Gaseous Carbon Waste Streams Utilization Status and Research Needs (2019) / Chapter Skim
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5 Biological Utilization of CO2 into Chemicals and Fuels
5 Biological Utilization of CO2 into Chemicals and Fuels
Pages 97-136
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From page 97... ...
This chapter addresses opportunities, challenges, and research needs relevant to utilization technologies that convert CO2 into various products through photosynthetic and nonphotosynthetic means. PHOTOSYNTHETIC APPROACHES TO CARBON DIOXIDE UTILIZATION Approaches Based on Algae While nonphotosynthetic processes for carbon waste gas utilization can produce high biomass yields, these systems suffer from poor life-cycle analyses when compared to photosynthetic methodologies (Vieira et al., 2013)
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The competitive landscape for algal cultivation would shift should these commercial applications prove competitive with current agricultural products such as soy or corn. Despite these advantages, there are also significant challenges to algae cultivation.
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These variables have a considerable impact on capital and operations costs for biological conversion technologies, and it is important to account for resource use and environmental impact when comparing algae cultivation to conventional agriculture or other activities. Approaches Based on Green Algae Green algae, a common term for eukaryotic single cellular photosynthetic organisms deriving from several phyla, are a highly diverse group of algae representing many thousands of identified species.
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. Examples of co-products that may be valorized to improve the economics of biofuel production from algae include dietary protein, polyunsaturated fatty acids, and pigments.
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This suggests green algae has the potential to supplement or replace crops for animal feed with vastly reduced demands for arable land, but studies have not been validated at scale. Polyunsaturated fatty acids.
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, and protein secretion. Furthermore, like many green algae, C
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New and improved genetic tools for green algae genetic manipulation remain a key need for the advancement of algae biomass applications. This includes development of genetic insertion (homologous recombination)
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are generally too low to make commercialization of the technology appealing, however, and these technologies have been demonstrated primarily on small, academic research scales. To move these technologies toward commercialization, it will be critical to reduce the cost of production and further advance tools for genetic manipulation and metabolic engineering of cyanobacteria.
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Free fatty acid production in 7002 results in reduced negative impacts on the cell, and production can be further improved by the nonnative expression of RuBisCO leading to >130 mg/L free fatty acids being produced (Ruffing, 2014)
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PCC 6803 5.5 g/L Isobutanol Synechococcus elongatus PCC 7942 0.5 g/L n-Butanol Clostridium acetobutylicum 2 mg/L/h Synechococcus elongatus PCC 7942 Fatty acids Synechocystis sp. PCC 6803 >130 mg/L Synechococcus sp.
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Synechococcus elongatus PCC 7942 0.8 g/L (after 6 days) Synechocystis sp.
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Bisabolene has applications as a biodiesel candidate and production relies on c arbon flow through the MEP pathway and the expression of a single heterologous gene, (E) -α-bisabolene synthase.
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Malonyl-CoA natively feeds into fatty acid biosynthesis. This production can be aided by streamlining carbon flux to malonyl-CoA, expressing malonyl-CoA reductase, and optimizing nicotinamide adenine dinucleotide phophaste (NADPH)
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Metabolic Engineering of Cyanobacteria One of the primary challenges of chemical production in cyanobacteria is the low titers that result from most pathways. Production is usually limited to the milligram per liter (mg/L)
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While these carbon sources effectively improve titers, this additional carbon is not always directed toward chemical production and is often lost as biomass. Careful rewiring of carbon integration into metabolism can help improve this carbon utilization and act as a carbon sink to improve CO2 fixation (Oliver and Atsumi, 2015; van der Woude et al., 2014)
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One of the greatest challenges to metabolic engineering efforts for cyanobacteria is the careful tailoring of tools to each strain. Key Considerations for Photosynthetic Approaches Whether they involve the cultivation of green algae or cyanobacteria, photosynthetic approaches to carbon dioxide utilization raise a unique set of considerations relevant to their costs, benefits, environmental impacts, and social acceptability.
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3 is an example of an open raceway pond. 1 Photo by IGV Biotech.
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Bed liners are a requirement in some jurisdictions to meet water treatment codes, and the use of any genetically modified species may also mandate such barriers. The cost of bed liners commonly dominates the capital costs of a raceway pond, particularly with increasing size (Rogers et al., 2014)
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Eutrophication is already a serious environmental concern in many areas due to agricultural runoff and municipal wastewater. To prevent algae cultivation from further compounding this problem and even partially address eutrophication, some have advocated the use of municipal and agricultural wastewater as sources of nutrients for algal cultivation (Woertz et al., 2009)
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For example, products derived from coal-fired flue gas are likely to contain more contaminants, making these sources better suited for biofuel applications than for the production of protein for animal feed. NONPHOTOSYNTHETIC APPROACHES TO CARBON DIOXIDE UTILIZATION Commercializing microbial production has always been an economic challenge due to the high cost of carbon feedstocks and low product yields.
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Some nonphotosynthetic organisms can take advantage of low-cost, low-emissions electrons from renewable energy sources, and many can be cultivated with cheap and ubiquitous gaseous carbon feedstocks comprised of hydrogen, CO, and CO2, such as industrial waste gas, biogas, or syngas. There are also significant challenges that need to be addressed in order for technologies based on nonphotosynthetic organisms to reach maturity.
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From page 118... ...
However, metabolic engineering was not applied to the host strain, and reactor conditions were not extensively optimized, especially for scale-up. More research is necessary to overcome the low degree of 2 See https://www.hexaresearch.com/press-release/global-acetone-market.
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. Researchers have investigated the fermentation conditions required to boost ethanol production, such as by optimizing metal cofactor concentrations, pH, and trace nutrients, and sought to optimize bioreactor designs for peak gas-to-liquid volumetric transfer efficiencies (Daniell et al., 2012)
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The issue with this method is that the electron shuttles, such as neutral red, are expensive, add complexity to product recovery, and are unstable and toxic within the host organism. Current research is focused on two main approaches: directly transferring electrons from electrodes to microorganisms and indirectly transferring electrons via electron donors.
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Because the direct bioelectrochemical model does not utilize electron shuttles, the host is required to be in immediate contact with the electrode in order for outer membrane electron transfer proteins to interact with the electrode surface. This represents a critical challenge of this method as there are physical limitations to the amount of electrode surface area that can be made available.
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While production was boosted to 0.78 g/L/h, significantly more research is needed to further advance bioelectrochemical reactor systems, in particular to improve product recovery. Indirect Electron Transfer via Electrochemically Synthesized Electron Donors Electrons can be indirectly transferred from electrodes to microorganisms via electrochemically synthesized electron donors such as H2, formate, ammonia, sulfide, or iron (Lovely and Nevin, 2013)
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Methods based on photosynthetic green algae are relatively mature; while they may be optimized to improve their cost competitiveness, there are opportunities, assuming the appropriate policy and social environment, to create valuable products. Cyanobacteria are a particularly promising platform for biological conversion because of their photosynthetic efficiency and genetic manipulability, but additional research is needed in order to improve titers and make cyanobacteria-based processes scalable and economically viable.
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Fundamental Research Bench Scale Proof of Concept Pilot Scale Demonstration Scale Limited Commercial Implementation Broad Implementation BIO-ELECTROCHEMICAL GREEN ALGAE CYANOBACTERIA CHEMO-LITHOTROPHS SYSTEMS FIGURE 5-3 Stages of research activity for target products of carbon dioxide waste stream biological utilization. The higher density on the diagram indicates larger amounts of research activity in that stage.
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PUFAs: Docosahexaenoic acid (DHA) Scalability, social acceptance Pigments: Astaxanthin Limited market size Cyanobacteria Growth rate, smaller number of synthetic biology tools Fuels: Ethanol, butanol, fatty acids, Low productivity and titers, lack of heptadecane, limonene, bisabolene genetic manipulation tools, scale-up Chemicals: 2,3-butanediol, Low productivity and titers, lack of 1,3-propanediol, ethylene, lactate, genetic manipulation tools, scale-up 3-HP, 3-HB, 4-HB, isoprene, squalene, farnesene Chemolithotrophs Acetogens (feedstock: CO2)
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Genome-Scale Modeling and Improvement of Metabolic Efficiency Research is needed to develop and improve methods for in-depth computational modeling, genetic manipulation, biochemical validation, and fermentative demonstration. This could improve metabolic flux, including carbon dioxide uptake and incorporation, photosynthetic efficiency, metabolic streamlining, and product accumulation.
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Finding 5-5 Polyunsaturated fatty acids are a promising and potentially lucrative product of algal biomass. Pigments may be another valuable algae-based product.
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The life-cycle implications of necessary high purity contaminants present in carbon waste streams will need to be considered.
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2014. Exploring metabolic engineering design principles for the photosynthetic production of lactic acid by Synechocystis sp.
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2017. Improve ment of squalene production from CO2 in Synechococcus elongatus PCC 7942 by metabolic engineering and scalable production in a photobioreactor.
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Metabolic Engineering 34:97-103.
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1997. Power plant flue gas as a source of CO2 for microalgae cultivation: economic impact of different process options.
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2017. Metabolic engineering of the pentose phosphate pathway for enhanced limonene production in the cyanobacterium Synechocysti s sp.
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PCC6803 via heterologous expression of a catabolic pathway from lactic acid- and enterobacteria. Metabolic Engineering 20:121-130.
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2014. Generation and evaluation of a genome-scale metabolic network model of Synechococcus elongatus PCC7942.
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2011. Metabolic engineering of Escherichia coli for direct production of 1,4-butanediol.
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