Available online at www.sciencedirect.com
ScienceDirect Editorial overview: Energy biotechnology Eleftherios Terry Papoutsakis and Jack T Pronk Current Opinion in Biotechnology 2015, 33:viii–xi For a complete overview see the Issue Available online 18th May 2015 http://dx.doi.org/10.1016/j.copbio.2015.04.001 0958-1669/# 2015 Elsevier Ltd. All rights reserved.
Eleftherios Terry Papoutsakis
Department of Chem. & Biomolecular Engineering, Department of Biological Sciences, University of Delaware, Delaware Biotechnology Institute, Newark, DE 19711, USA e-mail: [email protected] Dr. Papoutsakis received his Diploma in Chemical Engineering from the Nat. Technical Univ. of Athens, Greece, and his MS and PhD from Purdue University, IN (USA). Following the completion of his PhD, he started his academic career at Rice University in Houston, Texas as an Assistant Professor, and in 1987 moved to Northwstern University, Evanston, IL, USA, where he was promoted to full professor and eventually appointed to a Walter P. Murphy Chair Professorship. In 2007, he moved to the University of Delaware as Eugene DuPont Chair Professor. Papoutsakis’ group is active and has made important contributions in the areas of clostridia genetics and metabolic engineering; animal-cell biotechnology & stem-cell bioengineering with emphasis in hematopoietic stem-cell engineering. He is widely recognized as a leader in metabolic engineering of the industrial anaerobes clostridia as well as in genome engineering. His lab is interested in developing strains of industrial importance in the biorenewable arena, with emphasis on complex, nonpathway dependent traits.
Biofuels research in the 2015 economic environment: when the going gets tough The importance of metabolic engineering, synthetic biology, and the molecular and biophysical dissection of cell physiology for the development of technologies to produce fuel molecules by biological routes is firmly established. This collection of reviews captures some of the most active areas of research on microbial biofuel production, including promising, emerging concepts. Biofuel production from renewable biomass has attracted an enormous attention worldwide for its promise to pave the way towards energy sustainability. Despite the large and intense efforts and investments of the last 10+ years, it has proven to be difficult to achieve industrial-biofuels production beyond biogas, ethanol and plant-seed derived biodiesel. Production of biofuels from cellulosics is recently facing additional headwinds from two sources. One is the increasing abundance of natural gas worldwide and especially in the US, which has resulted in dramatically reduced methane prices. A more recent one is the dramatic drop in prices of petroleum oil, by more than 50% in the last few months. These developments accentuate the need to relentlessly improve the economics of biofuel production from renewables by addressing high substrate costs, low yields, low titers, and high separation costs. Business as usual clearly is not an option in this competitive environment. Instead, there is a clear need to re-assess the successes and re-direct the priorities to solve the essential issues that prevent bioenergy technologies from reaching the marketplace. We are optimistic that these issues can be resolved, and that intensive innovation of bioenergy technologies will contribute to a sustainable energy generation model for the world. Virtually all issues that limit industrial-scale production of biofuels are explored in this collection: what is the basis of cell toxicity that limits titers of biofuel molecules in microbial cultures; how can we reduce substrate costs (which account for more than 65% of the total production costs); how can we explore new abundant substrates, such as sunlight, organic waste streams, biogas and natural gas, and how can we simplify process configurations by integration of feedstock pretreatment, fermentation and product recovery. At the same time, pathway engineering, including the modification of existing pathways, the development of totally synthetic pathways, and the optimization of new and old pathways for in vivo product fluxes, remains an active area of R&D, and several papers of this collection address critical issues in this field of bioenergy production.
Optimizing the capabilities of native producers based on physiological, energetic and ecological analyses In terms of product volume, ‘first-generation’, yeast-based bioethanol production is currently the largest pure-culture industrial biotechnology Current Opinion in Biotechnology 2015, 33:viii–xi
www.sciencedirect.com
Editorial overview Papoutsakis and Pronk ix
Jack T Pronk
Department of Biotechnology, Delft University of Technology, Delft, The Netherlands e-mail: [email protected] Jack Pronk (1963) holds an MSc from Leiden (Plant Molecular Biology) and a PhD from Delft (Microbiology). In 1991, he joined the Delft Industrial Microbiology group as assistant professor. In 1999, the TU Delft appointed him Antoni van Leeuwenhoek Full Professor. Jack was co-founder and, from 2002 to 2013, scientific director of the Kluyver Centre, a public–private partnership focused on Genomics research on key industrial microorganisms. Jack’s research integrates quantitative analysis of metabolism and its regulation in industrial microorganisms with improvement of their performance via advanced metabolic engineering and laboratory evolution. Research in his group on understanding and engineering of central carbon, redox and energy metabolism in Saccharomyces cerevisiase has contributed to the development of this yeast into a multipurpose cell factory for production of a wide range of compounds.
process. Ethanol production, in which the ethanol yield on the carbohydrate feedstock (e.g., corn starch or cane sugar) is a key cost-determining parameter, has already been intensively optimized over the past decades. The contribution by Gombert and Van Maris ‘‘Improving conversion yield of fermentable sugars into fuel ethanol in 1st generation yeast-based production processes’’ [1] demonstrates that, even in this highly optimized process, engineering of central metabolism has recently led to new strategies for further improvement of conversion efficiencies. In particular, metabolic engineering of redox and energy metabolism in yeast has recently led to significant improvements of the ethanol yield, based on reduction of glycerol and biomass production, respectively. In their exceptional review ‘‘Ethanol production by engineered thermophiles’’ Olson et al. [2] analyze the current state of the art in the development of strains to enable ethanol production at high temperature. Thermophilic ethanol production has been viewed for many years as an important technological goal that could lead to substantially reduced production costs largely deriving from the reduced ethanol separation at higher temperatures. Their analysis shows that while cell engineering has improved the productivities of thermophilic ethanol producers, engineering nonethanol producing thermophiles remains a major challenge. Fast et al. [3] ‘‘Mixed substrate fermentation: novel options for yield improvement in biofuels production’’ engage detailed yield calculations and analysis of the literature to argue for the concept of mixotrophy as essential for the successful engage acetogens in biofuel production. Acetogens are anaerobic prokaryotes that utilize the celebrated Wood-Ljungdhal pathway to grow on mixtures of CO2, CO and H2 (all are syngas components) while producing small amounts of acetate and ethanol and traces a few more metabolites. The ability to fix CO2 and use these syngas components has the potential to not only use waste gases and reduce CO2 emissions from fermentation processes, but, significantly, it can dramatically increase the yields of metabolites from carbohydrates when gases are used as co-substrates. Most currently investigated processes for biofuels production are based on aseptic, pure-culture processes, often involving genetically engineered microorganisms. Mooij et al. ‘‘Ecology-based selective environments as solution to contamination in microalgal cultivation’’ [4] outline a new, radically different approach. Rather than engineering natural producers for improved production, these authors advocate the use of open cultivation systems, in which carefully designed process conditions and feeding regimes favor the selection of high-producing microbial communities. They illustrate this concept by discussing the selection of lipid-high-producing algae (‘survival of the fattest’) via dynamic feeding regimes and conclude that use of simple, open cultivation systems and low cost feedstocks offers attractive possibilities to dramatically reduce the costs for biofuels production.
Synthetic product-formation pathways: chemistries, bottlenecks and implementation challenges Metabolic engineering offers unique possibilities to express pathways for efficient production of naturally occurring compounds in hosts with attractive properties for large-scale industrial production. An industrially relevant and deceptively simple example of this approach is the expression of pathways for the production butanol isomers in bakers’ yeast. In ‘‘Metabolic engineering of Saccharomyces cerevisiae for production of butanol isomers’’ Generoso et al. [5] present an excellent overview of recent progress in research on this intensively studied subject. Their contribution identifies www.sciencedirect.com
Current Opinion in Biotechnology 2015, 33:viii–xi
x Energy biotechnology
subcellular compartmentalization, cofactor usage and project tolerance as key challenges that need to be addressed in order to enable cost-effective, large-scale production of butanol biofuels by yeast. ‘Hooking up’ product pathways to photoautotrophic metabolism holds the long-term perspective of enabling light-driven production of biofuels from carbon dioxide. In the contribution ‘‘Engineering cyanobacteria for direct biofuel production from CO2’’, that is complementary to the microbial-community engineering strategy advocated by Mooij et al., Savakis and Helllingwerf [6] discuss the metabolic engineering of cyanobacteria for biofuels production. While, in recent years, production of many relevant compounds has been demonstrated in genetically engineered strains of cyanobacteria, the authors conclude that product titers and yields are as yet too low to consider them industrially relevant for production of biofuels and commodity chemicals. Lee et al. [7] in their review ‘‘Metabolic engineering for the production of hydrocarbon fuels’’, summarize the fascinating chemistry of biological production of hydrocarbon molecules. This is a relatively new field, but recent advances have demonstrated the power of metabolic engineering and synthetic biology to enable the microbial synthesis of complex hydrocarbon molecules, a task that has been viewed as exotic and unattainable even a few years back.
microorganisms for consolidated bioprocessing’’ Den Haan et al. [9] give an inspiring overview of the current status in this highly relevant domain of biofuels research. While clearly specifying the challenges involved in functional expression of multi-component cellulolytic systems in heterologous hosts and under harsh industrial conditions, the authors are optimistic that these challenges will prove to be surmountable. Innovations in the exploration of natural gas have led to a dramatically increased availability of methane and methanol, with concomitant price reductions. Intensification of biogas production may lead to additional, ecologically sustainable sources of these reduced one-carbon (C1) compounds. Synthetic methylotrophy refers to the development of recombinant microbial strains that can utilize reduced C1 chemicals, and notably methane and methanol. Using these C1 substrates to produce useful metabolites such as biofuels and commodity chemicals could have a tremendous economic impact. While the strictly aerobic native methylotrophs can effectively utilize C1 chemicals, they do not produce extracellular metabolites and cannot be easily engineered. In their review, ‘‘Synthetic methylotrophy: engineering the production of biofuels and chemicals based on the biology of aerobic methanol utilization’’ Whitaker et al. [10] discuss strategies to make Escherichia coli an effective methylotroph as a basis for a new platform technology.
Product toxicity and downstream processing One of the essential challenges in achieving high product yields and titers from engineered complex (i.e., multigene) pathways is expression of expression of genes and their proteins for achieving a balanced pathway with ‘smooth’ carbon and electron flow. Here ‘smooth’ refers to the situation whereby the pathway operates with good flux of chemicals (in the context of comparable natural pathways), without essential limitations or accumulation of intermediates. In nature, in most case, evolutionary selection has optimized the coordinated expression of the genes in metabolic pathways, but this is not an easy task for synthetic or ‘enhanced’ natural pathways. Jones et al. [8] have worked for years on this problem, and in their review ‘‘Metabolic pathway balancing and its role in the production of biofuels and chemicals’’ they discuss strategies for tuning the expression of pathway genes for arriving quickly to the desirable pathway ‘flow’.
Synthetic biology to enable novel substrate capabilities Fast, efficient use of low-cost feedstocks is of paramount importance for economically viable production of biofuels. In the case of (ligno)cellulosic feedstocks, substantial cost reductions can be realized when cellulose hydrolysis and product formation are integrated into a single organism (a concept known as ‘consolidated bioprocessing’). In ‘‘Progress and challenges in the engineering of non-cellulolytic Current Opinion in Biotechnology 2015, 33:viii–xi
Cray et al. [11] have written an excellent review, ‘‘Chaotropicity: a key factor in product tolerance of biofuelproducing microorganisms’’, on a subject of broad and fundamental importance for understanding and engineering tolerance to toxic chemicals. Metabolites that are desirable as fuel molecules, commodity or specialty chemicals are typically mildly to severely toxic to cells, thus inhibiting cell growth and/or viability and preventing the achievement of the necessary high product titers. They argue that understanding the detailed mechanisms of chaotropicity, by which inhibitory molecules destabilize macromolecular systems, will enable practical solutions to the critical issue of product toxicity by rational cell engineering. Replacement of conventional jet fuels and other apolar chemicals by biotechnological alternatives requires their cost-effective microbial production and recovery from large-scale industrial processes. In ‘‘Recent advances in the microbial production and recovery of apolar molecules’’, Cuellar and Van der Wielen [12] review recent progress in this field. Their contribution goes beyond progress in pathway engineering and product tolerance by also specifically focusing on the specific challenges involved in the recovery of apolar compounds from microbial cultures. An important message from this contribution is that rapid progress in metabolic engineering www.sciencedirect.com
Editorial overview Papoutsakis and Pronk xi
should be combined and integrated with process innovation in the down-stream processing of the resulting complex, multi-phase fermentation broths at full industrial scale.
6.
Savakis P, Hellingwerf KJ: Engineering cyanobacteria for direct biofuel production from CO2. Curr Opin Biotechnol 2015, 33:8-14.
7.
Lee SY, Kim HM, Cheon S: Metabolic engineering for the production of hydrocarbon fuels. Curr Opin Biotechnol 2015, 33:15-22.
References
8.
Jones JA, Toparlak O¨D, Koffas MAG: Metabolic pathway balancing and its role in the production of biofuels and chemicals. Curr Opin Biotechnol 2015, 33:52-59.
9.
den Haan R, van Rensburg E, Rose SH, Go¨rgens JF, van Zyl WH: Progress and challenges in the engineering of non-cellulolytic microorganisms for consolidated bioprocessing. Curr Opin Biotechnol 2015, 33:32-38.
1.
2.
3.
Gombert AK, van Maris AJA: Improving conversion yield of fermentable sugars into fuel ethanol in 1st generation yeast-based production processes. Curr Opin Biotechnol 2015, 33:81-86. Olson DG, Sparling R, Lynd LR: Ethanol production by engineered thermophiles. Curr Opin Biotechnol 2015, 33:130-141. Fast AG, Schmidt ED, Jones SW, Tracy BP: Acetogenic mixotrophy: novel options for yield improvement in biofuels and biochemicals production. Curr Opin Biotechnol 2015, 33:60-72.
4.
Mooij PR, Stouten GR, van Loosdrecht MCM, Kleerebezem R: Ecology-based selective environments as solution to contamination in microalgal cultivation. Curr Opin Biotechnol 2015, 33:46-51.
5.
Generoso WC, Schadeweg V, Oreb M, Boles E: Metabolic engineering of Saccharomyces cerevisiae for production of butanol isomers. Curr Opin Biotechnol 2015, 33:1-7.
www.sciencedirect.com
10. Whitaker WB, Sandoval NR, Bennett RK, Fast AG, Papoutsakis ET: Synthetic methylotrophy: engineering the production of biofuels and chemicals based on the biology of aerobic methanol utilization. Curr Opin Biotechnol 2015, 33:165-175. 11. Cray JA, Stevenson A, Ball P, Bankar SB, Eleutherio ECA, Ezeji TC, Singhal RS, Thevelein JM, Timson DJ, Hallsworth JE: Chaotropicity: a key factor in product tolerance of biofuel-producing microorganisms. Curr Opin Biotechnol 2015, 33:228-259. 12. Cuellar MC, van der Wielen LAM: Recent advances in the microbial production and recovery of apolar molecules. Curr Opin Biotechnol 2015, 33:39-45.
Current Opinion in Biotechnology 2015, 33:viii–xi
ScienceDirect Editorial overview: Energy biotechnology Eleftherios Terry Papoutsakis and Jack T Pronk Current Opinion in Biotechnology 2015, 33:viii–xi For a complete overview see the Issue Available online 18th May 2015 http://dx.doi.org/10.1016/j.copbio.2015.04.001 0958-1669/# 2015 Elsevier Ltd. All rights reserved.
Eleftherios Terry Papoutsakis
Department of Chem. & Biomolecular Engineering, Department of Biological Sciences, University of Delaware, Delaware Biotechnology Institute, Newark, DE 19711, USA e-mail: [email protected] Dr. Papoutsakis received his Diploma in Chemical Engineering from the Nat. Technical Univ. of Athens, Greece, and his MS and PhD from Purdue University, IN (USA). Following the completion of his PhD, he started his academic career at Rice University in Houston, Texas as an Assistant Professor, and in 1987 moved to Northwstern University, Evanston, IL, USA, where he was promoted to full professor and eventually appointed to a Walter P. Murphy Chair Professorship. In 2007, he moved to the University of Delaware as Eugene DuPont Chair Professor. Papoutsakis’ group is active and has made important contributions in the areas of clostridia genetics and metabolic engineering; animal-cell biotechnology & stem-cell bioengineering with emphasis in hematopoietic stem-cell engineering. He is widely recognized as a leader in metabolic engineering of the industrial anaerobes clostridia as well as in genome engineering. His lab is interested in developing strains of industrial importance in the biorenewable arena, with emphasis on complex, nonpathway dependent traits.
Biofuels research in the 2015 economic environment: when the going gets tough The importance of metabolic engineering, synthetic biology, and the molecular and biophysical dissection of cell physiology for the development of technologies to produce fuel molecules by biological routes is firmly established. This collection of reviews captures some of the most active areas of research on microbial biofuel production, including promising, emerging concepts. Biofuel production from renewable biomass has attracted an enormous attention worldwide for its promise to pave the way towards energy sustainability. Despite the large and intense efforts and investments of the last 10+ years, it has proven to be difficult to achieve industrial-biofuels production beyond biogas, ethanol and plant-seed derived biodiesel. Production of biofuels from cellulosics is recently facing additional headwinds from two sources. One is the increasing abundance of natural gas worldwide and especially in the US, which has resulted in dramatically reduced methane prices. A more recent one is the dramatic drop in prices of petroleum oil, by more than 50% in the last few months. These developments accentuate the need to relentlessly improve the economics of biofuel production from renewables by addressing high substrate costs, low yields, low titers, and high separation costs. Business as usual clearly is not an option in this competitive environment. Instead, there is a clear need to re-assess the successes and re-direct the priorities to solve the essential issues that prevent bioenergy technologies from reaching the marketplace. We are optimistic that these issues can be resolved, and that intensive innovation of bioenergy technologies will contribute to a sustainable energy generation model for the world. Virtually all issues that limit industrial-scale production of biofuels are explored in this collection: what is the basis of cell toxicity that limits titers of biofuel molecules in microbial cultures; how can we reduce substrate costs (which account for more than 65% of the total production costs); how can we explore new abundant substrates, such as sunlight, organic waste streams, biogas and natural gas, and how can we simplify process configurations by integration of feedstock pretreatment, fermentation and product recovery. At the same time, pathway engineering, including the modification of existing pathways, the development of totally synthetic pathways, and the optimization of new and old pathways for in vivo product fluxes, remains an active area of R&D, and several papers of this collection address critical issues in this field of bioenergy production.
Optimizing the capabilities of native producers based on physiological, energetic and ecological analyses In terms of product volume, ‘first-generation’, yeast-based bioethanol production is currently the largest pure-culture industrial biotechnology Current Opinion in Biotechnology 2015, 33:viii–xi
www.sciencedirect.com
Editorial overview Papoutsakis and Pronk ix
Jack T Pronk
Department of Biotechnology, Delft University of Technology, Delft, The Netherlands e-mail: [email protected] Jack Pronk (1963) holds an MSc from Leiden (Plant Molecular Biology) and a PhD from Delft (Microbiology). In 1991, he joined the Delft Industrial Microbiology group as assistant professor. In 1999, the TU Delft appointed him Antoni van Leeuwenhoek Full Professor. Jack was co-founder and, from 2002 to 2013, scientific director of the Kluyver Centre, a public–private partnership focused on Genomics research on key industrial microorganisms. Jack’s research integrates quantitative analysis of metabolism and its regulation in industrial microorganisms with improvement of their performance via advanced metabolic engineering and laboratory evolution. Research in his group on understanding and engineering of central carbon, redox and energy metabolism in Saccharomyces cerevisiase has contributed to the development of this yeast into a multipurpose cell factory for production of a wide range of compounds.
process. Ethanol production, in which the ethanol yield on the carbohydrate feedstock (e.g., corn starch or cane sugar) is a key cost-determining parameter, has already been intensively optimized over the past decades. The contribution by Gombert and Van Maris ‘‘Improving conversion yield of fermentable sugars into fuel ethanol in 1st generation yeast-based production processes’’ [1] demonstrates that, even in this highly optimized process, engineering of central metabolism has recently led to new strategies for further improvement of conversion efficiencies. In particular, metabolic engineering of redox and energy metabolism in yeast has recently led to significant improvements of the ethanol yield, based on reduction of glycerol and biomass production, respectively. In their exceptional review ‘‘Ethanol production by engineered thermophiles’’ Olson et al. [2] analyze the current state of the art in the development of strains to enable ethanol production at high temperature. Thermophilic ethanol production has been viewed for many years as an important technological goal that could lead to substantially reduced production costs largely deriving from the reduced ethanol separation at higher temperatures. Their analysis shows that while cell engineering has improved the productivities of thermophilic ethanol producers, engineering nonethanol producing thermophiles remains a major challenge. Fast et al. [3] ‘‘Mixed substrate fermentation: novel options for yield improvement in biofuels production’’ engage detailed yield calculations and analysis of the literature to argue for the concept of mixotrophy as essential for the successful engage acetogens in biofuel production. Acetogens are anaerobic prokaryotes that utilize the celebrated Wood-Ljungdhal pathway to grow on mixtures of CO2, CO and H2 (all are syngas components) while producing small amounts of acetate and ethanol and traces a few more metabolites. The ability to fix CO2 and use these syngas components has the potential to not only use waste gases and reduce CO2 emissions from fermentation processes, but, significantly, it can dramatically increase the yields of metabolites from carbohydrates when gases are used as co-substrates. Most currently investigated processes for biofuels production are based on aseptic, pure-culture processes, often involving genetically engineered microorganisms. Mooij et al. ‘‘Ecology-based selective environments as solution to contamination in microalgal cultivation’’ [4] outline a new, radically different approach. Rather than engineering natural producers for improved production, these authors advocate the use of open cultivation systems, in which carefully designed process conditions and feeding regimes favor the selection of high-producing microbial communities. They illustrate this concept by discussing the selection of lipid-high-producing algae (‘survival of the fattest’) via dynamic feeding regimes and conclude that use of simple, open cultivation systems and low cost feedstocks offers attractive possibilities to dramatically reduce the costs for biofuels production.
Synthetic product-formation pathways: chemistries, bottlenecks and implementation challenges Metabolic engineering offers unique possibilities to express pathways for efficient production of naturally occurring compounds in hosts with attractive properties for large-scale industrial production. An industrially relevant and deceptively simple example of this approach is the expression of pathways for the production butanol isomers in bakers’ yeast. In ‘‘Metabolic engineering of Saccharomyces cerevisiae for production of butanol isomers’’ Generoso et al. [5] present an excellent overview of recent progress in research on this intensively studied subject. Their contribution identifies www.sciencedirect.com
Current Opinion in Biotechnology 2015, 33:viii–xi
x Energy biotechnology
subcellular compartmentalization, cofactor usage and project tolerance as key challenges that need to be addressed in order to enable cost-effective, large-scale production of butanol biofuels by yeast. ‘Hooking up’ product pathways to photoautotrophic metabolism holds the long-term perspective of enabling light-driven production of biofuels from carbon dioxide. In the contribution ‘‘Engineering cyanobacteria for direct biofuel production from CO2’’, that is complementary to the microbial-community engineering strategy advocated by Mooij et al., Savakis and Helllingwerf [6] discuss the metabolic engineering of cyanobacteria for biofuels production. While, in recent years, production of many relevant compounds has been demonstrated in genetically engineered strains of cyanobacteria, the authors conclude that product titers and yields are as yet too low to consider them industrially relevant for production of biofuels and commodity chemicals. Lee et al. [7] in their review ‘‘Metabolic engineering for the production of hydrocarbon fuels’’, summarize the fascinating chemistry of biological production of hydrocarbon molecules. This is a relatively new field, but recent advances have demonstrated the power of metabolic engineering and synthetic biology to enable the microbial synthesis of complex hydrocarbon molecules, a task that has been viewed as exotic and unattainable even a few years back.
microorganisms for consolidated bioprocessing’’ Den Haan et al. [9] give an inspiring overview of the current status in this highly relevant domain of biofuels research. While clearly specifying the challenges involved in functional expression of multi-component cellulolytic systems in heterologous hosts and under harsh industrial conditions, the authors are optimistic that these challenges will prove to be surmountable. Innovations in the exploration of natural gas have led to a dramatically increased availability of methane and methanol, with concomitant price reductions. Intensification of biogas production may lead to additional, ecologically sustainable sources of these reduced one-carbon (C1) compounds. Synthetic methylotrophy refers to the development of recombinant microbial strains that can utilize reduced C1 chemicals, and notably methane and methanol. Using these C1 substrates to produce useful metabolites such as biofuels and commodity chemicals could have a tremendous economic impact. While the strictly aerobic native methylotrophs can effectively utilize C1 chemicals, they do not produce extracellular metabolites and cannot be easily engineered. In their review, ‘‘Synthetic methylotrophy: engineering the production of biofuels and chemicals based on the biology of aerobic methanol utilization’’ Whitaker et al. [10] discuss strategies to make Escherichia coli an effective methylotroph as a basis for a new platform technology.
Product toxicity and downstream processing One of the essential challenges in achieving high product yields and titers from engineered complex (i.e., multigene) pathways is expression of expression of genes and their proteins for achieving a balanced pathway with ‘smooth’ carbon and electron flow. Here ‘smooth’ refers to the situation whereby the pathway operates with good flux of chemicals (in the context of comparable natural pathways), without essential limitations or accumulation of intermediates. In nature, in most case, evolutionary selection has optimized the coordinated expression of the genes in metabolic pathways, but this is not an easy task for synthetic or ‘enhanced’ natural pathways. Jones et al. [8] have worked for years on this problem, and in their review ‘‘Metabolic pathway balancing and its role in the production of biofuels and chemicals’’ they discuss strategies for tuning the expression of pathway genes for arriving quickly to the desirable pathway ‘flow’.
Synthetic biology to enable novel substrate capabilities Fast, efficient use of low-cost feedstocks is of paramount importance for economically viable production of biofuels. In the case of (ligno)cellulosic feedstocks, substantial cost reductions can be realized when cellulose hydrolysis and product formation are integrated into a single organism (a concept known as ‘consolidated bioprocessing’). In ‘‘Progress and challenges in the engineering of non-cellulolytic Current Opinion in Biotechnology 2015, 33:viii–xi
Cray et al. [11] have written an excellent review, ‘‘Chaotropicity: a key factor in product tolerance of biofuelproducing microorganisms’’, on a subject of broad and fundamental importance for understanding and engineering tolerance to toxic chemicals. Metabolites that are desirable as fuel molecules, commodity or specialty chemicals are typically mildly to severely toxic to cells, thus inhibiting cell growth and/or viability and preventing the achievement of the necessary high product titers. They argue that understanding the detailed mechanisms of chaotropicity, by which inhibitory molecules destabilize macromolecular systems, will enable practical solutions to the critical issue of product toxicity by rational cell engineering. Replacement of conventional jet fuels and other apolar chemicals by biotechnological alternatives requires their cost-effective microbial production and recovery from large-scale industrial processes. In ‘‘Recent advances in the microbial production and recovery of apolar molecules’’, Cuellar and Van der Wielen [12] review recent progress in this field. Their contribution goes beyond progress in pathway engineering and product tolerance by also specifically focusing on the specific challenges involved in the recovery of apolar compounds from microbial cultures. An important message from this contribution is that rapid progress in metabolic engineering www.sciencedirect.com
Editorial overview Papoutsakis and Pronk xi
should be combined and integrated with process innovation in the down-stream processing of the resulting complex, multi-phase fermentation broths at full industrial scale.
6.
Savakis P, Hellingwerf KJ: Engineering cyanobacteria for direct biofuel production from CO2. Curr Opin Biotechnol 2015, 33:8-14.
7.
Lee SY, Kim HM, Cheon S: Metabolic engineering for the production of hydrocarbon fuels. Curr Opin Biotechnol 2015, 33:15-22.
References
8.
Jones JA, Toparlak O¨D, Koffas MAG: Metabolic pathway balancing and its role in the production of biofuels and chemicals. Curr Opin Biotechnol 2015, 33:52-59.
9.
den Haan R, van Rensburg E, Rose SH, Go¨rgens JF, van Zyl WH: Progress and challenges in the engineering of non-cellulolytic microorganisms for consolidated bioprocessing. Curr Opin Biotechnol 2015, 33:32-38.
1.
2.
3.
Gombert AK, van Maris AJA: Improving conversion yield of fermentable sugars into fuel ethanol in 1st generation yeast-based production processes. Curr Opin Biotechnol 2015, 33:81-86. Olson DG, Sparling R, Lynd LR: Ethanol production by engineered thermophiles. Curr Opin Biotechnol 2015, 33:130-141. Fast AG, Schmidt ED, Jones SW, Tracy BP: Acetogenic mixotrophy: novel options for yield improvement in biofuels and biochemicals production. Curr Opin Biotechnol 2015, 33:60-72.
4.
Mooij PR, Stouten GR, van Loosdrecht MCM, Kleerebezem R: Ecology-based selective environments as solution to contamination in microalgal cultivation. Curr Opin Biotechnol 2015, 33:46-51.
5.
Generoso WC, Schadeweg V, Oreb M, Boles E: Metabolic engineering of Saccharomyces cerevisiae for production of butanol isomers. Curr Opin Biotechnol 2015, 33:1-7.
www.sciencedirect.com
10. Whitaker WB, Sandoval NR, Bennett RK, Fast AG, Papoutsakis ET: Synthetic methylotrophy: engineering the production of biofuels and chemicals based on the biology of aerobic methanol utilization. Curr Opin Biotechnol 2015, 33:165-175. 11. Cray JA, Stevenson A, Ball P, Bankar SB, Eleutherio ECA, Ezeji TC, Singhal RS, Thevelein JM, Timson DJ, Hallsworth JE: Chaotropicity: a key factor in product tolerance of biofuel-producing microorganisms. Curr Opin Biotechnol 2015, 33:228-259. 12. Cuellar MC, van der Wielen LAM: Recent advances in the microbial production and recovery of apolar molecules. Curr Opin Biotechnol 2015, 33:39-45.
Current Opinion in Biotechnology 2015, 33:viii–xi
