Most technology development of alternative fuels production focuses on photosynthetic routes due to the endless supply of light and straightforward extraction of energy from it. However, there is also a wide range of processes and products for non-photosynthetic pathways in renewable fuel production. A class of these, electrofuels, can utilize an extensive range of microorganism and energy sources to produce variety of fuel types. Hawkins et al. (2013) discussed the key process and elements of carbon fixation for electrofuel production. The current blueprint of electrofuel projects focuses on autotrophic microorganisms using carbon fixation pathways to consume CO2 directly for the production of energy-dense liquid fuels. Electrofuel production requires complementary expertise of multiple fields such as synthetic biology, metabolic engineering, and microbiology to produce the desire type of fuel from the CO2-fixing species. Detailed biochemical characterization of each autotrophic species with cycle type, target product, and enzyme varieties can help to improve the efficiency of fuel production. —Chieh-Hsin Chen
Hawkins, A.S., McTernan, P.M., Lian, H., Kelly, R.M., Adams, M.W., 2013. Biological conversion of carbon dioxide and hydrogen into liquid fuels and industrial chemicals. Current opinion in biotechnology 24, 376−384
Electrofuel production is the process of using non-photosynthetic microorganisms to convert CO2 directly to energy-dense fuel. There are multiple sources of the electrons needed to power electrofuel process, including H2, formate, carbon monoxide, and electricity. In the paper, the authors focus on the use of hydrogen gas as source of reducing power for CO2 fixation, with hydrogen acting as a reducing agent for electron carriers and production of pyruvate. Though both aerobic and anaerobic process the microorganisms produce storable fuel.
There are currently six different biological pathways occurs spontaneously for carbon fixation. Each pathway has unique features arising from its molecular and biological context, and they differ in efficiency. One of the most common types of CO2-fixation is Calvin-Benson-Bassham (CBB) cycle found in plants, algae, and many bacteria. For example, Ralstonia eutropha is a metabolically diverse autotrophic bacterium that can grow on CO2 and H2; it is able to store excess carbon as polyhydroxyalkanoates (PHA). Wood-Ljundahl (W-L), 3-hudroxypropionate (3HP), 4-hydroxybutyrate (4HB), reductive tricarboxylic acid(rTCA) and dicarboxylates (DC) are also various pathways of CO2-fixation. One important characteristic to the various CO2-fixation pathways is the tolerance of the enzyme and redox carrier molecules to oxygen. Some pathways are found in anaerobic organisms, and these pathways utilize oxygen-sensitive reduced ferredoxins as electron carriers.
The efficiency of CO2-fixation depends on multiple factors. On average, it is found that W-L pathway and rTCA cycle were the most energy efficient routes, while CBB comes in as least efficient. One of the analyses of carbon fixation reaction is examining the ATP requirement and the reduction potential of electron carriers. For example, ferredoxin has much lower reduction potential than NADPH, but the extra energetic contribution from the difference is insignificant compared to the large difference in ATP cost for pyruvate formation between cycles. Another analysis looked into the individual reactions in carbon fixation pathways. There are many half-reactions in the process of carbon fixation, and some of the processes like carboxylation and carboxyl reduction reaction are energetically unfavorable. To increase the efficiency and reduce ATP requirements, some pathways manage to couple unfavorable reactions to exergonic reaction other than ATP hydrolysis. One of the faster ways to speed up a reaction is the use of enzymes; combining enzymes in novel ways may improve the efficiency of the pathway or reduce the energetic cost. The conversion of CO2and hydrogen to electrofuel is also dependent on the use of enzymes, carbonic anhydrase and hydrogenase.
Hydrogenase catalyzes the reversible conversion of molecular hydrogen and protons in the presence of electron carrier. Based on the molecular structure, hydrogenase can be categorized: [NiFe]-hydrogenase, [FeFe]-hydrogenase, [Fe]-hydrogenase. Different microorganisms appear to utilize different type; [NiFe]-hydrogenase is widespread among bacteria and archaea, [FeFe]-hydrogenase is found in anaerobic bacteric and eukaryote, and [Fe]-hydrogenase is found only in certain archaea. In CBB cycle hydrogenase helps catalyzing the oxidation of hydrogen for the reduction of NADP+, where in 3Hp and 4HB cycle hydrogenase catalyze the oxidation of reduced ferredoxin and generates an in gradiet that is used for ATP synthase. Other than the use of hydrogenase, enzyme that enhances the concentration of CO2 also plays a huge role in carbon fixation.
Any fuel or organic product from CO2 that relies on carbon fixation depends on the concentration of CO2 in the environment. Thus the mechanism of increasing the concentration of CO2 has developed to compensate the low ambient CO2 concentration. CBB cycles utilizing RubisCO, 2 CO2-fixing enzyme that has low attraction for CO2 and does not differentiate well between CO2 and the competing substrate O2, especially need this distinct mechanism to avoid unfavorable lost of carbon in photorespiration. Carbon concentrating mechanisms have only been described for CBB cycle because 3HP/4HB uses bicarbonate (HCO3) instead of directly using CO2. However, exploitation of carbon fixing enzyme for microbial eletrofuels can improve the engineering of enzyme.
The key to carbon fixation is the electrofuel-processing microorganism. The host development requires genetic engineer as well as knowledge of gene regulation and metabolism in the target host cell. Electrofuel is promising relatively to poor efficient photosynthetic biofuels, but only a handful of electrofuel organism are developed or reported.