Zeldes et al ). Actinobacteria have been detected in hot springs, including representatives of the genera Couchiplanes , Glycomyces and Mycobacterium (Valverrde et al ) and Actinospica , Amycolatopsis and Rhodococcus strains (Kusuma & Goodfellow, pers.…”

Impact of growth mode, phase, and rate on the metabolic state of the extremely thermophilic archaeon Pyrococcus furiosus

“…Although extreme thermophiles were isolated and described nearly 50 years ago, their known diversity expanded considerably in the 1980s and 1990s as discoveries made it clear that thermal microbial biotopes contained a broad range of physiological and metabolic features. Recent advances in the development of molecular genetics tools for extreme thermophiles have focused their interest on the prospects of using these microorganisms as metabolic engineering platforms for bio‐based fuels and chemicals (Zeldes et al, ). Indeed, the extremely thermophilic archaeon, Pyrococcus furiosus (Fiala & Stetter, ) has been engineered to produce 3‐hydroxypropionate (3‐HP), n‐butanol, acetoin, ethanol, and to produce H 2 from formate and carbon monoxide (Adams & Kelly, ).…”

Reaction kinetic analysis of the 3-hydroxypropionate/4-hydroxybutyrate CO2 fixation cycle in extremely thermoacidophilic archaea

“…First, it functions at high temperatures, allowing it to be used in an extremely thermophilic host with concomitant minimal risk of contamination and reduced cooling costs (Keller et al, 2015; Zeldes et al, 2015). Second, the 3HP/4HB cycle can function in either an aerobic or anaerobic host, unlike the DC/4HB and reductive acetyl-CoA pathways, which are found exclusively in anaerobic organisms (Fast and Papoutsakis, 2012).…”