Safe-Harboring based novel genetic toolkit for Nannochloropsis salina CCMP1776: Efficient overexpression of transgene via CRISPR/Cas9-Mediated Knock-in at the transcriptional hotspot

Progress and challenges in CRISPR/Cas applications in microalgae

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) technologies have emerged as powerful tools for precise genome editing, leading to a revolution in genetic research and biotechnology across diverse organisms including microalgae. Since the 1950s, microalgal production has evolved from initial cultivation under controlled conditions to advanced metabolic engineering to meet industrial demands. However, effective genetic modification in microalgae has faced significant challenges, including issues with transformation efficiency, limited target selection, and genetic differences between species, as interspecies genetic variation limits the use of genetic tools from one species to another. This review summarized recent advancements in CRISPR systems applied to microalgae, with a focus on improving gene editing precision and efficiency, while addressing organism-specific challenges. We also discuss notable successes in utilizing the class 2 CRISPR-associated (Cas) proteins, including Cas9 and Cas12a, as well as emerging CRISPR-based approaches tailored to overcome microalgal cellular barriers. Additionally, we propose future perspectives for utilizing CRISPR/Cas strategies in microalgal biotechnology.

The bioactivities and biotechnological production approaches of carotenoids derived from microalgae and cyanobacteria

Microalgae and cyanobacteria are a rich source of carotenoids that are well known for their potent bioactivities, including antioxidant, anti-cancer, anti-proliferative, anti-inflammatory, and anti-obesity properties. Recently, many interests have also been focused on the biological activities of these microalgae/cyanobacteria-derived carotenoids, such as fucoxanthin and β-carotene potential to be the salutary nutraceuticals, on treating or preventing human common diseases (e.g., cancers). This is due to their special chemical structures that demonstrate unique bioactive functions, in which the biologically active discrepancies might attribute to the different spatial configurations of their molecules. In addition, their abundance and bioaccessibilities make them more popularly applied in food and pharmaceutical industries, as compared to the macroalgal/fungal-derived ones. This review is focused on the recent studies on the bioactivities of fucoxanthin and some carotenoids derived from microalgae and cyanobacteria in relationship with human health and diseases, with emphasis on their potential applications as natural antioxidants. Various biotechnological approaches employed to induce the production of these specific carotenoids from the culture of microalgae/cyanobacteria are also critically reviewed. These well-developed and emerging biotechnologies present promise to be applied in food and pharmaceutical industries to facilitate the efficient manufacture of the bioactive carotenoid products derived from microalgae and cyanobacteria.

Turning the industrially relevant marine alga Nannochloropsis red: one move for multifaceted benefits

Nannochloropsis oceanica is an industrially relevant marine microalga rich in eicosapentaenoic acid (EPA, a valuable ω-3 polyunsaturated fatty acid), yet the algal production potential remains to be unlocked. Here we engineered N. oceanica to synthesize the high-value carotenoid astaxanthin independent of high-light (HL) induction for achieving multifaceted benefits. By screening β-carotenoid ketolases and hydroxylases of various origins, and strategically manipulating compartmentalization, fusion patterns, and linkers of the enzyme pair, a remarkable 133-fold increase in astaxanthin content was achieved in N. oceanica. Iterative metabolic engineering efforts led to further increases in astaxanthin synthesis up to 7.3 mg g-1, the highest reported for microalgae under nonstress conditions. Astaxanthin was found in the photosystem components and allowed the alga HL resistance and augmented EPA production. Besides, we achieved co-production of astaxanthin and EPA by the engineered alga through a fed-batch cultivation approach. Our findings unveil the untapped potential of N. oceanica as a robust, light-driven chassis for constitutive astaxanthin synthesis and provide feasible strategies for the concurrent production of multiple high-value biochemicals from CO2, thereby paving the way for sustainable biotechnological applications of this alga.

Enhancing fatty acid and omega-3 production in Schizochytrium sp. using developed safe-harboring expression system

BACKGROUND

Schizochytrium, a group of eukaryotic marine protists, is an oleaginous strain, making it a highly promising candidate for the production of lipid-derived products such as biofuels and omega-3 fatty acids. However, the insufficient advancement of genetic engineering tools has hindered further advancements. Therefore, the development and application of genetic engineering tools for lipid enhancement are crucial for industrial production.

RESULTS

Transgene expression in Schizochytrium often encounters challenges such as instability due to positional effects. To overcome this, we developed a safe-harbor transgene expression system. Initially, the sfGFP gene was integrated randomly, and high-expressing transformants were identified using fluorescence-activated cell sorting. Notably, HRsite 2, located approximately 3.2 kb upstream of cytochrome c, demonstrated enhanced sfGFP expression and homologous recombination efficiency. We then introduced the 3-ketoacyl-ACP reductase (KR) gene at HRsite 2, resulting in improved lipid and docosahexaenoic acid (DHA) production. Transformants with KR at HRsite 2 exhibited stable growth, increased glucose utilization, and a higher lipid content compared to those with randomly integrated transgenes. Notably, these transformants showed a 25% increase in DHA content compared to the wild-type strain.

CONCLUSION

This study successfully established a robust homologous recombination system in Schizochytrium sp. by identifying a reliable safe harbor site for gene integration. The targeted expression of the KR gene at this site not only enhanced DHA production but also maintained growth and glucose consumption rates, validating the efficacy of the safe-harbor approach. This advancement in synthetic biology and metabolic engineering paves the way for more efficient biotechnological applications in Schizochytrium sp.

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.

Pioneering DNA assembling techniques and their applications in eukaryotic microalgae

Assembling DNA fragments is a fundamental manipulation of cloning microalgal genes and carrying out microalgal synthetic biological studies. From the earliest DNA recombination to current trait and metabolic pathway engineering, we are always accompanied by homology-based DNA assembling. The improvement and modification of pioneering DNA assembling techniques and the combinational applications of the available assembling techniques have diversified and complicated the literature environment and aggravated our identification of the core and pioneering methodologies. Identifying the core assembling methodologies and using them appropriately and flourishing them even are important for researchers. A group of microalgae have been evolving as the models for both industrial applications and biological studies. DNA assembling requires researchers to know the methods available and their improvements and evolvements. In this review, we summarized the pioneering (core; leading) DNA assembling techniques developed previously, extended these techniques to their modifications, improvements and their combinations, and highlighted their applications in eukaryotic microalgae. We predicted that the gene(s) will be assembled into a functional cluster (e.g., those involving in a metabolic pathway, and stacked on normal microalgal chromosomes, their artificial episomes and looming artificial chromosomes. It should be particularly pointed out that the techniques mentioned in this review are classified according to the strategy used to assemble the final construct.

CRISPR-based bioengineering in microalgae for production of industrially important biomolecules

Microalgae, as photosynthetic organisms, have the potential to produce biomolecules for use in food, feed, cosmetics, nutraceuticals, fuel, and other applications. Faster growth rates and higher protein and lipid content make microalgae a popular chassis for many industrial applications. However, challenges such as low productivity and high production costs have limited their commercialization. To overcome these challenges, bioengineering approaches such as genetic engineering, metabolic engineering, and synthetic biology have been employed to improve the productivity and quality of microalgae-based products. Genetic engineering employing genome editing tools like CRISPR/Cas allows precise and targeted genetic modifications. CRISPR/Cas systems are presently used to modify the genetic makeup of microalgae for enhanced production of specific biomolecules. However, these tools are yet to be explored explicitly in microalgae owing to some limitations. Despite the progress made in CRISPR-based bioengineering approaches, there is still a need for further research to optimize the production of microalgae-based products. This includes improving the efficiency of genome editing tools, understanding the regulatory mechanisms of microalgal metabolism, and optimizing growth conditions and cultivation strategies. Additionally, addressing the ethical, social, and environmental concerns associated with genetic modification of microalgae is crucial for the responsible development and commercialization of microalgae-based products. This review summarizes the advancements of CRISPR-based bioengineering for production of industrially important biomolecules and provides key considerations to use CRISPR/Cas systems in microalgae. The review will help researchers to understand the progress and to initiate genome editing experiments in microalgae.

Harnessing CRISPR-Cas for oomycete genome editing

Oomycetes are a group of microorganisms that include pathogens responsible for devastating diseases in plants and animals worldwide. Despite their importance, the development of genome editing techniques for oomycetes has progressed more slowly than for model microorganisms. Here, we review recent breakthroughs in clustered regularly interspaced short palindromic repeats (CRISPR)-Cas technologies that are expanding the genome editing toolbox for oomycetes - from the original Cas9 study to Cas12a editing, ribonucleoprotein (RNP) delivery, and complementation. We also discuss some of the challenges to applying CRISPR-Cas in oomycetes and potential ways to overcome them. Advances in CRISPR-Cas technologies are being used to illuminate the biology of oomycetes, which ultimately can guide the development of tools for managing oomycete diseases.

Genome engineering via gene editing technologies in microalgae

CRISPR-Cas has revolutionized genetic modification with its comparative simplicity and accuracy, and it can be used even at the genomic level. Microalgae are excellent feedstocks for biofuels and nutraceuticals because they contain high levels of fatty acids, carotenoids, and other metabolites; however, genome engineering for microalgae is not yet as developed as for other model organisms. Microalgal engineering at the genetic and metabolic levels is relatively well established, and a few genomic resources are available. Their genomic information was used for a "safe harbor" site for stable transgene expression in microalgae. This review proposes further genome engineering schemes including the construction of sgRNA libraries, pan-genomic and epigenomic resources, and mini-genomes, which can together be developed into synthetic biology for carbon-based engineering in microalgae. Acetyl-CoA is at the center of carbon metabolic pathways and is further reviewed for the production of molecules including terpenoids in microalgae.

Advances in engineering algae for biofuel production

While algae demonstrate potential as a sustainable fuel source, low productivities limit the economic realization of algal biofuels. High-throughput strain engineering, omics-informed genome-scale modeling, and microbiome engineering are key technologies for enabling algal biofuels. High-throughput strain engineering efforts generate improved traits, including high biomass productivity and lipid content, in diverse algal species. Genome-scale models, constructed with the aid of omics data, provide insight into metabolic limitations and guide rational algal strain engineering efforts. As outdoor cultivation systems introduce exogenous organisms, microbiome engineering seeks to eliminate harmful organisms and introduce beneficial species. Optimizing algal biomass production and lipid content using these technologies may overcome the productivity barrier for the commercialization of algal biofuels.

Metabolic engineering of the oleaginous alga Nannochloropsis for enriching eicosapentaenoic acid in triacylglycerol by combined pulling and pushing strategies

The marine alga Nannochloropsis oceanica has been considered as a promising photosynthetic cell factory for synthesizing eicosapentaenoic acid (EPA), yet the accumulation of EPA in triacylglycerol (TAG) is restricted to an extreme low level. Poor channeling of EPA to TAG was observed in N. oceanica under TAG induction conditions, likely due to the weak activity of endogenous diacylglycerol acyltransferases (DGATs) on EPA-CoA. Screening over thirty algal DGATs revealed potent enzymes acting on EPA-CoA. Whilst overexpressing endogenous DGATs had no or slight effect on EPA abundance in TAG, introducing selected DGATs with strong activity on EPA-CoA, particularly the Chlamydomonas-derived CrDGTT1, which resided at the outermost membrane of the chloroplast and provided a strong pulling power to divert EPA to TAG for storage and protection, led to drastic increases in EPA abundance in TAG and TAG-derived EPA level in N. oceanica. They were further promoted by additional overexpression of an elongase gene involved in EPA biosynthesis, reaching 5.9- and 12.3-fold greater than the control strain, respectively. Our results together demonstrate the concept of applying combined pulling and pushing strategies to enrich EPA in algal TAG and provide clues for the enrichment of other desired fatty acids in TAG as well.