Eukaryotic microalgae as hosts for light-driven heterologous isoprenoid production

Strategies for producing high value small molecules in microalgae

Eukaryotic microalgae are a diverse group of organisms that can be used for the sustainable production of a wide range of high value compounds, including lipids, flavours and dyes, bioplastics, and cosmetics. Optimising total biomass production often does not lead to optimal product yield and more sophisticated biphasic growth strategies are needed, introducing specific stresses to induce product synthesis. Genetic tools have been used to increase yields of natural products or to introduce new pathways to algae, and wider deployment of these tools offers promising routes for commercial production of high value compounds utilising minimal inputs.

The Microalgae Chlamydomonas for Bioremediation and Bioproduct Production

The extensive metabolic diversity of microalgae, coupled with their rapid growth rates and cost-effective production, position these organisms as highly promising resources for a wide range of biotechnological applications. These characteristics allow microalgae to address crucial needs in the agricultural, medical, and industrial sectors. Microalgae are proving to be valuable in various fields, including the remediation of diverse wastewater types, the production of biofuels and biofertilizers, and the extraction of various products from their biomass. For decades, the microalga Chlamydomonas has been widely used as a fundamental research model organism in various areas such as photosynthesis, respiration, sulfur and phosphorus metabolism, nitrogen metabolism, and flagella synthesis, among others. However, in recent years, the potential of Chlamydomonas as a biotechnological tool for bioremediation, biofertilization, biomass, and bioproducts production has been increasingly recognized. Bioremediation of wastewater using Chlamydomonas presents significant potential for sustainable reduction in contaminants and facilitates resource recovery and valorization of microalgal biomass, offering important economic benefits. Chlamydomonas has also established itself as a platform for the production of a wide variety of biotechnologically interesting products, such as different types of biofuels, and high-value-added products. The aim of this review is to achieve a comprehensive understanding of the potential of Chlamydomonas in these aspects, and to explore their interrelationship, which would offer significant environmental and biotechnological advantages.

Toward the Exploitation of Sustainable Green Factory: Biotechnology Use of Nannochloropsis spp

Securing food, energy, and raw materials for a growing population is one of the most significant challenges of our century. Algae play a central role as an alternative to plants. Wastewater and flue gas can secure nutrients and CO2 for carbon fixation. Unfortunately, algae domestication is necessary to enhance biomass production and reduce cultivation costs. Nannochloropsis spp. have increased in popularity among microalgae due to their ability to accumulate high amounts of lipids, including PUFAs. Recently, the interest in the use of Nannochloropsis spp. as a green bio-factory for producing high-value products increased proportionally to the advances of synthetic biology and genetic tools in these species. In this review, we summarized the state of the art of current nuclear genetic manipulation techniques and a few examples of their application. The industrial use of Nannochloropsis spp. has not been feasible yet, but genetic tools can finally lead to exploiting this full-of-potential microalga.

Towards Lipid from Microalgae: Products, Biosynthesis, and Genetic Engineering

Microalgae can convert carbon dioxide into organic matter through photosynthesis. Thus, they are considered as an environment-friendly and efficient cell chassis for biologically active metabolites. Microalgal lipids are a class of organic compounds that can be used as raw materials for food, feed, cosmetics, healthcare products, bioenergy, etc., with tremendous potential for commercialization. In this review, we summarized the commercial lipid products from eukaryotic microalgae, and updated the mechanisms of lipid synthesis in microalgae. Moreover, we reviewed the enhancement of lipids, triglycerides, polyunsaturated fatty acids, pigments, and terpenes in microalgae via environmental induction and/or metabolic engineering in the past five years. Collectively, we provided a comprehensive overview of the products, biosynthesis, induced strategies and genetic engineering in microalgal lipids. Meanwhile, the outlook has been presented for the development of microalgal lipids industries, emphasizing the significance of the accurate analysis of lipid bioactivity, as well as the high-throughput screening of microalgae with specific lipids.

The synthetic future of algal genomes

Algae are diverse organisms with significant biotechnological potential for resource circularity. Taking inspiration from fermentative microbes, engineering algal genomes holds promise to broadly expand their application ranges. Advances in genome sequencing with improvements in DNA synthesis and delivery techniques are enabling customized molecular tool development to confer advanced traits to algae. Efforts to redesign and rebuild entire genomes to create fit-for-purpose organisms currently being explored in heterotrophic prokaryotes and eukaryotic microbes could also be applied to photosynthetic algae. Future algal genome engineering will enhance yields of native products and permit the expression of complex biochemical pathways to produce novel metabolites from sustainable inputs. We present a historical perspective on advances in engineering algae, discuss the requisite genetic traits to enable algal genome optimization, take inspiration from whole-genome engineering efforts in other microbes for algal systems, and present candidate algal species in the context of these engineering goals.

Engineered ketocarotenoid biosynthesis in the polyextremophilic red microalga Cyanidioschyzon merolae 10D

The polyextremophilic Cyanidiophyceae are eukaryotic red microalgae with promising biotechnological properties arising from their low pH and elevated temperature requirements which can minimize culture contamination at scale. Cyanidioschyzon merolae 10D is a cell wall deficient species with a fully sequenced genome that is amenable to nuclear transgene integration by targeted homologous recombination. C. merolae maintains a minimal carotenoid profile and here, we sought to determine its capacity for ketocarotenoid accumulation mediated by heterologous expression of a green algal β-carotene ketolase (BKT) and hydroxylase (CHYB). To achieve this, a synthetic transgene expression cassette system was built to integrate and express Chlamydomonas reinhardtii (Cr) sourced enzymes by fusing native C. merolae transcription, translation and chloroplast targeting signals to codon-optimized coding sequences. Chloramphenicol resistance was used to select for the integration of synthetic linear DNAs into a neutral site within the host genome. CrBKT expression caused accumulation of canthaxanthin and adonirubin as major carotenoids while co-expression of CrBKT with CrCHYB generated astaxanthin as the major carotenoid in C. merolae. Unlike green algae and plants, ketocarotenoid accumulation in C. merolae did not reduce total carotenoid contents, but chlorophyll a reduction was observed. Light intensity affected global ratios of all pigments but not individual pigment compositions and phycocyanin contents were not markedly different between parental strain and transformants. Continuous illumination was found to encourage biomass accumulation and all strains could be cultivated in simulated summer conditions from two different extreme desert environments. Our findings present the first example of carotenoid metabolic engineering in a red eukaryotic microalga and open the possibility for use of C. merolae 10D for simultaneous production of phycocyanin and ketocarotenoid pigments.

Engineering Nannochloropsis oceanica for the production of diterpenoid compounds

Photosynthetic microalgae like Nannochloropsis hold enormous potential as sustainable, light-driven biofactories for the production of high-value natural products such as terpenoids. Nannochloropsis oceanica is distinguished as a particularly robust host with extensive genomic and transgenic resources available. Its capacity to grow in wastewater, brackish, and sea waters, coupled with advances in microalgal metabolic engineering, genome editing, and synthetic biology, provides an excellent opportunity. In the present work, we demonstrate how N. oceanica can be engineered to produce the diterpene casbene-an important intermediate in the biosynthesis of pharmacologically relevant macrocyclic diterpenoids. Casbene accumulated after stably expressing and targeting the casbene synthase from Daphne genkwa (DgTPS1) to the algal chloroplast. The engineered strains yielded production titers of up to 0.12 mg g-1 total dry cell weight (DCW) casbene. Heterologous overexpression and chloroplast targeting of two upstream rate-limiting enzymes in the 2-C-methyl- d-erythritol 4-phosphate pathway, Coleus forskohlii 1-deoxy- d-xylulose-5-phosphate synthase and geranylgeranyl diphosphate synthase genes, further enhanced the yield of casbene to a titer up to 1.80 mg g-1 DCW. The results presented here form a basis for further development and production of complex plant diterpenoids in microalgae.

Metabolic engineering and synthetic biology strategies for producing high-value natural pigments in Microalgae

Microalgae are microorganisms capable of producing bioactive compounds using photosynthesis. Microalgae contain a variety of high value-added natural pigments such as carotenoids, phycobilins, and chlorophylls. These pigments play an important role in many areas such as food, pharmaceuticals, and cosmetics. Natural pigments have a health value that is unmatched by synthetic pigments. However, the current commercial production of natural pigments from microalgae is not able to meet the growing market demand. The use of metabolic engineering and synthetic biological strategies to improve the production performance of microalgal cell factories is essential to promote the large-scale production of high-value pigments from microalgae. This paper reviews the health and economic values, the applications, and the synthesis pathways of microalgal pigments. Overall, this review aims to highlight the latest research progress in metabolic engineering and synthetic biology in constructing engineered strains of microalgae with high-value pigments and the application of CRISPR technology and multi-omics in this context. Finally, we conclude with a discussion on the bottlenecks and challenges of microalgal pigment production and their future development prospects.

Promoting Photosynthetic Production of Dammarenediol-II in Chlamydomonas reinhardtii via Gene Loading and Culture Optimization

Ginsenosides are major bioactive compounds found in Panax ginseng that exhibit various pharmaceutical properties. Dammarenediol-II, the nucleus of dammarane-type ginsenosides, is a promising candidate for pharmacologically active triterpenes. Dammarenediol-II synthase (DDS) cyclizes 2,3-oxidosqualene to produce dammarenediol-II. Based on the native terpenoids synthetic pathway, a dammarane-type ginsenosides synthetic pathway was established in Chlamydomonas reinhardtii by introducing P. ginseng PgDDS, CYP450 enzyme (PgCYP716A47), or/and Arabidopsis thaliana NADPH-cytochrome P450 reductase gene (AtCPR), which is responsible for producing dammarane-type ginsenosides. To enhance productivity, strategies such as "gene loading" and "culture optimizing" were employed. Multiple copies of transgene expression cassettes were introduced into the genome to increase the expression of the key rate-limiting enzyme gene, PgDDS, significantly improving the titer of dammarenediol-II to approximately 0.2 mg/L. Following the culture optimization in an opt2 medium supplemented with 1.5 mM methyl jasmonate under a light:dark regimen, the titer of dammarenediol-II increased more than 13-fold to approximately 2.6 mg/L. The C. reinhardtii strains engineered in this study constitute a good platform for the further production of ginsenosides in microalgae.

Engineered production of isoprene from the model green microalga Chlamydomonas reinhardtii

Isoprene is a clear, colorless, volatile 5-carbon hydrocarbon that is one monomer of all cellular isoprenoids and a platform chemical with multiple applications in industry. Many plants have evolved isoprene synthases (IspSs) with the capacity to liberate isoprene from dimethylallyl diphosphate (DMADP) as part of cellular thermotolerance mechanisms. Isoprene is hydrophobic and volatile, rapidly leaves plant tissues and is one of the main carbon emission sources from vegetation globally. The universality of isoprenoid metabolism allows volatile isoprene production from microbes expressing heterologous IspSs. Here, we compared heterologous overexpression from the nuclear genome and localization into the plastid of four plant terpene synthases (TPs) in the green microalga Chlamydomonas reinhardtii. Using sealed vial mixotrophic cultivation, direct quantification of isoprene production was achieved from the headspace of living cultures, with the highest isoprene production observed in algae expressing the Ipomoea batatas IspS. Perturbations of the downstream carotenoid pathway through keto carotenoid biosynthesis enhanced isoprene titers, which could be further enhanced by increasing flux towards DMADP through heterologous co-expression of a yeast isopentenyl-DP delta isomerase. Multiplexed controlled-environment testing revealed that cultivation temperature, rather than illumination intensity, was the main factor affecting isoprene yield from the engineered alga. This is the first report of heterologous isoprene production from a eukaryotic alga and sets a foundation for further exploration of carbon conversion to this commodity chemical.

Metabolic Engineering for Efficient Ketocarotenoid Accumulation in the Green Microalga Chlamydomonas reinhardtii

Astaxanthin is a valuable ketocarotenoid with various pharmaceutical and nutraceutical applications. Green microalgae harbor natural capacities for pigment accumulation due to their 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway. Recently, a redesigned ß-carotene ketolase (BKT) was found to enable ketocarotenoid accumulation in the model microalga Chlamydomonas reinhardtii, and transformants exhibited reduced photoinhibition under high-light. Here, a systematic screening by synthetic transgene design of carotenoid pathway enzymes and overexpression from the nuclear genome identified phytoene synthase (PSY/crtB) as a bottleneck for carotenoid accumulation in C. reinhardtii. Increased ß-carotene hydroxylase (CHYB) activity was found to be essential for engineered astaxanthin accumulation. A combined BKT, crtB, and CHYB expression strategy resulted in a volumetric astaxanthin production of 9.5 ± 0.3 mg L^-1 (4.5 ± 0.1 mg g^-1 CDW) in mixotrophic and 23.5 mg L^-1 (1.09 mg L^-1 h^-1) in high cell density conditions, a 4-fold increase compared to previous reports in C. reinhardtii. This work presents a systematic investigation of bottlenecks in astaxanthin accumulation in C. reinhardtii and the phototrophic green cell factory design for competitive use in industrial biotechnology.

Chloroplast engineering of the green microalgae Chlamydomonas reinhardtii for the production of HAA, the lipid moiety of rhamnolipid biosurfactants

Hydroxyalkanoyloxyalkanoates (HAA) are lipidic surfactants with a number of potential applications, but more remarkably, they are the biosynthetic precursors of rhamnolipids (RL), which are preferred biosurfactants thanks to their excellent physicochemical properties, biological activities, and environmental biodegradability. Because the natural highest producer of RLs is the pathogenic bacterium Pseudomonas aeruginosa, important efforts have been dedicated to transfer production to heterologous non-pathogenic microorganisms. Unicellular photosynthetic microalgae are emerging as important hosts for sustainable industrial biotechnology due to their ability to transform CO₂ efficiently into biomass and bioproducts of interest. Here, we have explored the potential of the eukaryotic green microalgae Chlamydomonas reinhardtii as a chassis to produce RLs. Chloroplast genome engineering allowed the stable functional expression of the gene encoding RhlA acyltransferase from P. aeruginosa, an enzyme catalyzing the condensation of two 3-hydroxyacyl acid intermediaries in the fatty acid synthase cycle, to produce HAA. Four congeners of varying chain lengths were identified and quantified by UHPLC-QTOF mass spectrometry and gas chromatography, including C₁₀-C₁₀ and C₁₀-C₈, and the less abundant C₁₀-C₁₂ and C₁₀-C₆ congeners. HAA was present in the intracellular fraction, but also showed increased accumulation in the extracellular medium. Moreover, HAA production was also observed under photoautotrophic conditions based on atmospheric CO₂. These results establish that RhlA is active in the chloroplast and is able to produce a new pool of HAA in a eukaryotic host. Subsequent engineering of microalgal strains should contribute to the development of an alternative clean, safe and cost-effective platform for the sustainable production of RLs. DATA AVAILABILITY: The data that support the findings of this study are available from the corresponding authors upon reasonable request.

Biomass generation and heterologous isoprenoid milking from engineered microalgae grown in anaerobic membrane bioreactor effluent

Wastewater (WW) treatment in anaerobic membrane bioreactors (AnMBR) is considered more sustainable than in aerobic reactors. However, outputs from AnMBR are a mixed methane and carbon dioxide gas stream as well as ammonium- (N) and phosphate- (P) containing waters. Using AnMBR outputs as inputs for photoautotrophic algal cultivation can strip the CO₂ while removing N and P from effluent which feed algal biomass generation. Recent advances in algal engineering have generated strains that produce high-value side products concomitant with biomass, although only shown in heavily domesticated, lab-adapted strains. Here, it was investigated whether engineered Chlamydomonas reinhardtii could be grown directly in AnMBR effluent with CO₂ concentrations found in AnMBR off-gas. The strain was found to proliferate over bacteria in the non-sterile effluent, consume N and P to levels that meet general discharge or reuse limits, and tolerate cultivation in modelled (extreme) outdoor environmental conditions prevalent along the central Red Sea coast. In addition to ∼2.4 g CDW L^-1 biomass production in 96 h, a high-value heterologous sesquiterpene co-product could be obtained from 'milking' up to 837 µg L^-1 culture in 96 h. This is the first demonstration of a combined bio-process that employs a heavily engineered algal strain to enhance the product generation potentials from AnMBR effluent treatment. This study shows it is possible to convert waste into value through use of engineered algae while also improving wastewater treatment economics through co-product generation.

Chlamydomonas reinhardtii mutants deficient for Old Yellow Enzyme 3 exhibit increased photooxidative stress

Old Yellow Enzymes (OYEs) are flavin-containing ene-reductases that have been intensely studied with regard to their biotechnological potential for sustainable chemical syntheses. OYE-encoding genes are found throughout the domains of life, but their physiological role is mostly unknown, one reason for this being the promiscuity of most ene-reductases studied to date. The unicellular green alga Chlamydomonas reinhardtii possesses four genes coding for OYEs, three of which we have analyzed biochemically before. Ene-reductase CrOYE3 stood out in that it showed an unusually narrow substrate scope and converted N-methylmaleimide (NMI) with high rates. This was recapitulated in a C. reinhardtii croye3 mutant that, in contrast to the wild type, hardly degraded externally added NMI. Here we show that CrOYE3-mediated NMI conversion depends on electrons generated photosynthetically by photosystem II (PSII) and that the croye3 mutant exhibits slightly decreased photochemical quenching in high light. Non-photochemical quenching is strongly impaired in this mutant, and it shows enhanced oxidative stress. The phenotypes of the mutant suggest that C. reinhardtii CrOYE3 is involved in the protection against photooxidative stress, possibly by converting reactive carbonyl species derived from lipid peroxides or maleimides from tetrapyrrole degradation.

A review on design-build-test-learn cycle to potentiate progress in isoprenoid engineering of photosynthetic microalgae

Currently, the generation of isoprenoid factories in microalgae relies on two strategies: 1) enhanced production of endogenous isoprenoids; or 2) production of heterologous terpenes by metabolic engineering. Nevertheless, low titers and productivity are still a feature of isoprenoid biotechnology and need to be addressed. In this context, the mechanisms underlying isoprenoid biosynthesis in microalgae and its relationship with central carbon metabolism are reviewed. Developments in microalgal biotechnology are discussed, and a new approach of integrated "design-build-test-learn" cycle is advocated to the trends, challenges and prospects involved in isoprenoid engineering. The emerging and promising strategies and tools are discussed for microalgal engineering in the future. This review encourages a systematic engineering perspective aimed at potentiating progress in isoprenoid engineering of photosynthetic microalgae.

Teaching an old ‘doc’ new tricks for algal biotechnology: Strategic filter use enables multi-scale fluorescent protein signal detection

Fluorescent proteins (FPs) are powerful reporters with a broad range of applications in gene expression and subcellular localization. High-throughput screening is often required to identify individual transformed cell lines in organisms that favor non-homologous-end-joining integration of transgenes into genomes, like in the model green microalga Chlamydomonas reinhardtii. Strategic transgene design, including genetic fusion of transgenes to FPs, and strain domestication have aided engineering efforts in this host but have not removed the need for screening large numbers of transformants to identify those with robust transgene expression levels. FPs facilitate transformant screening by providing a visual signal indicating transgene expression. However, limited combinations of FPs have been described in alga and inherent background fluorescence from cell pigments can hinder FP detection efforts depending on available infrastructure. Here, an updated set of algal nuclear genome-domesticated plasmid parts for seven FPs and six epitope tags were generated and tested in C. reinhardtii. Strategic filter selection was found to enable detection of up to five independent FPs signals from cyan to far-red separately from inherent chlorophyll fluorescence in live algae at the agar plate-level and also in protein electrophoresis gels. This work presents technical advances for algal engineering that can assist reporter detection efforts in other photosynthetic host cells or organisms with inherent background fluorescence.

Metabolic Engineering of the Isopentenol Utilization Pathway Enhanced the Production of Terpenoids in Chlamydomonas reinhardtii

Eukaryotic green microalgae show considerable promise for the sustainable light-driven biosynthesis of high-value fine chemicals, especially terpenoids because of their fast and inexpensive phototrophic growth. Here, the novel isopentenol utilization pathway (IUP) was introduced into Chlamydomonas reinhardtii to enhance the hemiterpene (isopentenyl pyrophosphate, IPP) titers. Then, diphosphate isomerase (IDI) and limonene synthase (MsLS) were further inserted for limonene production. Transgenic algae showed 8.6-fold increase in IPP compared with the wild type, and 23-fold increase in limonene production compared with a single MsLS expressing strain. Following the culture optimization, the highest limonene production reached 117 µg/L, when the strain was cultured in a opt2 medium supplemented with 10 mM isoprenol under a light: dark regimen. This demonstrates that transgenic algae expressing the IUP represent an ideal chassis for the high-value terpenoid production. The IUP will facilitate further the metabolic and enzyme engineering to enhance the terpenoid titers by significantly reducing the number of enzyme steps required for an optimal biosynthesis.

Farnesyl pyrophosphate compartmentalization in the green microalga Chlamydomonas reinhardtii during heterologous (E)-α-bisabolene production

Background

Eukaryotic algae have recently emerged as hosts for metabolic engineering efforts to generate heterologous isoprenoids. Isoprenoid metabolic architectures, flux, subcellular localization, and transport dynamics have not yet been fully elucidated in algal hosts.

Results

In this study, we investigated the accessibility of different isoprenoid precursor pools for C₁₅ sesquiterpenoid generation in the cytoplasm and chloroplast of Chlamydomonas reinhardtii using the Abies grandis bisabolene synthase (AgBS) as a reporter. The abundance of the C₁₅ sesquiterpene precursor farnesyl pyrophosphate (FPP) was not increased in the cytosol by co-expression and fusion of AgBS with different FPP synthases (FPPSs), indicating limited C₅ precursor availability in the cytoplasm. However, FPP was shown to be available in the plastid stroma, where bisabolene titers could be improved several-fold by FPPSs. Sesquiterpene production was greatest when AgBS-FPPS fusions were directed to the plastid and could further be improved by increasing the gene dosage. During scale-up cultivation with different carbon sources and light regimes, specific sesquiterpene productivities from the plastid were highest with CO₂ as the only carbon source and light:dark illumination cycles. Potential prenyl unit transporters are proposed based on bioinformatic analyses, which may be in part responsible for our observations.

Conclusions

Our findings indicate that the algal chloroplast can be harnessed in addition to the cytosol to exploit the full potential of algae as green cell factories for non-native sesquiterpenoid generation. Identification of a prenyl transporter may be leveraged for further extending this capacity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01910-5.

Over-expression of a cyanobacterial gene for 1-deoxy-d-xylulose-5-phosphate synthase in the chloroplast of Chlamydomonas reinhardtii perturbs chlorophyll: carotenoid ratios

Terpenoids are a diverse class of naturally occurring compounds consisting of more than 50,000 structurally different molecules and are found in all living organisms. Many terpenoid compounds, in particular those isolated from plants, have applications in various commercial sectors including medicine, agriculture and cosmetics. However, these high value terpenoids are produced in relatively small quantities in their natural hosts and their chemical synthesis for large scale production is costly and complicated. Therefore, there is much focus on producing these compounds in novel biological hosts using metabolic engineering technologies. As a photosynthetic system, the unicellular green alga C. reinhardtii is of particular interest as the most well-studied model alga with well-established molecular tools for genetic manipulation. However, the direct manipulation of terpenoid biosynthetic pathways in C. reinhardtii necessitates a thorough understanding of the basic terpenoid metabolism. To gain a better understanding of the methylerythritol phosphate (MEP) pathway that leads to terpenoid biosynthesis in the chloroplast of C. reinhardtii, hence this study has investigated the effect of over-expressing 1-deoxy-d-xylulose-5-phosphate synthase (DXS) on plastidic downstream terpenoids. We produced marker-free chloroplast transformants of C. reinhardtii lines that express an additional cyanobacterial gene for DXS. The analysis of terpenoid content for the transgenic line demonstrates that overexpressing DXS resulted in a two-fold decrease in the chlorophyll levels while carotenoid levels showed variable changes: zeaxanthin and antherxanthin levels increased several-fold, lutein levels dropped to approximately half, but β-carotene and violaxanthin did not show a significant change.

Natural carbon fixation and advances in synthetic engineering for redesigning and creating new fixation pathways

BACKGROUND

Autotrophic carbon fixation is the primary route through which organic carbon enters the biosphere, and it is a key step in the biogeochemical carbon cycle. The Calvin-Benson-Bassham pathway, which is predominantly found in plants, algae, and some bacteria (mainly cyanobacteria), was previously considered to be the sole carbon-fixation pathway. However, the discovery of a new carbon-fixation pathway in sulfurous green bacteria almost two decades ago encouraged further research on previously overlooked ancient carbon-fixation pathways in taxonomically and phylogenetically distinct microorganisms.

AIM OF REVIEW

In this review, we summarize the six known natural carbon-fixation pathways and outline the newly proposed additions to this list. We also discuss the recent achievements in synthetic carbon fixation and the importance of the metabolism of thermophilic microorganisms in this field.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Currently, at least six carbon-fixation routes have been confirmed in Bacteria and Archaea. Other possible candidate routes have also been suggested on the basis of emerging "omics" data analyses, expanding our knowledge and stimulating discussions on the importance of these pathways in the way organisms acquire carbon. Notably, the currently known natural fixation routes cannot balance the excessive anthropogenic carbon emissions in a highly unbalanced global carbon cycle. Therefore, significant efforts have also been made to improve the existing carbon-fixation pathways and/or design new efficient in vitro and in vivo synthetic pathways.

Engineering a powerful green cell factory for robust photoautotrophic diterpenoid production

Diterpenoids display a large and structurally diverse class of natural compounds mainly found as specialized plant metabolites. Due to their diverse biological functions they represent an essential source for various industrially relevant applications as biopharmaceuticals, nutraceuticals, and fragrances. However, commercial production utilizing their native hosts is inhibited by low abundances, limited cultivability, and challenging extraction, while the precise stereochemistry displays a particular challenge for chemical synthesis. Due to a high carbon flux through their native 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway towards photosynthetically active pigments, green microalgae hold great potential as efficient and sustainable heterologous chassis for sustainable biosynthesis of plant-derived diterpenoids. In this study, innovative synthetic biology and efficient metabolic engineering strategies were systematically combined to re-direct the metabolic flux through the MEP pathway for efficient heterologous diterpenoid synthesis in C. reinhardtii. Engineering of the 1-Deoxy-D-xylulose 5-phosphate synthase (DXS) as the main rate-limiting enzyme of the MEP pathway and overexpression of diterpene synthase fusion proteins increased the production of high-value diterpenoids. Applying fully photoautotrophic high cell density cultivations demonstrate potent and sustainable production of the high-value diterpenoid sclareol up to 656 mg L^-1 with a maximal productivity of 78 mg L^-1 day^-1 in a 2.5 L scale photobioreactor, which is comparable to sclareol titers reached by highly engineered yeast. Consequently, this work represents a breakthrough in establishing a powerful phototrophic green cell factory for the competetive use in industrial biotechnology.

Biocompatible fluorocarbon liquid underlays for in situ extraction of isoprenoids from microbial cultures

Microbial production of heterologous metabolites is now a mature technology in many host organisms, opening new avenues for green production processes of specialty chemicals. At lab scale, petroleum-based hydrophobic bio-compatible solvents like dodecane can be used as a second phase on top of microbial cultures to act as a physical sink for heterologous hydrocarbon products like isoprenoids. However, this approach has significant drawbacks at scale due to the difficulty of handling solvents and their potential contamination with unwanted byproducts of their manufacture. We discovered that synthetic perfluorocarbon liquids (FCs), commonly used for heat transfer, can also act as physical sinks for microbially produced isoprenoid compounds. FCs are stable, inert, and are amenable to direct liquid-liquid extraction with alcohols for rapid product isolation. These liquids are more dense than water and form a lower phase to microbial cultures rather than an upper phase as with other solvents. Their ability to form an under-layer or 'underlay' also enables the cultivation of microbes directly at the FC-culture medium interface via gravity settling, which could open their application for filamentous or mat-forming organisms. We present comparisons of the isoprenoid extraction potential of three commercial FCs: FC-3283, FC-40, and FC-770 with engineered green microalga cultures producing patchoulol, taxadiene, casbene, or 13R(+) manoyl oxide. We demonstrate that FCs are promising alternatives to traditional solvents and open new avenues in bio-process design for microbial heterologous metabolite milking.

Combinatorial Engineering Enables Photoautotrophic Growth in High Cell Density Phosphite-Buffered Media to Support Engineered Chlamydomonas reinhardtii Bio-Production Concepts

Chlamydomonas reinhardtii has emerged as a powerful green cell factory for metabolic engineering of sustainable products created from the photosynthetic lifestyle of this microalga. Advances in nuclear genome modification and transgene expression are allowing robust engineering strategies to be demonstrated in this host. However, commonly used lab strains are not equipped with features to enable their broader implementation in non-sterile conditions and high-cell density concepts. Here, we used combinatorial chloroplast and nuclear genome engineering to augment the metabolism of the C. reinhardtii strain UVM4 with publicly available genetic tools to enable the use of inorganic phosphite and nitrate as sole sources of phosphorous and nitrogen, respectively. We present recipes to create phosphite-buffered media solutions that enable high cell density algal cultivation. We then combined previously reported engineering strategies to produce the heterologous sesquiterpenoid patchoulol to high titers from our engineered green cell factories and show these products are possible to produce in non-sterile conditions. Our work presents a straightforward means to generate C. reinhardtii strains for broader application in bio-processes for the sustainable generation of products from green microalgae.

Combinatorial Engineering Enables Photoautotrophic Growth in High Cell Density Phosphite-Buffered Media to Support Engineered Chlamydomonas reinhardtii Bio-Production Concepts

Chlamydomonas reinhardtii has emerged as a powerful green cell factory for metabolic engineering of sustainable products created from the photosynthetic lifestyle of this microalga. Advances in nuclear genome modification and transgene expression are allowing robust engineering strategies to be demonstrated in this host. However, commonly used lab strains are not equipped with features to enable their broader implementation in non-sterile conditions and high-cell density concepts. Here, we used combinatorial chloroplast and nuclear genome engineering to augment the metabolism of the C. reinhardtii strain UVM4 with publicly available genetic tools to enable the use of inorganic phosphite and nitrate as sole sources of phosphorous and nitrogen, respectively. We present recipes to create phosphite-buffered media solutions that enable high cell density algal cultivation. We then combined previously reported engineering strategies to produce the heterologous sesquiterpenoid patchoulol to high titers from our engineered green cell factories and show these products are possible to produce in non-sterile conditions. Our work presents a straightforward means to generate C. reinhardtii strains for broader application in bio-processes for the sustainable generation of products from green microalgae.

Recent Progress on Systems and Synthetic Biology of Diatoms for Improving Algal Productivity

Microalgae have drawn much attention for their potential applications as a sustainable source for developing bioactive compounds, functional foods, feeds, and biofuels. Diatoms, as one major group of microalgae with high yields and strong adaptability to the environment, have shown advantages in developing photosynthetic cell factories to produce value-added compounds, including heterologous bioactive products. However, the commercialization of diatoms has encountered several obstacles that limit the potential mass production, such as the limitation of algal productivity and low photosynthetic efficiency. In recent years, systems and synthetic biology have dramatically improved the efficiency of diatom cell factories. In this review, we discussed first the genome sequencing and genome-scale metabolic models (GEMs) of diatoms. Then, approaches to optimizing photosynthetic efficiency are introduced with a focus on the enhancement of biomass productivity in diatoms. We also reviewed genome engineering technologies, including CRISPR (clustered regularly interspaced short palindromic repeats) gene-editing to produce bioactive compounds in diatoms. Finally, we summarized the recent progress on the diatom cell factory for producing heterologous compounds through genome engineering to introduce foreign genes into host diatoms. This review also pinpointed the bottlenecks in algal engineering development and provided critical insights into the future direction of algal production.

Harnessing the Algal Chloroplast for Heterologous Protein Production

Photosynthetic microbes are gaining increasing attention as heterologous hosts for the light-driven, low-cost production of high-value recombinant proteins. Recent advances in the manipulation of unicellular algal genomes offer the opportunity to establish engineered strains as safe and viable alternatives to conventional heterotrophic expression systems, including for their use in the feed, food, and biopharmaceutical industries. Due to the relatively small size of their genomes, algal chloroplasts are excellent targets for synthetic biology approaches, and are convenient subcellular sites for the compartmentalized accumulation and storage of products. Different classes of recombinant proteins, including enzymes and peptides with therapeutical applications, have been successfully expressed in the plastid of the model organism Chlamydomonas reinhardtii, and of a few other species, highlighting the emerging potential of transplastomic algal biotechnology. In this review, we provide a unified view on the state-of-the-art tools that are available to introduce protein-encoding transgenes in microalgal plastids, and discuss the main (bio)technological bottlenecks that still need to be addressed to develop robust and sustainable green cell biofactories.

Biomolecules from Microalgae and Cyanobacteria: Applications and Market Survey

Nowadays, microalgae and cyanobacteria have become a promising and sustainable source of useful products, thanks to their richness in bioactive metabolites of high value (antibiotics, toxins, pharmaceutically active compounds, plant growth regulators, and others). These photoautotroph microorganisms generate biomass using photosynthesis. This review, which distinguishes microalgae and Cyanobacteria, often called blue-green microalgae, aims to present their classification and taxonomic diversity as the ecological niches occupied by them. In addition, the usages of open ponds and photobioreactors to produce various microalgae and Cyanobacteria strains and the high-value bioactive compounds from these microorganisms are summarized. Finally, the numerous commercial applications of these phytoplanktons in different fields, such as food, dietary supplements, feed, cosmetic, and biofuel applications, are reviewed.

Microalgal metabolic engineering strategies for the production of fuels and chemicals

Microalgae are promising sustainable resources because of their ability to convert CO₂ into biofuels and chemicals directly. However, the industrial production and economic feasibility of microalgal bioproducts are still limited. As such, metabolic engineering approaches have been undertaken to enhance the productivities of microalgal bioproducts. In the last decade, impressive advances in microalgae metabolic engineering have been made by developing genetic engineering tools and multi-omics analysis. This review presents comprehensive microalgal metabolic pathways and metabolic engineering strategies for producing lipids, long chain-polyunsaturated fatty acids, terpenoids, and carotenoids. Additionally, promising metabolic engineering approaches specific to target products are summarized. Finally, this review discusses current challenges and provides future perspectives for the effective production of chemicals and fuels via microalgal metabolic engineering.

Improving microalgae for biotechnology - From genetics to synthetic biology - Moving forward but not there yet

Microalgae are a diverse group of photosynthetic organisms that can be exploited for the production of different compounds, ranging from crude biomass and biofuels to high value-added biochemicals and synthetic proteins. Traditionally, algal biotechnology relies on bioprospecting to identify new highly productive strains and more recently, on forward genetics to further enhance productivity. However, it has become clear that further improvements in algal productivity for biotechnology is impossible without combining traditional tools with the arising molecular genetics toolkit. We review recent advantages in developing high throughput screening methods, preparing genome-wide mutant libraries, and establishing genome editing techniques. We discuss how algae can be improved in terms of photosynthetic efficiency, biofuel and high value-added compound production. Finally, we critically evaluate developments over recent years and explore future potential in the field.

Microalgal lipids: biochemistry and biotechnology

Microalgae, including cyanobacteria, are a genetically diverse and biochemically diverse group of mostly photosynthetic organisms that can be found in nearly every ecosystem. They produce a broad range of compounds, including structural and bioactive lipids with various roles in the metabolism of the cell and potential applications in biotechnology. The majority of lipids are synthesized in the chloroplast using acetate to produce fatty acids and terpenoids via the acetate pathway and methylerythritol phosphate deoxy-xylulose phosphate pathway, respectively. This review will highlight the major groups of microalgal lipids as well as their applications in food, fuels, and specialty chemicals. Future directions include the development of microalgal chassis organisms and molecular tools to manipulate lipid synthesis for the enhanced production of target metabolites.

Review on hydrothermal liquefaction aqueous phase as a valuable resource for biofuels, bio-hydrogen and valuable bio-chemicals recovery

Hydrothermal liquefaction (HTL) of biomass results in the formation of bio-oil, aqueous phase (HTL-AP), bio-char, and gaseous products. Safer disposal of HTL-AP is difficult on an industrial scale since it comprises low molecular acid compounds. This review provides a comprehensive note on the recent articles published on the effective usage of HTL-AP for the recovery of valuable compounds. Thermo-chemical and biological processes are the preferred techniques for the recovery of biofuel, platform chemicals from HTL-AP. From this review, it was evident that the composition of HTL-AP and product recovery are the integrated pathways, which depend on each other. Substitute as reaction medium in HTL process, growth medium for algae and microbes are the most common mode of reuse and recycle of HTL-AP. Future research is needed to depict the mechanism of HTL process when HTL-AP is used as a reaction medium on an industrial scale. Need to find a solution for the hindrance in commercializing HTL process and recovery of value-added compounds from HTL-AP from lab scale to industry level. Integrated pathways on reuse and HTL-AP recycle helps in reduced environmental concerns and sustainable production of bio-products.

Strategies for enhancing terpenoids accumulation in microalgae

Terpenoids represent one of the largest class of chemicals in nature, which play important roles in food and pharmaceutical fields due to diverse biological and pharmacological activities. Microorganisms are recognized as a promising source of terpenoids due to its short growth cycle and sustainability. Importantly, microalgae can fix inorganic carbon through photosynthesis for the growth of themselves and the biosynthesis of various terpenoids. Moreover, microalgae possess effective biosynthesis pathways of terpenoids, both the eukaryotic mevalonic acid (MVA) pathway and the prokaryotic methyl-D-erythritol 4-phosphate (MEP) pathway. In recent years, various genetic engineering strategies have been applied to increase target terpenoid yields, including overexpression of the rate-limited enzymes and inhibition of the competing pathways. However, since gene-editing tools are only built in some model microalgae, fermentation strategies that are easier to be operated have been widely successful in promoting the production of terpenoids, such as changing culture conditions and addition of chemical additives. In addition, an economical and effective downstream process is also an important consideration for the industrial production of terpenoids, and the solvent extraction and the supercritical fluid extraction method are the most commonly used strategies, especially in the industrial production of β-carotene and astaxanthin from microalgae. In this review, recent advancements and novel strategies used for terpenoid production are concluded and discussed, and new insights to move the field forward are proposed. KEY POINTS: • The MEP pathway is more stoichiometrically efficient than the MVA pathway. • Advanced genetic engineering and fermentation strategies can increase terpene yield. • SFE has a higher recovery of carotenoids than solvent extraction.

A novel, robust and mating-competent Chlamydomonas reinhardtii strain with an enhanced transgene expression capacity for algal biotechnology

In the future, algae biotechnology could play an important role in sustainable development, especially with regard to the production of valuable chemicals. Among the established laboratory strains with efficient transgene expression, there are none that have demonstrated the required robustness for industrial applications, which generally require growth at larger scale. Here, we created a robust and mating-competent cell line of the green microalga Chlamydomonas reinhardtii, which also possesses a high transgene expression capacity. This strain shows a comparably high resistance to shear stress by accumulating increased amounts of biomass under these conditions. As a proof-of-concept, a high phototrophic productivity of cadaverine from CO₂ and nitrate was demonstrated by efficiently expressing a bacterial l-lysine decarboxylase. In contrast to other established strains, this novel chassis strain for phototrophic production schemes is equipped with the traits required for industrial applications, by combining mating-competence, cell wall-mediated robustness and high level transgene expression.

Droplet-based microfluidic screening and sorting of microalgal populations for strain engineering applications

The application of microfluidic technologies to microalgal research is particularly appealing since these approaches allow the precise control of the extracellular environment and offer a high-throughput approach to studying dynamic cellular processes. To expand the portfolio of applications, here we present a droplet-based microfluidic method for analysis and screening of Phaeodactylum tricornutum and Nannochloropsis gaditana, which can be integrated into a genetic transformation workflow. Following encapsulation of single cells in picolitre-sized droplets, fluorescence signals arising from each cell can be used to assess its phenotypic state. In this work, the chlorophyll fluorescence intensity of each cell was quantified and used to identify populations of P. tricornutum cells grown in different light conditions. Further, individual P. tricornutum or N. gaditana cells engineered to express green fluorescent protein were distinguished and sorted from wild-type cells. This has been exploited as a rapid screen for transformed cells within a population, bypassing a major bottleneck in algal transformation workflows and offering an alternative strategy for the identification of genetically modified strains.

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Droplet-based microfluidic systems are promising tools for algal single cell analysis.

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Improved intracellular fluorescence detection allows effective sorting of algae cells.

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The physiological status of single encapsulated algae cells can be determined.

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Sorting in microdroplets enables faster identification of transformed cells.

Toward the Enhancement of Microalgal Metabolite Production through Microalgae-Bacteria Consortia

Engineered mutualistic consortia of microalgae and bacteria may be a means of assembling a novel combination of metabolic capabilities with potential biotechnological advantages. Microalgae are promising organisms for the sustainable production of metabolites of commercial interest, such as lipids, carbohydrates, pigments, and proteins. Several studies reveal that microalgae growth and cellular storage of these metabolites can be enhanced significantly by co-cultivation with growth-promoting bacteria. This review summarizes the state of the art of microalgae-bacteria consortia for the production of microalgal metabolites. We discuss the current knowledge on microalgae-bacteria mutualism and the mechanisms of bacteria to enhance microalgae metabolism. Furthermore, the potential routes for a microalgae-bacteria biorefinery are outlined in an attempt to overcome the economic failures and negative energy balances of the existing production processes.

Gene Delivery Technologies with Applications in Microalgal Genetic Engineering

Microalgae and cyanobacteria are photosynthetic microbes that can be grown with the simple inputs of water, carbon dioxide, (sun)light, and trace elements. Their engineering holds the promise of tailored bio-molecule production using sustainable, environmentally friendly waste carbon inputs. Although algal engineering examples are beginning to show maturity, severe limitations remain in the transformation of multigene expression cassettes into model species and DNA delivery into non-model hosts. This review highlights common and emerging DNA delivery methods used for other organisms that may find future applications in algal engineering.

Plant synthetic biology for producing potent phyto-antimicrobials to combat antimicrobial resistance

Inappropriate and injudicious use of antimicrobial drugs in human health, hygiene, agriculture, animal husbandry and food industries has contributed significantly to rapid emergence and persistence of antimicrobial resistance (AMR), one of the serious global public health threats. The crisis of AMR versus slower discovery of newer antibiotics put forth a daunting task to control these drug-resistant superbugs. Several phyto-antimicrobials have been identified in recent years with direct-killing (bactericidal) and/or drug-resistance reversal (re-sensitization of AMR phenotypes) potencies. Phyto-antimicrobials may hold the key in combating AMR owing to their abilities to target major microbial drug-resistance determinants including cell membrane, drug-efflux pumps, cell communication and biofilms. However, limited distribution, low intracellular concentrations, eco-geographical variations, beside other considerations like dynamic environments, climate change and over-exploitation of plant-resources are major blockades in full potential exploration phyto-antimicrobials. Synthetic biology (SynBio) strategies integrating metabolic engineering, RNA-interference, genome editing/engineering and/or systems biology approaches using plant chassis (as engineerable platforms) offer prospective tools for production of phyto-antimicrobials. With expanding SynBio toolkit, successful attempts towards introduction of entire gene cluster, reconstituting the metabolic pathway or transferring an entire metabolic (or synthetic) pathway into heterologous plant systems highlight the potential of this field. Through this perspective review, we are presenting herein the current situation and options for addressing AMR, emphasizing on the significance of phyto-antimicrobials in this apparently post-antibiotic era, and effective use of plant chassis for phyto-antimicrobial production at industrial scales along with major SynBio tools and useful databases. Current knowledge, recent success stories, associated challenges and prospects of translational success are also discussed.

Future trends in synthetic biology in Asia

Synthetic biology research and technology translation has garnered increasing interest from the governments and private investors in Asia, where the technology has great potential in driving a sustainable bio-based economy. This Perspective reviews the latest developments in the key enabling technologies of synthetic biology and its application in bio-manufacturing, medicine, food and agriculture in Asia. Asia-centric strengths in synthetic biology to grow the bio-based economy, such as advances in genome editing and the presence of biofoundries combined with the availability of natural resources and vast markets, are also highlighted. The potential barriers to the sustainable development of the field, including inadequate infrastructure and policies, with suggestions to overcome these by building public-private partnerships, more effective multi-lateral collaborations and well-developed governance framework, are presented. Finally, the roles of technology, education and regulation in mitigating potential biosecurity risks are examined. Through these discussions, stakeholders from different groups, including academia, industry and government, are expectantly better positioned to contribute towards the establishment of innovation and bio-economy hubs in Asia.

Advantages of Heterotrophic Microalgae as a Host for Phytochemicals Production

Currently, most commercial recombinant technologies rely on host systems. However, each host has their own benefits and drawbacks, depending on the target products. Prokaryote host is lack of post-transcriptional and post-translational mechanisms, making them unsuitable for eukaryotic productions like phytochemicals. Even there are other eukaryote hosts (e.g., transgenic animals, mammalian cell, and transgenic plants), but those hosts have some limitations, such as low yield, high cost, time consuming, virus contamination, and so on. Thus, flexible platforms and efficient methods that can produced phytochemicals are required. The use of heterotrophic microalgae as a host system is interesting because it possibly overcome those obstacles. This paper presents a comprehensive review of heterotrophic microalgal expression host including advantages of heterotrophic microalgae as a host, genetic engineering of microalgae, genetic transformation of microalgae, microalgal engineering for phytochemicals production, challenges of microalgal hosts, key market trends, and future view. Finally, this review might be a directions of the alternative microalgae host for high-value phytochemicals production in the next few years.

The Algal Chloroplast as a Testbed for Synthetic Biology Designs Aimed at Radically Rewiring Plant Metabolism

Sustainable and economically viable support for an ever-increasing global population requires a paradigm shift in agricultural productivity, including the application of biotechnology to generate future crop plants. Current genetic engineering approaches aimed at enhancing the photosynthetic efficiency or composition of the harvested tissues involve relatively simple manipulations of endogenous metabolism. However, radical rewiring of central metabolism using new-to-nature pathways, so-called "synthetic metabolism", may be needed to really bring about significant step changes. In many cases, this will require re-programming the metabolism of the chloroplast, or other plastids in non-green tissues, through a combination of chloroplast and nuclear engineering. However, current technologies for sophisticated chloroplast engineering ("transplastomics") of plants are limited to just a handful of species. Moreover, the testing of metabolic rewiring in the chloroplast of plant models is often impractical given their obligate phototrophy, the extended time needed to create stable non-chimeric transplastomic lines, and the technical challenges associated with regeneration of whole plants. In contrast, the unicellular green alga, Chlamydomonas reinhardtii is a facultative heterotroph that allows for extensive modification of chloroplast function, including non-photosynthetic designs. Moreover, chloroplast engineering in C. reinhardtii is facile, with the ability to generate novel lines in a matter of weeks, and a well-defined molecular toolbox allows for rapid iterations of the "Design-Build-Test-Learn" (DBTL) cycle of modern synthetic biology approaches. The recent development of combinatorial DNA assembly pipelines for designing and building transgene clusters, simple methods for marker-free delivery of these clusters into the chloroplast genome, and the pre-existing wealth of knowledge regarding chloroplast gene expression and regulation in C. reinhardtii further adds to the versatility of transplastomics using this organism. Herein, we review the inherent advantages of the algal chloroplast as a simple and tractable testbed for metabolic engineering designs, which could then be implemented in higher plants.

Wastewater-borne microalga Chlamydomonas sp.: A robust chassis for efficient biomass and biomethane production applying low-N cultivation strategy

Biogas/biomethane generation from microalgae biomass via anaerobic fermentation is increasingly gaining attention as CO₂-neutral energy source. Intensive research has shown, however, that microalgae represent a rather challenging substrate for anaerobic digestion (AD) due to their high cell wall recalcitrance and unfavourable protein content. Previously, the utilization of nitrogen-limited (low-N) microalgal biomass for continuous AD-processes was demonstrated (as proof-of-concept) with remarkable biomethane productivity. The present study shows the efficient portability of the low-N cultivation/fermentation strategy on a robust, wastewater-borne microalga isolate that tolerates high temperature and light conditions and can perfectly cope with microbial contaminations. Continuous long-term anaerobic digestion was characterized by stable and efficient specific biogas and biomethane productivity (765 ± 20 and 478 ± 15 mL_Ng^-1 volatile solids (VS) d^-1, respectively), equivalent to volumetric methane productivity of 1912 mL_N L^-1d^-1. The present work underlines the applicability of low-N-biomass of wastewater-borne, robust microalgae as mono-substrate for highly efficient continuous methane generation.

Light-Independent Biological Conversion of CO2

Sevcan Erşan is a postdoctoral researcher at UCLA. Previously, she conducted postdoctoral research at the University of Hohenheim in Germany. She received her PhD in biotechnology from Yeditepe University, Turkey, and her bachelor's and master's degrees in food engineering from Istanbul Technical University, Turkey. She is experienced in waste utilization, bioprocessing technologies, and biological activities associated with phytochemicals. Her current research focuses on natural product chemistry and sustainable biotechnology. Junyoung Park is an assistant professor of Chemical and Biomolecular Engineering and co-director of the Metabolomics Center at UCLA. His research group focuses on systems-level analysis of metabolic networks to elucidate regulatory mechanisms and engineer metabolism. He aims to apply this knowledge to solving energy and environmental problems and curing human diseases such as cancer and diabetes. Before moving to Los Angeles, he conducted postdoctoral research at MIT. He received his bachelor's degrees in mathematics and bioengineering from UC San Diego and a master's and PhD in chemical engineering from Princeton University.

Turning a green alga red: engineering astaxanthin biosynthesis by intragenic pseudogene revival in Chlamydomonas reinhardtii

The green alga Chlamydomonas reinhardtii does not synthesize high-value ketocarotenoids like canthaxanthin and astaxanthin; however, a β-carotene ketolase (CrBKT) can be found in its genome. CrBKT is poorly expressed, contains a long C-terminal extension not found in homologues and likely represents a pseudogene in this alga. Here, we used synthetic redesign of this gene to enable its constitutive overexpression from the nuclear genome of C. reinhardtii. Overexpression of the optimized CrBKT extended native carotenoid biosynthesis to generate ketocarotenoids in the algal host causing noticeable changes the green algal colour to reddish-brown. We found that up to 50% of native carotenoids could be converted into astaxanthin and more than 70% into other ketocarotenoids by robust CrBKT overexpression. Modification of the carotenoid metabolism did not impair growth or biomass productivity of C. reinhardtii, even at high light intensities. Under different growth conditions, the best performing CrBKT overexpression strain was found to reach ketocarotenoid productivities up to 4.3 mg/L/day. Astaxanthin productivity in engineered C. reinhardtii shown here might be competitive with that reported for Haematococcus lacustris (formerly pluvialis) which is currently the main organism cultivated for industrial astaxanthin production. In addition, the extractability and bio-accessibility of these pigments were much higher in cell wall-deficient C. reinhardtii than the resting cysts of H. lacustris. Engineered C. reinhardtii strains could thus be a promising alternative to natural astaxanthin producing algal strains and may open the possibility of other tailor-made pigments from this host.

Aqueous enzymatic protein and lipid release from the microalgae Chlamydomonas reinhardtii

Advances in biochemical and molecular manipulation have led to increased biomass productivity and oil accumulation in the microalgae C. reinhardtii. However, scalable processes for the recovery of oil and other valuable biomolecules, such as protein, from C. reinhardtii are scarce. The use of aqueous enzymatic extraction, a non-solvent and environmentally friendly bioproduct recovery method, provides an opportunity to design an integrated process for oil and protein fractionation to reduce bioenergy and bioproducts costs. Based on the mechanistic understanding of biomolecule distribution and compartmentalization, an aqueous enzymatic treatment for the release of internally stored lipid bodies was designed. Application of a C. reinhardtii-produced protease, autolysin, for lysis of the microalgae cell wall was followed by a secondary treatment with trypsin for chloroplast disruption and lipid body release. Protein recovery after the primary treatment with autolysin indicated a 50.1 ± 4.2% release of total soluble protein and localization of lipid bodies still in the chloroplast. The development of a secondary enzyme treatment (trypsin) for chloroplast and lipid body lysis demonstrated a high percent of remaining lipids (73 ± 7%) released into the supernatant. The results indicate that the application of an enzymatic treatment scheme for protein and oil recovery is a promising alternative to traditional extraction processes.

Emerging Technologies to Enable Sustainable Controlled Environment Agriculture in the Extreme Environments of Middle East-North Africa Coastal Regions

Despite global shifts in attitudes toward sustainability and increasing awareness of human impact on the environment, projected population growth and climate change require technological adaptations to ensure food and resource security at a global scale. Although desert areas have long been proposed as ideal sites for solar electricity generation, only recently have efforts shifted toward development of specialized and regionally focused agriculture in these extreme environments. In coastal regions of the Middle East and North Africa (MENA), the most abundant resources are consistent intense sunlight and saline sea water. MENA coastal regions hold incredible untapped potential for agriculture driven by the combination of key emerging technologies in future greenhouse concepts: transparent infrared collecting solar panels and low energy salt water cooling. These technologies can be combined to create greenhouses that drive regionally relevant agriculture in this extreme environment, especially when the target crops are salt-tolerant plants and algal biomass. Future controlled environment agriculture concepts will not compete for municipal fresh water and can be readily integrated into local human/livestock/fisheries food chains. With strategic technological implementation, marginal lands in these environments could participate in production of biomass, sustainable energy generation, and the circular carbon economy. The goal of this perspective is to reframe the idea of these environments as extreme, to having incredible untapped development potential.

Development of Novel Riboswitches for Synthetic Biology in the Green Alga Chlamydomonas

Riboswitches are RNA regulatory elements that bind specific ligands to control gene expression. Because of their modular composition, where a ligand-sensing aptamer domain is combined with an expression platform, riboswitches offer unique tools for synthetic biology applications. Here we took a mutational approach to determine functionally important nucleotide residues in the thiamine pyrophosphate (TPP) riboswitch in the THI4 gene of the model alga Chlamydomonas reinhardtii, allowing us to carry out aptamer swap using THIC aptamers from Chlamydomonas and Arabidopsis thaliana. These chimeric riboswitches displayed a distinct specificity and dynamic range of responses to different ligands. Our studies demonstrate ease of assembly as 5'UTR DNA parts, predictability of output, and utility for controlled production of a high-value compound in Chlamydomonas. The simplicity of riboswitch incorporation in current design platforms will facilitate the generation of genetic circuits to advance synthetic biology and metabolic engineering of microalgae.

Emerging Technologies in Algal Biotechnology: Toward the Establishment of a Sustainable, Algae-Based Bioeconomy

Mankind has recognized the value of land plants as renewable sources of food, medicine, and materials for millennia. Throughout human history, agricultural methods were continuously modified and improved to meet the changing needs of civilization. Today, our rapidly growing population requires further innovation to address the practical limitations and serious environmental concerns associated with current industrial and agricultural practices. Microalgae are a diverse group of unicellular photosynthetic organisms that are emerging as next-generation resources with the potential to address urgent industrial and agricultural demands. The extensive biological diversity of algae can be leveraged to produce a wealth of valuable bioproducts, either naturally or via genetic manipulation. Microalgae additionally possess a set of intrinsic advantages, such as low production costs, no requirement for arable land, and the capacity to grow rapidly in both large-scale outdoor systems and scalable, fully contained photobioreactors. Here, we review technical advancements, novel fields of application, and products in the field of algal biotechnology to illustrate how algae could present high-tech, low-cost, and environmentally friendly solutions to many current and future needs of our society. We discuss how emerging technologies such as synthetic biology, high-throughput phenomics, and the application of internet of things (IoT) automation to algal manufacturing technology can advance the understanding of algal biology and, ultimately, drive the establishment of an algal-based bioeconomy.