C1-carbon sources for chemical and fuel production by microbial gas fermentation

Utilization of the liquid one carbon feedstocks methanol and formate for acetogenic bioproduction of chemicals and fuels

The fight against climate change requires consideration of carbon as a critical parameter in production systems, with the ultimate aim of creating a truly sustainable circular carbon economy. In this context, microbial bioproduction systems are a promising route to renewably generate value-added chemicals and fuels. Methanol and formate have recently gained interest as microbial one-carbon feedstocks, which can be produced sustainably from carbon dioxide and renewable energy, are easy to store and transport and readily dissolve in aqueous solutions. Acetogenic bacteria are strictly anaerobic microorganisms that can grow autotrophically on molecular hydrogen or use methanol, formate, and carbon monoxide as their sole carbon and energy sources via the Wood-Ljungdahl pathway, the most energetically efficient carbon fixation pathway known to date. Here, known variants of the Wood-Ljungdahl pathway, the physiology of a selection of methylotrophic and formatotrophic acetogens, and emphasize recent advancements in bioprocessing with respect to quantification of acetogen metabolism of methanol and formate as well as research aiming at establishing novel bioprocesses are reviewed. Additionally, the tools available for physiological and metabolic studies as well as for metabolic and genetic engineering are discussed. Finally, the features and constraints that govern the bioenergetics and stoichiometry of acetogen metabolism during growth on methanol and formate are reviewed, and future perspectives of the field discussed. The high energetic efficiency with which acetogens can convert methanol and formate into products renders them highly attractive platform hosts in the circular carbon economy.

Advances in microbial production of geraniol: from metabolic engineering to potential industrial applications

Geraniol, an acyclic monoterpene alcohol, has significant potential applications in various fields, including: food, cosmetics, biofuels, and pharmaceuticals. However, the current sources of geraniol mainly include plant tissue extraction or chemical synthesis, which are unsustainable and suffer severely from high energy consumption and severe environmental problems. The process of microbial production of geraniol has recently undergone vigorous development. Particularly, the sustainable construction of recombinant Escherichia coli (13.2 g/L) and Saccharomyces cerevisiae (5.5 g/L) laid a solid foundation for the microbial production of geraniol. In this review, recent advances in the development of geraniol-producing strains, including: metabolic pathway construction, key enzyme improvement, genetic modification strategies, and cytotoxicity alleviation, are critically summarized. Furthermore, the key challenges in scaling up geraniol production and future perspectives for the development of robust geraniol-producing strains are suggested. This review provides theoretical guidance for the industrial production of geraniol using microbial cell factories.

Engineering bacterial microcompartments to enable sustainable microbial bioproduction from C1 greenhouse gases

One-carbon (C1) greenhouse gases are the primary driver of global climate change. Fermenting these gases into higher-value products is an attractive strategy for climate action and sustainable development. C1 gas-fermenting bacteria are promising chassis organisms, but various technical challenges hinder scale-up to industrial production levels. Bacterial microcompartments (MCPs), proteinaceous organelles that encapsulate enzymatic pathways, may confer several metabolic benefits to increase the industrial potential of these bacteria. Many species of gas-fermenting bacteria are already predicted to natively produce MCPs. Here, we describe how these organelles can be identified and engineered to encapsulate pathways that convert C1 gases into valuable chemical products.

Seven critical challenges in synthetic one-carbon assimilation and their potential solutions

Synthetic C1 assimilation holds the promise of facilitating carbon capture while mitigating greenhouse gas emissions, yet practical implementation in microbial hosts remains relatively limited. Despite substantial progress in pathway design and prototyping, most efforts stay at the proof-of-concept stage, with frequent failures observed even under in vitro conditions. This review identifies seven major barriers constraining the deployment of synthetic C1 metabolism in microorganisms and proposes targeted strategies for overcoming these issues. A primary limitation is the low catalytic activity of carbon-fixing enzymes, particularly carboxylases, which restricts the overall pathway performance. In parallel, challenges in expressing multiple heterologous genes-especially those encoding metal-dependent or oxygen-sensitive enzymes-further hinder pathway functionality. At the systems level, synthetic C1 pathways often exhibit poor flux distribution, limited integration with the host metabolism, accumulation of toxic intermediates, and disruptions in redox and energy balance. These factors collectively reduce biomass formation and compromise product yields in biotechnological setups. Overcoming these interconnected challenges is essential for moving synthetic C1 assimilation beyond conceptual stages and enabling its application in scalable, efficient bioprocesses towards a circular bioeconomy.

Vibrio natriegens: Application of a Fast-Growing Halophilic Bacterium

The fast growth accompanied with high substrate consumption rates and a versatile metabolism paved the way to exploit Vibrio natriegens as unconventional host for biotechnological applications. Meanwhile, a wealth of knowledge on the physiology, the metabolism, and the regulation in this halophilic marine bacterium has been gathered. Sophisticated genetic engineering tools and metabolic models are available and have been applied to engineer production strains and first chassis variants of V. natriegens. In this review, we update the current knowledge on the physiology and the progress in the development of synthetic biology tools and provide an overview of recent advances in metabolic engineering of this promising host. We further discuss future challenges to enhance the application range of V. natriegens.

Biofuel production: exploring renewable energy solutions for a greener future

Biofuel production has emerged as a leading contender in the quest for renewable energy solutions, offering a promising path toward a greener future. This comprehensive state-of-the-art review delves into the current landscape of biofuel production, exploring its potential as a viable alternative to conventional fossil fuels. This study extensively examines various feedstock options, encompassing diverse sources such as plants, algae, and agricultural waste, and investigates the technological advancements driving biofuel production processes. This review highlights the environmental benefits of biofuels, emphasizing their capacity to significantly reduce greenhouse gas emissions compared to those of fossil fuels. Additionally, this study elucidates the role of biofuels in enhancing energy security by decreasing reliance on finite fossil fuel reserves, thereby mitigating vulnerabilities to geopolitical tensions and price fluctuations. The economic prospects associated with biofuel production are also elucidated, encompassing job creation, rural development, and the potential for additional revenue streams for farmers and landowners engaged in biofuel feedstock cultivation. While highlighting the promise of biofuels, the review also addresses the challenges and considerations surrounding their production. Potential issues such as land use competition, resource availability, and sustainability implications are critically evaluated. Responsible implementation, including proper land-use planning, resource management, and adherence to sustainability criteria, is emphasized as critical for the long-term viability of biofuel production. Moreover, the review underscores the importance of ongoing research and development efforts aimed at enhancing biofuel production efficiency, feedstock productivity, and conversion processes. Technological advancements hold the key to increasing biofuel yields, reducing production costs, and improving overall sustainability. This review uniquely synthesizes the latest advancements across the entire spectrum of biofuel production, from feedstock selection to end-use applications. It addresses critical research gaps by providing a comprehensive analysis of emerging technologies, sustainability metrics, and economic viability of various biofuel pathways. Unlike previous reviews, this work offers an integrated perspective on the interplay between technological innovation, environmental impact, and socio-economic factors in biofuel development, thereby providing a holistic framework for future research and policy directions in renewable energy.

Efficient production of itaconic acid from the single-carbon substrate methanol with engineered Komagataella phaffii

BACKGROUND

Amidst the escalating carbon dioxide levels resulting from fossil fuel consumption, there is a pressing need for sustainable, bio-based alternatives to underpin future global economies. Single-carbon feedstocks, derived from CO2, represent promising substrates for biotechnological applications. Especially, methanol is gaining prominence for bio-production of commodity chemicals.

RESULTS

In this study, we show the potential of Komagataella phaffii as a production platform for itaconic acid using methanol as the carbon source. Successful integration of heterologous genes from Aspergillus terreus (cadA, mttA and mfsA) alongside fine-tuning of the mfsA gene expression, led to promising initial itaconic acid titers of 28 g·L-1 after 5 days of fed-batch cultivation. Through the combined efforts of process optimization and strain engineering strategies, we further boosted the itaconic acid production reaching titers of 55 g·L-1 after less than 5 days of methanol feed, while increasing the product yield on methanol from 0.06 g·g-1 to 0.24 g·g-1.

CONCLUSION

Our results highlight the potential of K. phaffii as a methanol-based platform organism for sustainable biochemical production.

From sustainable feedstocks to microbial foods

Climate change-induced alterations in weather patterns, such as frequent and severe heatwaves, cold waves, droughts, floods, heavy rain and storms, are reducing crop yields and agricultural productivity. At the same time, greenhouse gases arising from food production and supply account for almost 30% of anthropogenic emissions. This vicious circle is producing a global food crisis. Sustainable food resources and production systems are needed now, and microbial foods are one possible solution. In this Perspective, we highlight the most promising technologies, and carbon and energy sources, for microbial food production.

Carbon dioxide reduction to high-value chemicals in microbial electrosynthesis system: Biological conversion and regulation strategies

Large emissions of atmospheric carbon dioxide (CO2) are causing climatic and environmental problems. It is crucial to capture and utilize the excess CO2 through diverse methods, among which the microbial electrosynthesis (MES) system has become an attractive and promising technology to mitigate greenhouse effects while reducing CO2 to high-value chemicals. However, the biological conversion and metabolic pathways through microbial catalysis have not been clearly elucidated. This review first introduces the main acetogenic bacteria for CO2 reduction and extracellular electron transfer mechanisms in MES. It then intensively analyzes the CO2 bioconversion pathways and carbon chain elongation processes in MES, together with energy supply and utilization. The factors affecting MES performance, including physical, chemical, and biological aspects, are summarized, and the strategies to promote and regulate bioconversion in MES are explored. Finally, challenges and perspectives concerning microbial electrochemical carbon sequestration are proposed, and suggestions for future research are also provided. This review provides theoretical foundation and technical support for further development and industrial application of MES for CO2 reduction.

Perspectives for Using CO2 as a Feedstock for Biomanufacturing of Fuels and Chemicals

Microbial cell factories offer an eco-friendly alternative for transforming raw materials into commercially valuable products because of their reduced carbon impact compared to conventional industrial procedures. These systems often depend on lignocellulosic feedstocks, mainly pentose and hexose sugars. One major hurdle when utilizing these sugars, especially glucose, is balancing carbon allocation to satisfy energy, cofactor, and other essential component needs for cellular proliferation while maintaining a robust yield. Nearly half or more of this carbon is inevitably lost as CO2 during the biosynthesis of regular metabolic necessities. This loss lowers the production yield and compromises the benefit of reducing greenhouse gas emissions-a fundamental advantage of biomanufacturing. This review paper posits the perspectives of using CO2 from the atmosphere, industrial wastes, or the exhausted gases generated in microbial fermentation as a feedstock for biomanufacturing. Achieving the carbon-neutral or -negative goals is addressed under two main strategies. The one-step strategy uses novel metabolic pathway design and engineering approaches to directly fix the CO2 toward the synthesis of the desired products. Due to the limitation of the yield and efficiency in one-step fixation, the two-step strategy aims to integrate firstly the electrochemical conversion of the exhausted CO2 into C1/C2 products such as formate, methanol, acetate, and ethanol, and a second fermentation process to utilize the CO2-derived C1/C2 chemicals or co-utilize C5/C6 sugars and C1/C2 chemicals for product formation. The potential and challenges of using CO2 as a feedstock for future biomanufacturing of fuels and chemicals are also discussed.

Lactate based caproate production with Clostridium drakei and process control of Acetobacterium woodii via lactate dependent in situ electrolysis

Syngas fermentation processes with acetogens represent a promising process for the reduction of CO₂ emissions alongside bulk chemical production. However, to fully realize this potential the thermodynamic limits of acetogens need to be considered when designing a fermentation process. An adjustable supply of H₂ as electron donor plays a key role in autotrophic product formation. In this study an anaerobic laboratory scale continuously stirred tank reactor was equipped with an All-in-One electrode allowing for in-situ H₂ generation via electrolysis. Furthermore, this system was coupled to online lactate measurements to control the co-culture of a recombinant lactate-producing Acetobacterium woodii strain and a lactate-consuming Clostridium drakei strain to produce caproate. When C. drakei was grown in batch cultivations with lactate as substrate, 1.6 g·L^-1 caproate were produced. Furthermore, lactate production of the A. woodii mutant strain could manually be stopped and reinitiated by controlling the electrolysis. Applying this automated process control, lactate production of the A. woodii mutant strain could be halted to achieve a steady lactate concentration. In a co-culture experiment with the A. woodii mutant strain and the C. drakei strain, the automated process control was able to dynamically react to changing lactate concentrations and adjust H₂ formation respectively. This study confirms the potential of C. drakei as medium chain fatty acid producer in a lactate-mediated, autotrophic co-cultivation with an engineered A. woodii strain. Moreover, the monitoring and control strategy presented in this study reinforces the case for autotrophically produced lactate as a transfer metabolite in defined co-cultivations for value-added chemical production.

Microbial electrosynthesis with Clostridium ljungdahlii benefits from hydrogen electron mediation and permits a greater variety of products

Microbial electrosynthesis (MES) is a very promising technology addressing the challenge of carbon dioxide recycling into organic compounds, which might serve as building blocks for the (bio)chemical industry. However, poor process control and understanding of fundamental aspects such as the microbial extracellular electron transfer (EET) currently limit further developments. In the model acetogen Clostridium ljungdahlii, both direct and indirect electron consumption via hydrogen have been proposed. However, without clarification neither targeted development of the microbial catalyst nor process engineering of MES are possible. In this study, cathodic hydrogen is demonstrated to be the dominating electron source for C. ljungdahlii at electroautotrophic MES allowing for superior growth and biosynthesis, compared to previously reported MES using pure cultures. Hydrogen availability distinctly controlled an either planktonic- or biofilm-dominated lifestyle of C. ljungdahlii. The most robust operation yielded higher planktonic cell densities in a hydrogen mediated process, which demonstrated the uncoupling of growth and biofilm formation. This coincided with an increase of metabolic activity, acetate titers, and production rates (up to 6.06 g L^-1 at 0.11 g L^-1 d^-1). For the first time, MES using C. ljungdahlii was also revealed to deliver other products than acetate in significant amounts: here up to 0.39 g L^-1 glycine or 0.14 g L^-1 ethanolamine. Hence, a deeper comprehension of the electrophysiology of C. ljungdahlii was shown to be key for designing and improving bioprocess strategies in MES research.

Quantification and mitigation of byproduct formation by low-glycerol-producing Saccharomyces cerevisiae strains containing Calvin-cycle enzymes

BACKGROUND

Anaerobic Saccharomyces cerevisiae cultures require glycerol formation to re-oxidize NADH formed in biosynthetic processes. Introduction of the Calvin-cycle enzymes phosphoribulokinase (PRK) and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) has been shown to couple re-oxidation of biosynthetic NADH to ethanol production and improve ethanol yield on sugar in fast-growing batch cultures. Since growth rates in industrial ethanol production processes are not constant, performance of engineered strains was studied in slow-growing cultures.

RESULTS

In slow-growing anaerobic chemostat cultures (D = 0.05 h-1), an engineered PRK/RuBisCO strain produced 80-fold more acetaldehyde and 30-fold more acetate than a reference strain. This observation suggested an imbalance between in vivo activities of PRK/RuBisCO and formation of NADH in biosynthesis. Lowering the copy number of the RuBisCO-encoding cbbm expression cassette from 15 to 2 reduced acetaldehyde and acetate production by 67% and 29%, respectively. Additional C-terminal fusion of a 19-amino-acid tag to PRK reduced its protein level by 13-fold while acetaldehyde and acetate production decreased by 94% and 61%, respectively, relative to the 15 × cbbm strain. These modifications did not affect glycerol production at 0.05 h-1 but caused a 4.6 fold higher glycerol production per amount of biomass in fast-growing (0.29 h-1) anaerobic batch cultures than observed for the 15 × cbbm strain. In another strategy, the promoter of ANB1, whose transcript level positively correlated with growth rate, was used to control PRK synthesis in a 2 × cbbm strain. At 0.05 h-1, this strategy reduced acetaldehyde and acetate production by 79% and 40%, respectively, relative to the 15 × cbbm strain, without affecting glycerol production. The maximum growth rate of the resulting strain equalled that of the reference strain, while its glycerol production was 72% lower.

CONCLUSIONS

Acetaldehyde and acetate formation by slow-growing cultures of engineered S. cerevisiae strains carrying a PRK/RuBisCO bypass of yeast glycolysis was attributed to an in vivo overcapacity of PRK and RuBisCO. Reducing the capacity of PRK and/or RuBisCO was shown to mitigate this undesirable byproduct formation. Use of a growth rate-dependent promoter for PRK expression highlighted the potential of modulating gene expression in engineered strains to respond to growth-rate dynamics in industrial batch processes.

Enhancing acetone production from H2 and CO2 using supplemental electron acceptors in an engineered Moorella thermoacetica

Acetogens grow autotrophically and use hydrogen (H₂) as the energy source to fix carbon dioxide (CO₂). This feature can be applied to gas fermentation, contributing to a circular economy. A challenge is the gain of cellular energy from H₂ oxidation, which is substantially low, especially when acetate formation coupled with ATP production is diverted to other chemicals in engineered strains. Indeed, an engineered strain of the thermophilic acetogen Moorella thermoacetica that produces acetone lost autotrophic growth on H₂ and CO₂. We aimed to recover autotrophic growth and enhance acetone production, in which ATP production was assumed to be a limiting factor, by supplementing with electron acceptors. Among the four selected electron acceptors, thiosulfate and dimethyl sulfoxide (DMSO) enhanced both bacterial growth and acetone titers. DMSO was the most effective and was further analyzed. We showed that DMSO supplementation enhanced intracellular ATP levels, leading to increased acetone production. Although DMSO is an organic compound, it functions as an electron acceptor, not a carbon source. Thus, supplying electron acceptors is a potential strategy to complement the low ATP production caused by metabolic engineering and to improve chemical production from H₂ and CO₂.

Embracing a low-carbon future by the production and marketing of C1 gas protein

Food scarcity and environmental deterioration are two major problems that human populations currently face. Fortunately, the disruptive innovation of raw food materials has been stimulated by the rapid evolution of biomanufacturing. Therefore, it is expected that the new trends in technology will not only alter the natural resource-dependent food production systems and the traditional way of life but also reduce and assimilate the greenhouse gases released into the atmosphere. This review article summarizes the metabolic pathways associated with C1 gas conversion and the production of single-cell protein for animal feed. Moreover, the protein function, worldwide authorization, market access, and methods to overcome challenges in C1 gas assimilation microbial cell factory construction are also provided. With widespread attention and increasing policy support, the production of C1 gas protein will bring more opportunities and make tremendous contributions to our sustainable future.

Recent trends of biotechnological production of polyhydroxyalkanoates from C1 carbon sources

Growing concerns over the use of limited fossil fuels and their negative impacts on the ecological niches have facilitated the exploration of alternative routes. The use of conventional plastic material also negatively impacts the environment. One such green alternative is polyhydroxyalkanoates, which are biodegradable, biocompatible, and environmentally friendly. Recently, researchers have focused on the utilization of waste gases particularly those belonging to C1 sources derived directly from industries and anthropogenic activities, such as carbon dioxide, methane, and methanol as the substrate for polyhydroxyalkanoates production. Consequently, several microorganisms have been exploited to utilize waste gases for their growth and biopolymer accumulation. Methylotrophs such as Methylobacterium organophilum produced highest amount of PHA up to 88% using CH₄ as the sole carbon source and 52-56% with CH₃OH. On the other hand Cupriavidus necator, produced 71-81% of PHA by utilizing CO and CO₂ as a substrate. The present review shows the potential of waste gas valorization as a promising solution for the sustainable production of polyhydroxyalkanoates. Key bottlenecks towards the usage of gaseous substrates obstructing their realization on a large scale and the possible technological solutions were also highlighted. Several strategies for PHA production using C1 gases through fermentation and metabolic engineering approaches are discussed. Microbes such as autotrophs, acetogens, and methanotrophs can produce PHA from CO₂, CO, and CH₄. Therefore, this article presents a vision of C1 gas into bioplastics are prospective strategies with promising potential application, and aspects related to the sustainability of the system.

Recent progress in the engineering of C1-utilizing microbes

The global climate crisis has led to the transition toward the sustainable production of chemicals and fuels with a low carbon footprint. Microbial utilization of one-carbon (C1) substrates, such as carbon dioxide, carbon monoxide, methane, formate, and methanol, may be a promising replacement for the current fossil fuel-based industry. However, natural C1-utilizing microbes are currently unsuitable for industrial applications because of their slow growth and low carbon conversion efficiency, which results in low productivity and yield. Here, we review the recent achievements in engineering C1-utilizing microbes with improved carbon assimilation efficiency and describe the development of synthetic microorganisms by introducing natural C1 assimilation pathways in non-C1-utilizing microbes. Finally, we outline the future directions for realizing the industrial potential of C1-utilizing microbes.

Engineering a Rhodopsin-Based Photo-Electrosynthetic System in Bacteria for CO2 Fixation

A key goal of synthetic biology is to engineer organisms that can use solar energy to convert CO₂ to biomass, chemicals, and fuels. We engineered a light-dependent electron transfer chain by integrating rhodopsin and an electron donor to form a closed redox loop, which drives rhodopsin-dependent CO₂ fixation. A light-driven proton pump comprising Gloeobacter rhodopsin (GR) and its cofactor retinal have been assembled in Ralstonia eutropha (Cupriavidus necator) H16. In the presence of light, this strain fixed inorganic carbon (or bicarbonate) leading to 20% growth enhancement, when formate was used as an electron donor. We found that an electrode from a solar panel can replace organic compounds to serve as the electron donor, mediated by the electron shuttle molecule riboflavin. In this new autotrophic and photo-electrosynthetic system, GR is augmented by an external photocell for reductive CO₂ fixation. We demonstrated that this hybrid photo-electrosynthetic pathway can drive the engineered R. eutropha strain to grow using CO₂ as the sole carbon source. In this system, a bioreactor with only two inputs, light and CO₂, enables the R. eutropha strain to perform a rhodopsin-dependent autotrophic growth. Light energy alone, supplied by a solar panel, can drive the conversion of CO₂ into biomass with a maximum electron transfer efficiency of 20%.

Synthetic metabolism for in vitro acetone biosynthesis driven by ATP regeneration

In vitro ketone production continues to be a challenge due to the biochemical features of the enzymes involved-even when some of them have been extensively characterized (e.g. thiolase from Clostridium acetobutylicum), the assembly of synthetic enzyme cascades still face significant limitations (including issues with protein aggregation and multimerization). Here, we designed and assembled a self-sustaining enzyme cascade with acetone yields close to the theoretical maximum using acetate as the only carbon input. The efficiency of this system was further boosted by coupling the enzymatic sequence to a two-step ATP-regeneration system that enables continuous, cost-effective acetone biosynthesis. Furthermore, simple methods were implemented for purifying the enzymes necessary for this synthetic metabolism, including a first-case example on the isolation of a heterotetrameric acetate:coenzyme A transferase by affinity chromatography.

Recent advances and challenges in the bioconversion of acetate to value-added chemicals

Acetate is a major byproduct of the bioconversion of the greenhouse gas carbon dioxide, pretreatment of lignocellulose biomass, and microbial fermentation. The utilization and valorization of acetate have been emphasized in transforming waste to clean energy and value-added platform chemicals, contributing to the development of a closed carbon loop toward a low-carbon circular bio-economy. Acetate has been used to produce several platform chemicals, including succinate, 3-hydroxypropionate, and itaconic acid, highlighting the potential of acetate to synthesize many biochemicals and biofuels. On the other hand, the yields and titers have not reached the theoretical maximum. Recently, recombinant strain development and pathway regulation have been suggested to overcome this limitation. This review provides insights into the important constraints limiting the yields and titers of the biochemical and metabolic pathways of bacteria capable of metabolizing acetate for acetate bioconversion. The current developments in recombinant strain engineering are also discussed.

The Potential of Sequential Fermentations in Converting C1 Substrates to Higher-Value Products

Today production of (bulk) chemicals and fuels almost exclusively relies on petroleum-based sources, which are connected to greenhouse gas release, fueling climate change. This increases the urgence to develop alternative bio-based technologies and processes. Gaseous and liquid C1 compounds are available at low cost and often occur as waste streams. Acetogenic bacteria can directly use C1 compounds like CO, CO₂, formate or methanol anaerobically, converting them into acetate and ethanol for higher-value biotechnological products. However, these microorganisms possess strict energetic limitations, which in turn pose limitations to their potential for biotechnological applications. Moreover, efficient genetic tools for strain improvement are often missing. However, focusing on the metabolic abilities acetogens provide, they can prodigiously ease these technological disadvantages. Producing acetate and ethanol from C1 compounds can fuel via bio-based intermediates conversion into more energy-demanding, higher-value products, by deploying aerobic organisms that are able to grow with acetate/ethanol as carbon and energy source. Promising new approaches have become available combining these two fermentation steps in sequential approaches, either as separate fermentations or as integrated two-stage fermentation processes. This review aims at introducing, comparing, and evaluating the published approaches of sequential C1 fermentations, delivering a list of promising organisms for the individual fermentation steps and giving an overview of the existing broad spectrum of products based on acetate and ethanol. Understanding of these pioneering approaches allows collecting ideas for new products and may open avenues toward making full use of the technological potential of these concepts for establishment of a sustainable biotechnology.

Biocatalytic conversion of sunlight and carbon dioxide to solar fuels and chemicals

This review discusses the progress in the assembly of photosynthetic biohybrid systems using enzymes and microbes as the biocatalysts which are capable of utilising light to reduce carbon dioxide to solar fuels. We begin by outlining natural photosynthesis, an inspired biomachinery to develop artificial photosystems, and the rationale and motivation to advance and introduce biological substrates to create more novel, and efficient, photosystems. The case studies of various approaches to the development of CO2-reducing microbial semi-artificial photosystems are also summarised, showcasing a variety of methods for hybrid microbial photosystems and their potential. Finally, approaches to investigate the relatively ambiguous electron transfer mechanisms in such photosystems are discussed through the presentation of spectroscopic techniques, eventually leading to what this will mean for the future of microbial hybrid photosystems.

Advances in cathode designs and reactor configurations of microbial electrosynthesis systems to facilitate gas electro-fermentation

In gas fermentation, a range of chemolithoautotrophs fix single-carbon (C1) gases (CO₂ and CO) when H₂ or other reductants are available. Microbial electrosynthesis (MES) enables CO₂ reduction by generating H₂ or reducing equivalents with the sole input of renewable electricity. A combined approach as gas electro-fermentation is attractive for the sustainable production of biofuels and biochemicals utilizing C1 gases. Various platform compounds such as acetate, butyrate, caproate, ethanol, butanol and bioplastics can be produced. However, technological challenges pertaining to the microbe-material interactions such as poor gas-liquid mass transfer, low biomass and biofilm coverage on cathode, low productivities still exist. We are presenting a review on latest developments in MES focusing on the configuration and design of cathodes that can address the challenges and support the gas electro-fermentation. Overall, the opportunities for advancing CO and CO₂-based biochemicals and biofuels production in MES with suitable cathode/reactor design are prospected.

Thermodynamic approach to foresee experimental CO2 reduction to organic compounds

Anaerobic gas fermentation is a promising approach to transform carbon dioxide (CO₂) into chemical building blocks. However, the main operational conditions to enhance the process and its selectivity are still unknown. The main objective of this study was to trigger chain elongation from a joint perspective of thermodynamic and experimental assessment. Thermodynamics revealed that acetic acid formation was the most spontaneous reaction, followed by n-caproic and n-butyric acids, while the doorway for alcohols production was bounded by the selected conditions. Best parameters combinations were applied in three 0.12 L fermenters. Experimentally, n-caproic acid formation was boosted at pH 7, 37 °C, Acetate:Ethanol mass ratio of 1:3 and low H₂ partial pressure. Though these conditions did not match with those required to produce their main substrates, the unification of both perspectives yielded the highest n-caproic acid concentration (>11 g L^-1) so far from simple substrates, accounting for 77 % of the total products.

A clean in-frame knockout system for gene deletion in Acetobacterium woodii

Acetogenic bacteria produce acetate following the fixation of CO₂ via the Wood-Ljungdahl pathway. As such, they represent excellent process organisms for the production of novel chemicals and fuels from this waste greenhouse gas. Acetobacterium woodii is the model acetogen and numerous studies have been conducted investigating its biochemistry, gas consumption and use as a production chassis. However, there are a dearth of available tools for A. woodii gene modification which limits the research options available for genetic studies. Here, the previously proposed Clostridia Roadmap is implemented in A. woodii leading to the derivation of a knockout system for the generation of clean, in-frame deletions. The replicon of the Gram-positive plasmid pCD6 that originated in Clostridioides difficile was identified as being replication-defective in A. woodii, a property that was exploited to construct a pseudo-suicide knockout plasmid which was used to generate an auxotrophic, pyrE mutant. This allowed the subsequent use of a heterologous pyrE gene (from Clostridium acetobutylicum) as a counter selection marker and the deletion of a number of genes by allelic exchange. Specific mutants generated were affected in growth on glucose, fructose and ethanol as a consequence of deletion of fruA, pstG and adhE, respectively.

Metabolic engineering of Clostridium ljungdahlii for the production of hexanol and butanol from CO2 and H2

BACKGROUND

The replacement of fossil fuels and petrochemicals with sustainable alternatives is necessary to mitigate the effects of climate change and also to counteract diminishing fossil resources. Acetogenic microorganisms such as Clostridium spp. are promising sources of fuels and basic chemical precursors because they efficiently utilize CO and CO₂ as carbon source. However the conversion into high titers of butanol and hexanol is challenging.

RESULTS

Using a metabolic engineering approach we transferred a 17.9-kb gene cluster via conjugation, containing 13 genes from C. kluyveri and C. acetobutylicum for butanol and hexanol biosynthesis, into C. ljungdahlii. Plasmid-based expression resulted in 1075 mg L^-1 butanol and 133 mg L^-1 hexanol from fructose in complex medium, and 174 mg L^-1 butanol and 15 mg L^-1 hexanol from gaseous substrate (20% CO₂ and 80% H₂) in minimal medium. Product formation was increased by the genomic integration of the heterologous gene cluster. We confirmed the expression of all 13 enzymes by targeted proteomics and identified potential rate-limiting steps. Then, we removed the first-round selection marker using CRISPR/Cas9 and integrated an additional 7.8 kb gene cluster comprising 6 genes from C. carboxidivorans. This led to a significant increase in the hexanol titer (251 mg L^-1) at the expense of butanol (158 mg L^-1), when grown on CO₂ and H₂ in serum bottles. Fermentation of this strain at 2-L scale produced 109 mg L^-1 butanol and 393 mg L^-1 hexanol.

CONCLUSIONS

We thus confirmed the function of the butanol/hexanol biosynthesis genes and achieved hexanol biosynthesis in the syngas-fermenting species C. ljungdahlii for the first time, reaching the levels produced naturally by C. carboxidivorans. The genomic integration strain produced hexanol without selection and is therefore suitable for continuous fermentation processes.

Design and evaluation of gas fermentation systems for CO2 reduction to C2 and C4 fatty acids: Non-genetic metabolic regulation with pressure, pH and reaction time

Addressing the carbon emissions through microbial mediated fermentation is an emerging interest. Custom designed and fabricated gas fermentation (GF) systems were evaluated to optimize the headspace pressure, pH (6.5, 7.5, and 8.5), fermentation time, and substrate concentration by employing enriched homoacetogenic chemolithoautotrophs in non-genetic approach. Headspace pressure showed marked influence on the metabolic conversion of inorganic carbon to acetic and butyric acids with 26% higher productivity than the control (atmospheric pressure). Maximum volatile fatty acid (VFA) yield of 3.7 g/L was observed at alkaline pH (8.5) under 2 bar pressure at carbon load of 10 g/L, 96 h). Acetic (3.0 g/L) and butyric (0.7 g/L) acids were the major products upon conversion of 85% of the inorganic substrate. A better in-situ buffering (β = 0.048) at pH 8.5 along with higher reductive current (RCC: -4.4 mA) depicted better performance of GF towards CO₂ reduction.

Production of Hexanol as the Main Product Through Syngas Fermentation by Clostridium carboxidivorans P7

The production of hexanol from syngas by acetogens has gained attention as a replacement for petroleum-derived hexanol, which is widely used in the chemical synthesis and plastic industries. However, acetogenic bacteria generally produce C2 compounds (e.g., acetate and ethanol) as the main products. In this study, the gas fermentation conditions favorable for hexanol production were investigated at different temperatures (30-37°C) and CO gas contents (30-70%) in batch gas fermentation. Hexanol production increased from 0.02 to 0.09 g/L when the cultivation temperature was lowered from 37 to 30°C. As the CO content increased from 30 to 70%, the CO consumption rate and hexanol production (yield, titer, and ratio of C6 compound to total products) increased with the CO content. When 70% CO gas was repeatedly provided by flushing the headspace of the bottles at 30°C, the total alcohol production increased to 4.32 g/L at the expense of acids. Notably, hexanol production (1.90 g/L) was higher than that of ethanol (1.20 g/L) and butanol (1.20 g/L); this is the highest level of hexanol produced in gas fermentation to date and the first report of hexanol as the main product. Hexanol production was further enhanced to 2.34 g/L when 2 g/L ethanol was supplemented at the beginning of 70% CO gas refeeding fermentation. Particularly, hexanol productivity was significantly enhanced to 0.18 g/L/day while the supplemented ethanol was consumed, indicating that the conversion of ethanol to acetyl-CoA and reducing equivalents positively affected hexanol production. These optimized culture conditions (gas fermentation at 30°C and refeeding with 70% CO gas) and ethanol supplementation provide an effective and sustainable approach for bio-hexanol production.

Polyhydroxyalkanoate Production by Caenibius tardaugens from Steroidal Endocrine Disruptors

The α-proteobacterium Caenibius tardaugens can use estrogens and androgens as the sole carbon source. These compounds are steroidal endocrine disruptors that are found contaminating soil and aquatic ecosystems. Here, we show that C. tardaugens, which has been considered as a valuable biocatalyst for aerobic steroidal hormone decontamination, is also able to produce polyhydroxyalkanoates (PHA), biodegradable and biocompatible polyesters of increasing biotechnological interest as a sustainable alternative to classical oil-derived polymers. Steroid catabolism yields a significant amount of propionyl-CoA that is metabolically directed towards PHA production through condensation into 3-ketovaleryl-CoA, rendering a PHA rich in 3-hydroxyvalerate. To the best of our knowledge, this is the first report where PHAs are produced from steroids as carbon sources.

Adapting isotopic tracer and metabolic flux analysis approaches to study C1 metabolism

Single-carbon (C1, or one-carbon) substrates are promising feedstocks for sustainable biofuel and biochemical production. Crucial to the goal of engineering C1-utilizing strains for improved production is a quantitative understanding of the organization, regulation and rates of the reactions that underpin C1 metabolism. ¹³C Metabolic flux analysis (MFA) is a well-established platform for interrogating these questions with multi-carbon substrates, and uses the differential labeling of metabolites that results from feeding a substrate with position-specific incorporation of ¹³C in order to infer quantitative fluxes and pathway topology. Adapting isotopic tracer approaches to C1 metabolism, where position-specific substrate labeling is impossible, requires additional experimental considerations. Here we review recent studies that have developed isotopic tracer approaches to overcome the challenge of uniform metabolite labeling and provide quantitative insight into C1 metabolism.

Plastic accumulation during COVID-19: call for another pandemic; bioplastic a step towards this challenge?

Plastic pollution has become a serious transboundary challenge to nature and human health, with estimation of reports published - predicting a twofold increase in plastic waste by 2030. However, due to the COVID-19 pandemic, the excessive use of single-use plastics (including face masks, gloves and personal protective equipment) would possibly exacerbate such forecasts. The transition towards eco-friendly alternatives like bio-based plastics and new emerging sustainable technologies would be vital to deal with future pandemics, even though the use or consumption of plastics has greatly enhanced our quality of life; it is however critical to move towards bioplastics. We cannot deny the fact that bioplastics have some challenges and shortcomings, but still, it is an ideal option for opt. The circular economy is the need of the hour for waste management. Along with all these practices, individual accountability, corporate intervention and government policy are also needed to prevent us from moving from one crisis to the next. Only through cumulative efforts, we will be able to cope up with this problem. This article collected scattered information and data about accumulation of plastic during COVID-19 worldwide. Additionally, this paper illustrates the substitution of petroleum-based plastics with bio-based plastics. Different aspects are discussed, ranging from advantages to challenges in the way of bioplastics.

Strategies for Biosynthesis of C1 Gas-derived Polyhydroxyalkanoates: A review

Biosynthesis of polyhydroxyalkanoates (PHAs) from C1 gases is highly desirable in solving problems such as climate change and microplastic pollution. PHAs are biopolymers synthesized in microbial cells and can be used as alternatives to petroleum-based plastics because of their biodegradability. Because 50% of the cost of PHA production is due to organic carbon sources and salts, the utilization of costless C1 gases as carbon sources is expected to be a promising approach for PHA production. In this review, strategies for PHA production using C1 gases through fermentation and metabolic engineering are discussed. In particular, autotrophs, acetogens, and methanotrophs are strains that can produce PHA from CO₂, CO, and CH₄. In addition, integrated bioprocesses for the efficient utilization of C1 gases are introduced. Biorefinery processes from C1 gas into bioplastics are prospective strategies with promising potential and feasibility to alleviate environmental issues.

Biomethanation of Carbon Monoxide by Hyperthermophilic Artificial Archaeal Co-Cultures

Climate neutral and sustainable energy sources will play a key role in future energy production. Biomethanation by gas to gas conversion of flue gases is one option with regard to renewable energy production. Here, we performed the conversion of synthetic carbon monoxide (CO)-containing flue gases to methane (CH4) by artificial hyperthermophilic archaeal co-cultures, consisting of Thermococcus onnurineus and Methanocaldococcus jannaschii, Methanocaldococcus vulcanius, or Methanocaldococcus villosus. Experiments using both chemically defined and complex media were performed in closed batch setups. Up to 10 mol% CH4 was produced by converting pure CO or synthetic CO-containing industrial waste gases at a high rate using a co-culture of T. onnurineus and M. villosus. These findings are a proof of principle and advance the fields of Archaea Biotechnology, artificial microbial ecosystem design and engineering, industrial waste-gas recycling, and biomethanation.

Butyrate production in the acetogen Eubacterium limosum is dependent on the carbon and energy source

Eubacterium limosum KIST612 is one of the few acetogenic bacteria that has the genes encoding for butyrate synthesis from acetyl-CoA, and indeed, E. limosum KIST612 is known to produce butyrate from CO but not from H2  + CO2 . Butyrate production from CO was only seen in bioreactors with cell recycling or in batch cultures with addition of acetate. Here, we present detailed study on growth of E. limosum KIST612 on different carbon and energy sources with the goal, to find other substrates that lead to butyrate formation. Batch fermentations in serum bottles revealed that acetate was the major product under all conditions investigated. Butyrate formation from the C1 compounds carbon dioxide and hydrogen, carbon monoxide or formate was not observed. However, growth on glucose led to butyrate formation, but only in the stationary growth phase. A maximum of 4.3 mM butyrate was observed, corresponding to a butyrate:glucose ratio of 0.21:1 and a butyrate:acetate ratio of 0.14:1. Interestingly, growth on the C1 substrate methanol also led to butyrate formation in the stationary growth phase with a butyrate:methanol ratio of 0.17:1 and a butyrate:acetate ratio of 0.33:1. Since methanol can be produced chemically from carbon dioxide, this offers the possibility for a combined chemical-biochemical production of butyrate from H2 + CO2 using this acetogenic biocatalyst. With the advent of genetic methods in acetogens, butanol production from methanol maybe possible as well.

Impacts of Syngas Composition on Anaerobic Fermentation

Energy consumption places growing demands on modern lifestyles, which have direct impacts on the world’s natural environment. To attain the levels of sustainability required to avoid further consequences of changes in the climate, alternatives for sustainable production not only of energy but also materials and chemicals must be pursued. In this respect, syngas fermentation has recently attracted much attention, particularly from industries responsible for high levels of greenhouse gas emissions. Syngas can be obtained by thermochemical conversion of biomass, animal waste, coal, municipal solid wastes and other carbonaceous materials, and its composition depends on biomass properties and gasification conditions. It is defined as a gaseous mixture of CO and H2 but, depending on those parameters, it can also contain CO2, CH4 and secondary components, such as tar, oxygen and nitrogenous compounds. Even so, raw syngas can be used by anaerobic bacteria to produce biofuels (ethanol, butanol, etc.) and biochemicals (acetic acid, butyric acid, etc.). This review updates recent work on the influence of biomass properties and gasification parameters on syngas composition and details the influence of these secondary components and CO/H2 molar ratio on microbial metabolism and product formation. Moreover, the main challenges, opportunities and current developments in syngas fermentation are highlighted in this review.

Residual Gas for Ethanol Production by Clostridium carboxidivorans in a Dual Impeller Stirred Tank Bioreactor (STBR)

Recycling residual industrial gases and residual biomass as substrates to biofuel production by fermentation is an important alternative to reduce organic wastes and greenhouse gases emission. Clostridium carboxidivorans can metabolize gaseous substrates as CO and CO2 to produce ethanol and higher alcohols through the Wood-Ljungdahl pathway. However, the syngas fermentation is limited by low mass transfer rates. In this work, a syngas fermentation was carried out in serum glass bottles adding different concentrations of Tween® 80 in ATCC® 2713 culture medium to improve gas-liquid mass transfer. We observed a 200% increase in ethanol production by adding 0.15% (v/v) of the surfactant in the culture medium and a 15% increase in biomass production by adding 0.3% (v/v) of the surfactant in the culture medium. The process was reproduced in stirred tank bioreactor with continuous syngas low flow, and a maximum ethanol productivity of 0.050 g/L.h was achieved.

Autotrophic growth and ethanol production enabled by diverting acetate flux in the metabolically engineered Moorella thermoacetica

Gas fermentation is a promising biological process for the conversion of CO₂ or syngas into valuable chemicals. Homoacetogens are microorganisms growing autotrophically using CO₂ and H₂ or CO and metabolizing them to form acetate coupled with energy conservation. The challenge in the metabolic engineering of the homoacetogens is divergence of the acetate formation, whose intermediate is acetyl-CoA, to a targeted chemical with sufficient production of adenosine triphosphate (ATP). In this study, we report that an engineered strain of the thermophilic homoacetogen Moorella thermoacetica, in which a pool of acetyl-CoA is diverted to ethanol without ATP production, can maintain autotrophic growth on syngas. We estimated the ATP production in the engineered strains under different gaseous compositions by considering redox-balanced metabolism for ethanol and acetate formation. The culture test showed that the combination of retaining a level of acetate production and supplying the energy-rich CO allowed maintenance of the autotrophic growth during ethanol production. In contrast, autotrophy was collapsed by complete elimination of the acetate pathway or supplementation of H₂-CO₂. We showed that the intracellular level of ATP was significantly lowered on H₂-CO₂ in consistent with the incompetence. In the meantime, the complete disruption of the acetate pathway resulted in the redox imbalance to produce ethanol from CO, albeit a small loss in the ATP production. Thus, preservation of a fraction of acetate formation is required to maintain sufficient ATP and balanced redox in CO-containing gases for ethanol production.

Metabolic engineering for the production of butanol, a potential advanced biofuel, from renewable resources

Butanol is an important chemical and potential fuel. For more than 100 years, acetone-butanol-ethanol (ABE) fermentation of Clostridium strains has been the most successful process for biological butanol production. In recent years, other microbes have been engineered to produce butanol as well, among which Escherichia coli was the best one. Considering the crude oil price fluctuation, minimizing the cost of butanol production is of highest priority for its industrial application. Therefore, using cheaper feedstocks instead of pure sugars is an important project. In this review, we summarized butanol production from different renewable resources, such as industrial and food waste, lignocellulosic biomass, syngas and other renewable resources. This review will present the current progress in this field and provide insights for further engineering efforts on renewable butanol production.

Metabolic engineering strategies to enable microbial utilization of C1 feedstocks

One-carbon (C1) substrates are preferred feedstocks for the biomanufacturing industry and have recently gained attention owing to their natural abundance, low production cost and availability as industrial by-products. However, native pathways to utilize these substrates are absent in most biotechnologically relevant microorganisms. Recent advances in synthetic biology, genome engineering and laboratory evolution are enabling the first steps towards the creation of synthetic C1-utilizing microorganisms. Here, we briefly review the native metabolism of methane, methanol, CO₂, CO and formate, and how these C1-utilizing pathways can be engineered into heterologous hosts. In addition, this review analyses the potential, the challenges and the perspectives of C1-based biomanufacturing.

Reconciling the Sustainable Manufacturing of Commodity Chemicals with Feasible Technoeconomic Outcomes : Assessing the investment case for heat integrated aerobic gas fermentation

The manufacturing industry must diverge from a ‘take, make and waste’ linear production paradigm towards more circular economies. Truly sustainable, circular economies are intrinsically tied to renewable resource flows, where vast quantities need to be available at a central point of consumption. Abundant, renewable carbon feedstocks are often structurally complex and recalcitrant, requiring costly pretreatment to harness their potential fully. As such, the heat integration of supercritical water gasification (SCWG) and aerobic gas fermentation unlocks the promise of renewable feedstocks such as lignin. This study models the technoeconomics and life cycle assessment (LCA) for the sustainable production of the commodity chemicals, isopropanol and acetone, from gasified Kraft black liquor. The investment case is underpinned by rigorous process modelling informed by published continuous gas fermentation experimental data. Time series analyses support the price forecasts for the solvent products. Furthermore, a Monte Carlo simulation frames an uncertain boundary for the technoeconomic model. The technoeconomic assessment (TEA) demonstrates that production of commodity chemicals priced at ~US$1000 per tonne is within reach of aerobic gas fermentation. In addition, owing to the sequestration of biogenic carbon into the solvent products, negative greenhouse gas (GHG) emissions are achieved within a cradle-to-gate LCA framework. As such, the heat integrated aerobic gas fermentation platform has promise as a best-in-class technology for the production of a broad spectrum of renewable commodity chemicals.

Dynamic metabolic modelling predicts efficient acetogen-gut bacterium cocultures for CO-to-butyrate conversion

AIMS

While gas-fermenting acetogens have been engineered to secrete non-native metabolites such as butyrate, acetate remains the most thermodynamically favourable product. An alternative to metabolic engineering is to exploit native capabilities for CO-to-acetate conversion by coculturing an acetogen with a second bacterium that provides efficient acetate-butyrate conversion.

METHODS AND RESULTS

We used dynamic metabolic modelling to computationally evaluate the CO-to-butyrate conversion capabilities of candidate coculture systems by exploiting the diversity of human gut bacteria for anaerobic synthesis of butyrate from acetate and ethanol. A preliminary screening procedure based on flux balance analysis was developed to identify 48 gut bacteria which satisfied minimal growth rate and acetate-to-butyrate conversion requirements when cultured on minimal medium containing acetate and a simple sugar not consumed by the paired acetogen. A total of 170 acetogen/gut bacterium/sugar combinations were dynamically simulated for continuous growth using a 70/30 CO/CO₂ feed gas mixture and minimal medium computationally determined for each combination.

CONCLUSIONS

While coculture systems involving the acetogens Eubacterium limosum or Blautia producta yielded low butyrate productivities and CO-to-ethanol conversion had minimal impact on system performance, dynamic simulations predicted a large number of promising coculture designs with Clostridium ljungdahlii or C. autoethanogenum as the CO-to-acetate converter. Pairings with the gut bacterium Clostridium hylemonae or Roseburia hominis were particularly promising due to their ability to generate high butyrate productivities over a range of dilution rates with a variety of sugars. The higher specific acetate secretion rate of C. ljungdahlii proved more beneficial than the elevated growth rate of C. autoethanogenum for coculture butyrate productivity.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our study demonstrated that metabolic modelling could provide useful insights into coculture design that can guide future experimental studies. More specifically, our predictions generated several favourable designs, which could serve as the first coculture systems realized experimentally.

Metabolic engineering of Moorella thermoacetica for thermophilic bioconversion of gaseous substrates to a volatile chemical

Gas fermentation is one of the promising bioprocesses to convert CO₂ or syngas to important chemicals. Thermophilic gas fermentation of volatile chemicals has the potential for the development of consolidated bioprocesses that can simultaneously separate products during fermentation. This study reports the production of acetone from CO₂ and H₂, CO, or syngas by introducing the acetone production pathway using acetyl–coenzyme A (Ac-CoA) and acetate produced via the Wood–Ljungdahl pathway in Moorella thermoacetica. Reducing the carbon flux from Ac-CoA to acetate through genetic engineering successfully enhanced acetone productivity, which varied on the basis of the gas composition. The highest acetone productivity was obtained with CO–H₂, while autotrophic growth collapsed with CO₂–H₂. By adding H₂ to CO, the acetone productivity from the same amount of carbon source increased compared to CO gas only, and the maximum specific acetone production rate also increased from 0.04 to 0.09 g-acetone/g-dry cell/h. Our development of the engineered thermophilic acetogen M. thermoacetica, which grows at a temperature higher than the boiling point of acetone (58 °C), would pave the way for developing a consolidated process with simplified and cost-effective recovery via condensation following gas fermentation.

Investigation of putative genes for the production of medium-chained acids and alcohols in autotrophic acetogenic bacteria

Gas fermentation is a technology for producing platform chemicals as well as fuels and one of the most promising alternatives to petrochemicals. Medium-chained acids and alcohols such as hexanoate and hexanol are particularly interesting due to their versatile application. This study elucidated the pathway of chain elongation in native C6 compound-producing acetogens. Essential genes of Clostridium carboxidivorans for synthesis of medium-chained acids and alcohols were identified in order to demonstrate their catalytic activity in the acetogenic model organism Acetobacterium woodii. Two such gene clusters were identified, which are responsible for conversion of acetyl-CoA to butyryl-CoA by reverse β-oxidation. Using RT-PCR it could be demonstrated that only genes of cluster 1 are expressed constitutively with simultaneous formation of C6 compounds. Based on genes from C. carboxidivorans, a modular hexanoyl-CoA synthesis (hcs) plasmid system was constructed and transferred into A. woodii. With the recombinant A. woodii strains AWO [pPta_hcs1], AWO [pPta_hcs2], AWO [pTet_hcs1], and AWO [pTet_hcs2] butyrate and hexanoate production under heterotrophic (1.22-4.15 mM hexanoate) and autotrophic conditions (0.48-1.56 mM hexanoate) with both hcs clusters could be detected. hcs Cluster 1 from C. carboxidivorans was transferred into the ABE-fermenting strain Clostridium saccharoperbutylacetonicum as well. For further analysis, genes were also cloned into the hcs plasmid system individually. The resulting recombinant C. saccharoperbutylacetonicum strains with just individual genes neither produced hexanoate nor hexanol, but the strains containing the entire gene cluster were capable of chain elongation. A production of 0.8 mM hexanoate and 5.2 mM hexanol in the fermentation with glucose could be observed.

Stepping on the Gas to a Circular Economy: Accelerating Development of Carbon-Negative Chemical Production from Gas Fermentation

Owing to rising levels of greenhouse gases in our atmosphere and oceans, climate change poses significant environmental, economic, and social challenges globally. Technologies that enable carbon capture and conversion of greenhouse gases into useful products will help mitigate climate change by enabling a new circular carbon economy. Gas fermentation usingcarbon-fixing microorganisms offers an economically viable and scalable solution with unique feedstock and product flexibility that has been commercialized recently. We review the state of the art of gas fermentation and discuss opportunities to accelerate future development and rollout. We discuss the current commercial process for conversion of waste gases to ethanol, including the underlying biology, challenges in process scale-up, and progress on genetic tool development and metabolic engineering to expand the product spectrum. We emphasize key enabling technologies to accelerate strain development for acetogens and other nonmodel organisms.