Enhanced CO2 fixation and lipid production of Chlorella vulgaris through the carbonic anhydrase complex

Improved Solubility and Stability of a Thermostable Carbonic Anhydrase via Fusion with Marine-Derived Intrinsically Disordered Solubility Enhancers

Carbonic anhydrase (CA), an enzyme catalyzing the reversible hydration reaction of carbon dioxide (CO2), is considered a promising biocatalyst for CO2 reduction. The α-CA of Thermovibrio ammonificans (taCA) has emerged as a compelling candidate due to its high thermostability, a critical factor for industrial applications. However, the low-level expression and poor in vitro solubility have hampered further utilization of taCA. Recently, these limitations have been addressed through the fusion of the NEXT tag, a marine-derived, intrinsically disordered small peptide that enhances protein expression and solubility. In this study, the solubility and stability of NEXT-taCA were further investigated. When the linker length between the NEXT tag and the taCA was shortened, the expression level decreased without compromising solubility-enhancing performance. A comparison between the NEXT tag and the NT11 tag demonstrated the NEXT tag's superiority in improving both the expression and solubility of taCA. While the thermostability of taCA was lower than that of the extensively engineered DvCA10, the NEXT-tagged taCA exhibited a 30% improvement in long-term thermostability compared to the untagged taCA, suggesting that enhanced solubility can contribute to enzyme thermostability. Furthermore, the bioprospecting of two intrinsically disordered peptides (Hcr and Hku tags) as novel solubility-enhancing fusion tags was explored, demonstrating their performance in improving the expression and solubility of taCA. These efforts will advance the practical application of taCA and provide tools and insights for enzyme biochemistry and bioengineering.

Biotechnological production of omega-3 fatty acids: current status and future perspectives

Omega-3 fatty acids, including alpha-linolenic acids (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), have shown major health benefits, but the human body's inability to synthesize them has led to the necessity of dietary intake of the products. The omega-3 fatty acid market has grown significantly, with a global market from an estimated USD 2.10 billion in 2020 to a predicted nearly USD 3.61 billion in 2028. However, obtaining a sufficient supply of high-quality and stable omega-3 fatty acids can be challenging. Currently, fish oil serves as the primary source of omega-3 fatty acids in the market, but it has several drawbacks, including high cost, inconsistent product quality, and major uncertainties in its sustainability and ecological impact. Other significant sources of omega-3 fatty acids include plants and microalgae fermentation, but they face similar challenges in reducing manufacturing costs and improving product quality and sustainability. With the advances in synthetic biology, biotechnological production of omega-3 fatty acids via engineered microbial cell factories still offers the best solution to provide a more stable, sustainable, and affordable source of omega-3 fatty acids by overcoming the major issues associated with conventional sources. This review summarizes the current status, key challenges, and future perspectives for the biotechnological production of major omega-3 fatty acids.

Mixotrophy, a more promising culture mode: Multi-faceted elaboration of carbon and energy metabolism mechanisms to optimize microalgae culture

Some mixotrophic microalgae appear to exceed the sum of photoautotrophy and heterotrophy in terms of biomass production. This paper mainly reviews the carbon and energy metabolism of microalgae to reveal the synergistic mechanisms of the mixotrophic mode from multiple aspects. It explains the shortcomings of photoautotrophic and heterotrophic growth, highlighting that the mixotrophic mode is not simply the sum of photoautotrophy and heterotrophy. Specifically, microalgae in mixotrophic mode can be divided into separate parts of photoautotrophic and heterotrophic cultures, and the synergistic parts of photoautotrophic culture enhance aerobic respiration and heterotrophic culture enhance the Calvin cycle. Additionally, this review argues that current deficiencies in mixotrophic culture can be improved by uncovering the synergistic mechanism of the mixotrophic mode, aiming to increase biomass growth and improve quality. This approach will enable the full utilization of advantagesin various fields, and provide research directions for future microalgal culture.

A biotechnological roadmap for decarbonization systems combined into bioenergy production: Prelude of environmental life-cycle assessment

Decarbonization has become a critical issue in recent years due to rising energy demands and diminishing oil resources. Decarbonization systems based on biotechnology have proven to be a cost-effective and environmentally benign technique of lowering carbon emissions. Bioenergy generation is an environmentally friendly technique for mitigating climate change in the energy industry, and it is predicted to play an important role in lowering global carbon emissions. This review essentially provides a new perspective on the unique biotechnological approaches and strategies based decarbonization pathways. Furthermore, the application of genetically engineered microbes in CO₂ biomitigation and energy generation is particularly emphasized. The production of biohydrogen and biomethane via anaerobic digestion techniques has been highlighted in the perspective. In this review, role of microorganisms in bioconversion of CO₂ into different types of bioproducts such as biochemical, biopolymers, biosolvents and biosurfactant was summarized. The current analysis, which includes an in-depth discussion of a biotechnology-based roadmap for the bioeconomy, provides a clear picture of sustainability, forthcoming challenges, and perspectives.

C1-based biomanufacturing: Advances, challenges and perspectives

One-carbon (C1) compounds have emerged as a key research focus due to the growth of metabolic engineering and synthetic biology as affordable and sustainable nonfood sugar feedstocks for energy-efficient and environmentally friendly biomanufacturing. This paper summarizes and discusses current developments in C1 compounds for biomanufacturing. First, two primary groups of microbes that use C1 compounds (native and synthetic) are introduced, and the traits, categorization, and functions of C1 microbes are summarized. Second, engineering strategies for C1 utilization are compiled and reviewed, including reconstruction of C1-utilization pathway, enzyme engineering, cofactor engineering, genome-scale modeling, and adaptive laboratory evolution. Third, a review of C1 compounds' uses in the synthesis of biofuels and high-value compounds is presented. Finally, potential obstacles to C1-based biomanufacturing are highlighted along with future research initiatives.

Rice straw-derived lipid production by HMF/furfural-tolerant oleaginous yeast generated by adaptive laboratory evolution

Research on producing medium- and long-chain hydrocarbons as drop-in biofuels has recently accelerated. In addition, lipids are emerging as precursors for biofuel production, and thus, microbial lipid production utilizing agrowastes is becoming a feasible platform technology. Nonetheless, microorganisms are often inhibited by furan aldehydes in biomass-derived hydrolysates. Accordingly, this study aimed to develop oleaginous yeast strains that can tolerate furan aldehydes for producing lipids as biofuel precursors. Rhodosporidium toruloides was selected as the target for adaptive laboratory evolution. The evolved strain, which was obtained from 16 rounds of subcultures, showed a 2.5-fold higher specific growth rate than the wild-type strain in the presence of furan aldehydes and slightly higher lipid production in rice straw hydrolysate. The results discussed in this study provide insights into the production of lipid production by oleaginous yeast utilizing agrowastes as feedstock to obtain drop-in biofuels and contribute to feasible strategies to address climate crises.

Microbial carbonic anhydrase mediated carbon capture, sequestration & utilization: A sustainable approach to delivering bio-renewables

In the recent scenario, anthropogenic interventions have alarmingly disrupted climatic conditions. The persistent change in the climate necessitates carbon neutrality. Efficient ways of carbon capture and sequestration could be employed for sustainable product generation. Carbonic anhydrase (CA) is an enzyme that reversibly catalyzes the conversion of carbon dioxide to bicarbonate ions, further utilized by cells for metabolic processes. Hence, utilizing CA from microbial sources for carbon sequestration and the corresponding delivery of bio-renewables could be the eco-friendly approach. Consequently, the microbial CA and amine-based carbon capture chemicals are synergistically applied to enhance carbon capture efficiency and eventual utilization. This review comprehends recent developments coupled with engineering techniques, especially in microbial CA, to create integrated systems for CO₂ sequestration. It envisages developing sustainable approaches towards mitigating environmental CO₂ from industries and fossil fuels to generate bio-renewables and other value-added chemicals.

Biotechnological Approaches for Biomass and Lipid Production Using Microalgae Chlorella and Its Future Perspectives

Heavy reliance on fossil fuels has been associated with increased climate disasters. As an alternative, microalgae have been proposed as an effective agent for biomass production. Several advantages of microalgae include faster growth, usage of non-arable land, recovery of nutrients from wastewater, efficient CO₂ capture, and high amount of biomolecules that are valuable for humans. Microalgae Chlorella spp. are a large group of eukaryotic, photosynthetic, unicellular microorganisms with high adaptability to environmental variations. Over the past decades, Chlorella has been used for the large-scale production of biomass. In addition, Chlorella has been actively used in various food industries for improving human health because of its antioxidant, antidiabetic, and immunomodulatory functions. However, the major restrictions in microalgal biofuel technology are the cost-consuming cultivation, processing, and lipid extraction processes. Therefore, various trials have been performed to enhance the biomass productivity and the lipid contents of Chlorella cells. This study provides a comprehensive review of lipid enhancement strategies mainly published in the last five years and aimed at regulating carbon sources, nutrients, stresses, and expression of exogenous genes to improve biomass production and lipid synthesis.

Simultaneous carbon dioxide sequestration and utilization for cadaverine production using dual promoters in engineered Escherichia coli strains

Human carbonic anhydrase II (hCAII) is a rapid-acting zinc-metalloenzyme that catalyzes CO₂ hydration reversibly, with encouraging applications in carbon capture, sequestration, and utilization (CCSU). However, biocatalyst durability is a major challenge. Herein, hCAII is emphasized in 4 different Escherichia coli strains and designated under dual promoters from sigma factor 70 (σ⁷⁰) and heat shock protein (HSP70A) to suppress the usage of inducer and stimulate activity in heat environments. As a result, hCAII under high-efficient dual promoters regulation retained high residual activity in CO₂ biomineralization of 68.8 % after 4 cycles at 40 °C. Moreover, co-expression of CA_C9 with lysine decarboxylase (CadA) simultaneously sequestered CO₂ release up to 95.7 % and increased cadaverine titer from 18.0 to 36.7 g/L by using E. coli MG1655. The remnant biomass from cadaverine synthesis sustained converting CO₂ to 57.9 mg-CaCO₃. Thus, the dual promoters design demonstrated the promising potential for CCSU through simultaneous CO₂ utilization and cadaverine synthesis.

Enhanced recombinant carbonic anhydrase in T7RNAP-equipped Escherichia coli W3110 for carbon capture storage and utilization (CCSU)

Sulfurihydrogenibium yellowstonense carbonic anhydrase (SyCA) is a well-known thermophilic CA for carbon mineralization. To broaden the applications of SyCA, the activity of SyCA was improved through stepwise engineering and in different cultural conditions, as well as extended to co-expression with other enzymes. The engineered W3110 strains with 4 different T7 RNA polymerase levels were employed for SyCA production. As a result, the best strain WT7L cultured in modified M9 medium with temperature shifted from 37 to 30 °C after induction increased SyCA activity to 9122 U/mL. The SyCA whole-cell biocatalyst was successfully applied for carbon capture and storage (CCS) of CaCO₃. Furthermore, SyCA was applied for low-carbon footprint synthesis of 5-aminolevulinic acid (5-ALA) and cadaverine (DAP) by coupling with ALA synthetase (ALAS) and lysine decarboxylase (CadA), suppressing CO₂ release to -6.1 g-CO₂/g-ALA and -2.53 g-CO₂/g-DAP, respectively. Harnessing a highly active SyCA offers a complete strategy for CCSU in a green process.

Recent advances in biofuel production through metabolic engineering

Rising global energy demands and climate crisis has created an unprecedented need for the bio-based circular economy to ensure sustainable development with the minimized carbon footprint. Along with conventional biofuels such as ethanol, microbes can be used to produce advanced biofuels which are equivalent to traditional fuels in their energy efficiencies and are compatible with already established infrastructure and hence can be directly blended in higher proportions without overhauling of the pre-existing setup. Metabolic engineering is at the frontiers to develop microbial chassis for biofuel bio-foundries to meet the industrial needs for clean energy. This review does a thorough inquiry of recent developments in metabolic engineering for increasing titers, rates, and yields (TRY) of biofuel production by engineered microorganisms.

Current progress in lipid-based biofuels: Feedstocks and production technologies

The expanding use of fossil fuels has caused concern in terms of both energy security and environmental issues. Therefore, attempts have been made worldwide to promote the development of renewable energy sources, among which biofuel is especially attractive. Compared to other biofuels, lipid-derived biofuels have a higher energy density and better compatibility with existing infrastructure, and their performance can be readily improved by adjusting the chemical composition of lipid feedstocks. This review thus addresses the intrinsic interactions between lipid feedstocks and lipid-based biofuels, including biodiesel, and renewable equivalents to conventional gasoline, diesel, and jet fuel. Advancements in lipid-associated biofuel technology, as well as the properties and applicability of various lipid sources in terms of biofuel production, are also discussed. Furthermore, current progress in lipid production and profile optimization in the context of plant lipids, microbial lipids, and animal fats are presented to provide a wider context of lipid-based biofuel technology.

Yeast metabolic engineering for carbon dioxide fixation and its application

As numerous industrial bioprocesses rely on yeast fermentation, developing CO₂-fixing yeast strains can be an attractive option toward sustainable industrial processes and carbon neutrality. Recent studies have shown that the expression of ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO) in yeasts, such as Saccharomyces cerevisiae and Kluyveromyces marxianus, enables mixotrophic CO₂ fixation and production of biofuels. Also, the expression of a synthetic Calvin-Benson-Bassham (CBB) cycle including RuBisCO in Pichia pastoris enables autotrophic growth on CO₂. This review highlights recent advances in metabolic engineering strategies to enable CO₂ fixation in yeasts. Also, we discuss the potentials of other natural and synthetic metabolic pathways independent of RuBisCO for developing CO₂-fixing yeast strains capable of producing value-added biochemicals.

Carbonic anhydrase for CO2 capture, conversion and utilization

Carbonic anhydrase (CA) enzymes, catalyzing the CO₂ hydration at a high turnover number, can be employed in expediting CO₂ capture, conversion and utilization to aid in carbon neutrality. Despite extensive research over the last decade, there remain challenges in CA-related technologies due to poor stability and suboptimal use of CAs. Herein, we discuss recent advances in CA stabilization by protein engineering and enzyme immobilization, and shed light on state-of-the-art of in vitro and in vivo CA-mediated CO₂ conversion for improved production of value-added chemicals using CO₂ as a feedstock.

Sustainable production of 4-aminobutyric acid (GABA) and cultivation of Chlorella sorokiniana and Chlorella vulgaris as circular economy

The 4-aminobutyric acid (GABA) is important to produce bio-nylon 4 in biorefineries. First, a glutamate decarboxylase (GAD) was propagated in three different Escherichia coli strains to achieve 100% conversion from 1 M monosodium glutamate after optimization of the process. To make the process greener and more efficient, in situ CO₂ adaptation and citrate feeding strategies to maintain the optimal pH value and 498 g/L of GABA was obtained. However, the process releases the equivalent amount of CO₂. Therefore, CO₂ generated from GABA production was completely sequestered in sodium hydroxide to form bicarbonate and applied in a coupling culture of Chlorella sorokiniana (CS) or Chlorella vulgaris (CV) to increase the biomass when combined with sodium bicarbonate and carbonic anhydrase. Further improvement of 1.65-fold biomass and 1.43-fold lipid content were occurred when supplying GABA to the culture. This integrative process provided the highest GABA production rate without CO₂ release, forming an eco-friendly and carbon-neutral technology.

Non-Photosynthetic CO2 Utilization to Increase Fatty Acid Production in Yarrowia lipolytica

Metabolic engineering of non-photosynthetic microorganisms to increase the utilization of CO₂ has been focused on as a green strategy to convert CO₂ into valuable products such as fatty acids. In this study, a CO₂ utilization pathway involving carbonic anhydrase and biotin carboxylase was formed to recycle CO₂ in the oleaginous yeast Yarrowia lipolytica, thereby increasing the production of fatty acids. In the recombinant strain in which the CO₂ utilization pathway was introduced, the production of fatty acids was 10.7 g/L, which was 1.5-fold higher than that of the wild-type strain. The resulting strain had a 1.4-fold increase in dry cell mass compared to the wild-type strain. In addition, linoleic acid was 47.7% in the fatty acid composition of the final strain, which was increased by 11.6% compared to the wild-type strain. These results can be applied as an essential technology for developing efficient and eco-friendly processes by directly utilizing CO₂.

Revisiting carbon, nitrogen, and phosphorus metabolisms in microalgae for wastewater treatment

Threats posed to humans - including environmental pollution, water scarcity, food shortages, and resource crises drive a new concept to think about wastewater and its treatment. Wastewater is not only a waste but also a source of energy, renewable and/or non-renewable resources, including water itself. The nutrient in wastewater should not only be removed but also need to be upcycled. Microalgae based wastewater treatment has attracted considerable interests because algae have the potential to efficiently redirect nutrients from wastewater to the accumulated algal biomass. Additionally, microalgae are commercialized in human consumption and animal feed owing to their high content of essential amino and fatty acids, vitamins, and pigments. The whole process establishes a circular economy, totally relying on the ability of microalgae to uptake and store nutrients in wastewater, such as carbon (C), nitrogen (N), and phosphorus (P). It makes the study of the mechanisms underlying the uptake and storage of nutrients in microalgae of great interest. This review specifically aims to summarize C, N, and P metabolisms in microalgae for a better understanding of the microalgae-based wastewater treatment from the nutrient uptake pathway, and examine the key physiological factors or the operating conditions related to nutrient metabolisms that may affect the treatment efficiency. At last, I discuss the potential approaches to enhance the overall treatment performance by adjusting the critical parameters for C, N, and P metabolisms.