A process economic assessment of hydrocarbon biofuels production using chemoautotrophic organisms

Surface Modification of a Graphite Felt Cathode with Amide-Coupling Enhances the Electron Uptake of Rhodobacter sphaeroides

Microbial electrosynthesis (MES) is a promising technology platform for the production of chemicals and fuels from CO2 and external conducting materials (i.e., electrodes). In this system, electroactive microorganisms, called electrotrophs, serve as biocatalysts for cathodic reaction. While several CO2-fixing microorganisms can reduce CO2 to a variety of organic compounds by utilizing electricity as reducing energy, direct extracellular electron uptake is indispensable to achieve highly energy-efficient reaction. In the work reported here, Rhodobacter sphaeroides, a CO2-fixing chemoautotroph and a potential electroactive bacterium, was adopted to perform a cathodic CO2 reduction reaction via MES. To promote direct electron uptake, the graphite felt cathode was modified with a combination of chitosan and carbodiimide compound. Robust biofilm formation promoted by amide functionality between R. sphaeroides and a graphite felt cathode showed significantly higher faradaic efficiency (98.0%) for coulomb to biomass and succinic acid production than those of the bare (34%) and chitosan-modified graphite cathode (77.8%), respectively. The results suggest that cathode modification using a chitosan/carbodiimide composite may facilitate electron utilization by improving direct contact between an electrode and R. sphaeroides.

Hydrogen-driven microbial biogas upgrading: Advances, challenges and solutions

As a clean and renewable energy, biogas is an important alternative to fossil fuels. However, the high carbon dioxide (CO₂) content in biogas limits its value as a fuel. 'Biogas upgrading' is an advanced process which removes CO₂ from biogas, thereby converting biogas to biomethane, which has a higher commercial value. Microbial technologies offer a sustainable and cost-effective way to upgrade biogas, removing CO₂ using hydrogen (H₂) as electron donor, generated by surplus electricity from renewable wind or solar energy. Hydrogenotrophic methanogens can be applied to convert CO₂ with H₂ to methane (CH₄), or alternatively, homoacetogens can convert both CO₂ and H₂ into value-added chemicals. Here, we comprehensively review the current state of biogas generation and utilization, and describe the advances in biological, H₂-dependent biogas upgrading technologies, with particular attention to key challenges associated with the processes, e.g., metabolic limitations, low H₂ transfer rate, and finite CO₂ conversion rate. We also highlight several new strategies for overcoming technical barriers to achieve efficient CO₂ conversion, including process optimization to eliminate metabolic limitation, novel reactor designs to improve H₂ transfer rate and utilization efficiency, and employing advanced genetic engineering tools to generate more efficient microorganisms. The insights offered in this review will promote further exploration into microbial, H₂-driven biogas upgrading, towards addressing the global energy crisis and climate change associated with use of fossil fuels.

Enhancing microbial production of biofuels by expanding microbial metabolic pathways

Fatty acid, isoprenoid, and alcohol pathways have been successfully engineered to produce biofuels. By introducing three genes, atfA, adhE, and pdc, into Escherichia coli to expand fatty acid pathway, up to 1.28 g/L of fatty acid ethyl esters can be achieved. The isoprenoid pathway can be expanded to produce bisabolene with a high titer of 900 mg/L in Saccharomyces cerevisiae. Short- and long-chain alcohols can also be effectively biosynthesized by extending the carbon chain of ketoacids with an engineered "+1" alcohol pathway. Thus, it can be concluded that expanding microbial metabolic pathways has enormous potential for enhancing microbial production of biofuels for future industrial applications. However, some major challenges for microbial production of biofuels should be overcome to compete with traditional fossil fuels: lowering production costs, reducing the time required to construct genetic elements and to increase their predictability and reliability, and creating reusable parts with useful and predictable behavior. To address these challenges, several aspects should be further considered in future: mining and transformation of genetic elements related to metabolic pathways, assembling biofuel elements and coordinating their functions, enhancing the tolerance of host cells to biofuels, and creating modular subpathways that can be easily interconnected.

A temporary immersion plant propagation bioreactor with decoupled gas and liquid flows for enhanced control of gas phase

Temporary immersion bioreactors (TIBs) are being used to propagate superior plant species on a commercial scale. We demonstrate a new TIB design, a Hydrostatic-driven TIB (Hy-TIB), where periodic raising and lowering the media reservoir maintains the advantages of temporary immersion of plant tissues without requiring large amounts of gas to move the media that is a characteristic of other TIB designs. The advantage of utilizing low volumes of gas mixtures (that are more expensive than air) is shown by a doubling of the growth rate of plant root cultures under elevated (40%) oxygen in air, and with CO2 supplementation showing improved phototrophic and photomixotrophic growth of seedless watermelon meristem cultures. The development of this bioreactor system involved overcoming contamination issues associated with utilizing very low gas flow rates and included utilizing microchip pressure sensors to diagnose unexpected changes in internal bioreactor pressure (± 20 Pa ∼0.0002 atm) caused by flexing of non-rigid plastic bag vessels. The overall design seeks to achieve versatility, scalability and minimum cost such that bioreactor technology can play an increasing role in the critical need to improve plant productivity in the face of increasing demand for food, reduced resources, and environmental degradation. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:337-345, 2016.

Metabolic engineering in chemolithoautotrophic hosts for the production of fuels and chemicals

The ability of autotrophic organisms to fix CO2 presents an opportunity to utilize this 'greenhouse gas' as an inexpensive substrate for biochemical production. Unlike conventional heterotrophic microorganisms that consume carbohydrates and amino acids, prokaryotic chemolithoautotrophs have evolved the capacity to utilize reduced chemical compounds to fix CO2 and drive metabolic processes. The use of chemolithoautotrophic hosts as production platforms has been renewed by the prospect of metabolically engineered commodity chemicals and fuels. Efforts such as the ARPA-E electrofuels program highlight both the potential and obstacles that chemolithoautotrophic biosynthetic platforms provide. This review surveys the numerous advances that have been made in chemolithoautotrophic metabolic engineering with a focus on hydrogen oxidizing bacteria such as the model chemolithoautotrophic organism (Ralstonia), the purple photosynthetic bacteria (Rhodobacter), and anaerobic acetogens. Two alternative strategies of microbial chassis development are considered: (1) introducing or enhancing autotrophic capabilities (carbon fixation, hydrogen utilization) in model heterotrophic organisms, or (2) improving tools for pathway engineering (transformation methods, promoters, vectors etc.) in native autotrophic organisms. Unique characteristics of autotrophic growth as they relate to bioreactor design and process development are also discussed in the context of challenges and opportunities for genetic manipulation of organisms as production platforms.