Transformation of Nonselectable Reporter Genes in Marine Diatoms

More than a photoreceptor: aureochromes are intrinsic to the diatom light-regulated transcriptional network

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Im SH, Lepetit B, Mosesso N, Shrestha S, Weiss L, Nymark M, Roellig R, Wilhelm C, Isono E, Kroth PG. 2024. Identification of promoter targets by Aureochrome 1a in the diatom Phaeodactylum tricornutum. Journal of Experimental Botany 75, 1834–1851.

The tonoplast localized protein PtNPF1 participates in the regulation of nitrogen response in diatoms

Diatoms are a highly successful group of phytoplankton, well adapted also to oligotrophic environments and capable of handling nutrient fluctuations in the ocean, particularly nitrate. The presence of a large vacuole is an important trait contributing to their adaptive features. It confers diatoms the ability to accumulate and store nutrients, such as nitrate, when they are abundant outside and then to reallocate them into the cytosol to meet deficiencies, in a process called luxury uptake. The molecular mechanisms that regulate these nitrate fluxes are still not known in diatoms. In this work, we provide new insights into the function of Phaeodactylum tricornutum NPF1, a putative low-affinity nitrate transporter. To accomplish this, we generated overexpressing strains and CRISPR/Cas9 loss-of-function mutants. Microscopy observations confirmed predictions that PtNPF1 is localized on the vacuole membrane. Furthermore, functional characterizations performed on knock-out mutants revealed a transient growth delay phenotype linked to altered nitrate uptake. Together, these results allowed us to hypothesize that PtNPF1 is presumably involved in modulating intracellular nitrogen fluxes, managing intracellular nutrient availability. This ability might allow diatoms to fine-tune the assimilation, storage and reallocation of nitrate, conferring them a strong advantage in oligotrophic environments.

Overexpression of PtVDL1 in Phaeodactylum tricornutum Increases Fucoxanthin Content under Red Light

Phaeodactylum tricornutum is a model diatom with significant biotechnological applications, including enhancing biomass, biofuel, and carotenoid production. Specifically, owing to the capacity of this organism to serve as a valuable source of essential raw materials for pharmaceuticals and nutraceuticals, ongoing research is actively focused on enhancing its productivity. One of the genes involved in various stages of fucoxanthin (Fx) biosynthesis, violaxanthin de-epoxidase like 1 (VDL1), has recently been identified. To validate the intracellular function of this gene and boost Fx production through overexpression, we established and examined three transgenic P. tricornutum lines characterized by elevated P. tricortunum VDL1 ( PtVDL1) expression and evaluate their cell growth and Fx productivity. These transgenic lines exhibited substantially increased PtVDL1 mRNA and protein levels compared to the wild type (WT). Notably, the enzyme substrate violaxanthin was entirely depleted and could not be detected in the transformants, whereas it remained at constant levels in the WT. Interestingly, under standard white light conditions, Fx productivity in the transformants remained unchanged; however, but after 48 h of exposure to red light, it increased by up to 15%. These results indicate that PtVDL1-overexpressing P. tricornutum has industrial potential, particularly for enhancing Fx production under red light conditions.

Targeted Gene Editing of Nuclear-Encoded Plastid Proteins in Phaeodactylum tricornutum via CRISPR/Cas9

Genome modifications in microalgae have emerged as a crucial and indispensable tool for research in fundamental and applied biology. In particular, CRISPR/Cas9 has gained significant recognition as a highly effective method for genome engineering in these photosynthetic organisms, enabling the targeted induction of mutations in specific regions of the genome. Here, we present a comprehensive protocol for generating knock-out mutants in the model diatom Phaeodactylum tricornutum using CRISPR/Cas9 by both biolistic transformation and bacterial conjugation. Our protocol outlines the step-by-step procedures and experimental conditions required to achieve successful genome editing, including the design and construction of guide RNAs, the delivery of CRISPR/Cas9 components into the algae cells, and the selection of the generated knockout mutants. Through the implementation of this protocol, researchers can harness the potential of CRISPR/Cas9 in P. tricornutum to advance the understanding of diatom biology and explore their potential applications in various fields.

Phaeodactylum tricornutum: An established model species for diatom molecular research and an emerging chassis for algal synthetic biology

Diatoms are prominent and highly diverse microalgae in aquatic environments. Compared with other diatom species, Phaeodactylum tricornutum is an "atypical diatom" displaying three different morphotypes and lacking the usual silica shell. Despite being of limited ecological relevance, its ease of growth in the laboratory and well-known physiology, alongside the steady increase in genome-enabled information coupled with effective tools for manipulating gene expression, have meant it has gained increased recognition as a powerful experimental model for molecular research on diatoms. We here present a brief overview of how over the last 25 years P. tricornutum has contributed to the unveiling of fundamental aspects of diatom biology, while also emerging as a new tool for algal process engineering and synthetic biology.

Highly Efficient Genetic Transformation Methods for the Marine Oleaginous Diatom Fistulifera solaris

The oleaginous diatom Fistulifera solaris is a promising producer of biofuel owing to the high content of the lipids. A genetic transformation technique by microparticle bombardment for this diatom was already established. However, the transformation efficiency was significantly lower than those of other diatoms. Devoting efforts to advance the genetic modifications of this diatom is crucial to unlock its full potential. In this study, we optimized the microparticle bombardment protocol, and newly established a multi-pulse electroporation protocol for this diatom. The nutrient-rich medium in the pre-culture stage played an essential role to increase the transformation efficiency of the bombardment method. On the other hand, use of the nutrient-rich medium in the electroporation experiments resulted in decreasing the efficiency because excess nutrient salts could hamper to establish the best conductivity condition. Adjustments on the number and voltage of the poring pulses were also critical to obtain the best balance between cell viability and efficient pore formation. Under the optimized conditions, the transformation efficiencies of microparticle bombardment and multi-pulse electroporation were 111 and 82 per 108 cells, respectively (37 and 27 times higher than the conventional bombardment method). With the aid of the optimized protocol, we successfully developed the transformant clone over-expressing the endogenous fat storage-inducing transmembrane protein (FIT)-like protein, which was previously found in the genome of the oleaginous diatom F. solaris and the oleaginous eustigmatophyte Nannochloropsis gaditana. This study provides powerful techniques to investigate and further enhance the metabolic functions of F. solaris by genetic engineering.

Quantitative elemental imaging in eukaryotic algae

All organisms, fundamentally, are made from the same raw material, namely the elements of the periodic table. Biochemical diversity is achieved by how these elements are utilized, for what purpose, and in which physical location. Determining elemental distributions, especially those of trace elements that facilitate metabolism as cofactors in the active centers of essential enzymes, can determine the state of metabolism, the nutritional status, or the developmental stage of an organism. Photosynthetic eukaryotes, especially algae, are excellent subjects for quantitative analysis of elemental distribution. These microbes utilize unique metabolic pathways that require various trace nutrients at their core to enable their operation. Photosynthetic microbes also have important environmental roles as primary producers in habitats with limited nutrient supplies or toxin contaminations. Accordingly, photosynthetic eukaryotes are of great interest for biotechnological exploitation, carbon sequestration, and bioremediation, with many of the applications involving various trace elements and consequently affecting their quota and intracellular distribution. A number of diverse applications were developed for elemental imaging, allowing subcellular resolution, with X-ray fluorescence microscopy (XFM, XRF) being at the forefront, enabling quantitative descriptions of intact cells in a non-destructive method. This Tutorial Review summarizes the workflow of a quantitative, single-cell elemental distribution analysis of a eukaryotic alga using XFM.

CRISPR/Cas9-Mediated Genome Editing via Homologous Recombination in a Centric Diatom Chaetoceros muelleri

Chaetoceros, the most abundant genus of marine planktonic diatoms, can be used in mariculture. An effective genetic transformation system with a short transformation period was established in Chaetoceros muelleri by electroporation in our previous study. In this study, a sequence-specific clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 vector applicable for C. muelleri was constructed, and the expressions of sgRNA, resistance gene, and Cas9 gene were driven by the endogenous promoters U6, acetyl-CoA acetyltransferase, and fucoxanthin chlorophyll a/c binding protein, respectively, in the vector. Nitrate reductase (NR) and urease (URE) genes were edited in C. muelleri, and the NR knockout and NR/URE double-knockout lines displayed the strict auxotrophic phenotype. In addition, the DNA double-strand break was repaired by homologous recombination when a donor DNA was introduced. CRISPR/Cas9 technology was successfully applied to C. muelleri with an editing efficiency of up to 86%, providing a molecular tool for the study of basic biology in C. muelleri and its synthetic biology applications.

The first genetic engineered system for ovothiol biosynthesis in diatoms reveals a mitochondrial localization for the sulfoxide synthase OvoA

Diatoms represent one of the most abundant groups of microalgae in the ocean and are responsible for approximately 20% of photosynthetically fixed CO₂ on Earth. Due to their complex evolutionary history and ability to adapt to different environments, diatoms are endowed with striking molecular biodiversity and unique metabolic activities. Their high growth rate and the possibility to optimize their biomass make them very promising 'biofactories' for biotechnological applications. Among bioactive compounds, diatoms can produce ovothiols, histidine-derivatives, endowed with unique antioxidant and anti-inflammatory properties, and occurring in many marine invertebrates, bacteria and pathogenic protozoa. However, the functional role of ovothiols biosynthesis in organisms remains almost unexplored. In this work, we have characterized the thiol fraction of Phaeodactylum tricornutum, providing the first evidence of the presence of ovothiol B in pennate diatoms. We have used P. tricornutum to overexpress the 5-histidylcysteine sulfoxide synthase ovoA, the gene encoding the key enzyme involved in ovothiol biosynthesis and we have discovered that OvoA localizes in the mitochondria, a finding that uncovers new concepts in cellular redox biochemistry. We have also obtained engineered biolistic clones that can produce higher amount of ovothiol B compared to wild-type cells, suggesting a new strategy for the eco-sustainable production of these molecules.

Overexpression of a novel gene (Pt2015) endows the commercial diatom Phaeodactylum tricornutum high lipid content and grazing resistance

BACKGROUND

The marine diatom Phaeodactylum tricornutum is a commercially viable species due to its bioactive substances and lipid productivity. Increasing attention has been paid to the isolation or genetic modification of species or strains with a rapid growth rate and large quantities of lipids. Furthermore, contamination of microzooplankton has been one of the major constraints in P. tricornutum large-scale cultivation, which adversely affects growth and greatly impedes the course of biomass production industrialization.

RESULTS

Here, based on our previous transcriptomics of P. tricornutum, we found a novel gene (ID: 7202015, hereafter called Pt2015) which affects morphotype of P. tricornutum. Pt2015 protein is located in the plastid, which is highly homologous to part of the sequences of exosome component. The morphotype of the Pt2015 knockout strain (termed 2015KO) using CRISPR/Cas9 method is fusiform, but the Pt2015 overexpression strain (termed oeT) demonstrates a majority triradiate morphotype (approximately 95%) which is stable and has been cultured for more than 200 generations. In addition, the oeT strain demonstrated a similar growth rate to the WT and simultaneously accumulated larger lipids droplets that increased by approximately 30% compared to that of the WT. More importantly, the grazing rate of the amoebae cultured in the oeT strain significantly decreased in comparison with that cultured in WT, suggesting that the oeT can effectively avoid being eaten by microzooplankton.

CONCLUSIONS

Therefore, the oeT strain not only improves our understanding of morphotype conversion in diatoms but also demonstrates potential applications for large-scale cultivation of P. tricornutum.

Light-harvesting complex gene regulation by a MYB-family transcription factor in the marine diatom, Phaeodactylum tricornutum

Unicellular photoautotrophs adapt to variations in light intensity by changing the abundance of light harvest pigment-protein complexes (LHCs) on time scales of hours to days. This process requires a feedback signal between the plastid (where light intensity is sensed) to the nucleus (where the genes for LHCs are encoded). The signals must include heretofore unidentified transcription factors that modify the expression level of the LHCs. Analysis of the nuclear genome of the model diatom Phaeodactylum tricornutum revealed that all the lhc genes have potential binding sites for transcription factors belonging to the MYB-family proteins. Functional studies involving antisense RNA interference of a hypothetical protein with a MYB DNA-binding domain were performed. The resultant strains with altered photosynthetic and physiological characteristics lost their ability to acclimate to changes in irradiance; i.e., cellular chlorophyll content became independent of growth irradiance. Our results strongly suggest that the inter-organellar signaling cascade was disrupted, and the cell could no longer communicate the environmental signal from the plastid to the nucleus. Here, we identify, for the first time, an LHC Regulating Myb (LRM) transcription factor, which we propose is involved in lhc gene regulation and photoacclimation mechanisms in response to changes in light intensity.

Multi-omics approaches and genetic engineering of metabolism for improved biorefinery and wastewater treatment in microalgae

Microalgae, a group of photosynthetic microorganisms rich in diverse and novel bioactive metabolites, have been explored for the production of biofuels, high value-added compounds as food and feeds, and pharmaceutical chemicals as agents with therapeutic benefits. This article reviews the development of omics resources and genetic engineering techniques including gene transformation methodologies, mutagenesis, and genome-editing tools in microalgae biorefinery and wastewater treatment. The introduction of these enlisted techniques has simplified the understanding of complex metabolic pathways undergoing microalgal cells. The multiomics approach of the integrated omics datasets, big data analysis, and machine learning for the discovery of objective traits and genes responsible for metabolic pathways was reviewed. Recent advances and limitations of multiomics analysis and genetic bioengineering technology to facilitate the improvement of microalgae as the dual role of wastewater treatment and biorefinery feedstock production are discussed. This article is protected by copyright. All rights reserved.

Episome-Based Gene Expression Modulation Platform in the Model Diatom Phaeodactylum tricornutum

Chemically inducible gene expression systems have been an integral part of the advanced synthetic genetic circuit design and are employed for precise dynamic control over genetically engineered traits. However, the current systems for controlling transgene expression in most algae are limited to endogenous promoters that respond to different environmental factors. We developed a highly efficient, tunable, and reversible episome-based transcriptional control system in the model diatom alga, Phaeodactylum tricornutum. We assessed the time- and dose-response dynamics of each expression system using a reporter protein (eYFP) as a readout. Using our circuit configuration, we found two inducible expression systems with a high dynamic range and confirmed the suitability of an episome expression platform for synthetic biological applications in diatoms. These systems are controlled by the presence of β-estradiol and digoxin. Addition of either chemical to transgenic strains activates transcription with a dynamic range of up to ∼180-fold and ∼90-fold, respectively. We demonstrated that our episome-based transcriptional control systems are tunable and reversible in a dose- and time-dependent manner. Using droplet digital polymerase chain reaction (PCR), we also confirmed that inducer-dependent transcriptional activation starts within minutes of inducer application without any detectable transcript in the uninduced controls. The system described here expands the molecular and synthetic biology toolkits in algae and will facilitate future gene discovery and metabolic engineering efforts.

Biochemical and molecular properties of LHCX1, the essential regulator of dynamic photoprotection in diatoms

Light harvesting is regulated by a process triggered by the acidification of the thylakoid lumen, known as nonphotochemical "energy-dependent quenching" (qE). In diatoms, qE is controlled by the light-harvesting complex (LHC) protein LHCX1, while the LHC stress-related (LHCSR) and photosystem II subunit S proteins are essential for green algae and plants, respectively. Here, we report a biochemical and molecular characterization of LHCX1 to investigate its role in qE. We found that, when grown under intermittent light, Phaeodactylum tricornutum forms very large qE, due to LHCX1 constitutive upregulation. This "super qE" is abolished in LHCX1 knockout mutants. Biochemical and spectroscopic analyses of LHCX1 reveal that this protein might differ in the character of binding pigments relative to the major pool of light-harvesting antenna proteins. The possibility of transient pigment binding or not binding pigments at all is discussed. Targeted mutagenesis of putative protonatable residues (D95 and E205) in transgenic P. tricornutum lines does not alter qE capacity, showing that they are not involved in sensing lumen pH, differently from residues conserved in LHCSR3. Our results suggest functional divergence between LHCX1 and LHCSR3 in qE modulation. We propose that LHCX1 evolved independently to facilitate dynamic tracking of light fluctuations in turbulent waters. The evolution of LHCX(-like) proteins in organisms with secondary red plastids, such as diatoms, might have conferred a selective advantage in the control of dynamic photoprotection, ultimately resulting in their ecological success.

Optimized Proteolistic Protocol for the Delivery of the Cas9 Protein in Phaeodactylum tricornutum

The CRISPR/Cas9 system coupled with proteolistics is a DNA-free nuclear transformation method based on the introduction of ribonucleoprotein (RNP) complexes into cells. The method has been set up for diatoms as an alternative to genetic transformation via biolistics and has the advantages of reducing off-target mutations, limiting the working time of the Cas9 endonuclease, and overcoming the occurrence of random insertions of the transgene in the genome. We present a point-by-point description of the protocol with modifications that make it more cost-effective, by reducing the amount of the enzyme while maintaining a comparable efficiency to the original protocol, and with an increased concentration of the selective drug which allows to reduce false positives.

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.

The Critical Studies of Fucoxanthin Research Trends from 1928 to June 2021: A Bibliometric Review

Fucoxanthin is a major carotenoid in brown macroalgae and diatoms that possesses a broad spectrum of health benefits. This review evaluated the research trends of the fucoxanthin field from 1928 to June 2021 using the bibliometric method. The present findings unraveled that the fucoxanthin field has grown quickly in recent years with a total of 2080 publications. Japan was the most active country in producing fucoxanthin publications. Three Japan institutes were listed in the top ten productive institutions, with Hokkaido University being the most prominent institutional contributor in publishing fucoxanthin articles. The most relevant subject area on fucoxanthin was the agricultural and biological sciences category, while most fucoxanthin articles were published in Marine Drugs. A total of four research concepts emerged based on the bibliometric keywords analysis: “bioactivities”, “photosynthesis”, “optimization of process’’, and “environment”. The “bioactivities” of fucoxanthin was identified as the priority in future research. The current analysis highlighted the importance of collaboration and suggested that global collaboration could be the key to valorizing and efficiently boosting the consumer acceptability of fucoxanthin. The present bibliometric analysis offers valuable insights into the research trends of fucoxanthin to construct a better future development of this treasurable carotenoid.

Identification of Loci Enabling Stable and High-Level Heterologous Gene Expression

Efficient and reliable genome engineering technologies have yet to be developed for diatoms. The delivery of DNA in diatoms results in the random integration of multiple copies, quite often leading to heterogeneous gene activity, as well as host instability. Transgenic diatoms are generally selected on the basis of transgene expression or high enzyme activity, without consideration of the copy number or the integration locus. Here, we propose an integrated pipeline for the diatom, Phaeodactylum tricornutum, that accurately quantifies transgene activity using a β-glucuronidase assay and the number of transgene copies integrated into the genome through Droplet Digital PCR (ddPCR). An exhaustive and systematic analysis performed on 93 strains indicated that 42% of them exhibited high β-glucuronidase activity. Though most were attributed to high transgene copy numbers, we succeeded in isolating single-copy clones, as well as sequencing the integration loci. In addition to demonstrating the impact of the genomic integration site on gene activity, this study identifies integration sites for stable transgene expression in Phaeodactylum tricornutum.

Method Development Progress in Genetic Engineering of Thraustochytrids

Thraustochytrids are unicellular, heterotrophic marine eukaryotes. Some species are known to store surplus carbon as intracellular lipids, and these also contain the long-chain polyunsaturated fatty acid docosahexaenoic acid (DHA). Most vertebrates are unable to synthesize sufficient amounts of DHA, and this fatty acid is essential for, e.g., marine fish, domesticated animals, and humans. Thraustochytrids may also produce other commercially valuable fatty acids and isoprenoids. Due to the great potential of thraustochytrids as producers of DHA and other lipid-related molecules, a need for more knowledge on this group of organisms is needed. This necessitates the ability to do genetic manipulation of the different strains. Thus far, this has been obtained for a few strains, while it has failed for other strains. Here, we systematically review the genetic transformation methods used for different thraustochytrid strains, with the aim of aiding studies on strains not yet successfully transformed. The designs of transformation cassettes are also described and compared. Moreover, the potential problems when trying to establish transformation protocols in new thraustochytrid species/strains are discussed, along with suggestions utilized in other organisms to overcome similar challenges. The approaches discussed in this review could be a starting point when designing protocols for other non-model organisms.

Phylogenomic fingerprinting of tempo and functions of horizontal gene transfer within ochrophytes

Horizontal gene transfer (HGT) is an important source of novelty in eukaryotic genomes. This is particularly true for the ochrophytes, a diverse and important group of algae. Previous studies have shown that ochrophytes possess a mosaic of genes derived from bacteria and eukaryotic algae, acquired through chloroplast endosymbiosis and from HGTs, although understanding of the time points and mechanisms underpinning these transfers has been restricted by the depth of taxonomic sampling possible. We harness an expanded set of ochrophyte sequence libraries, alongside automated and manual phylogenetic annotation, in silico modeling, and experimental techniques, to assess the frequency and functions of HGT across this lineage. Through manual annotation of thousands of single-gene trees, we identify continuous bacterial HGT as the predominant source of recently arrived genes in the model diatom Phaeodactylum tricornutum Using a large-scale automated dataset, a multigene ochrophyte reference tree, and mathematical reconciliation of gene trees, we note a probable elevation of bacterial HGTs at foundational points in diatom evolution, following their divergence from other ochrophytes. Finally, we demonstrate that throughout ochrophyte evolutionary history, bacterial HGTs have been enriched in genes encoding secreted proteins. Our study provides insights into the sources and frequency of HGTs, and functional contributions that HGT has made to algal evolution.

ppGpp influences protein protection, growth and photosynthesis in Phaeodactylum tricornutum

Chloroplasts retain elements of a bacterial stress response pathway that is mediated by the signalling nucleotides guanosine penta- and tetraphosphate ((p)ppGpp). In the model flowering plant Arabidopsis, ppGpp acts as a potent regulator of plastid gene expression and influences photosynthesis, plant growth and development. However, little is known about ppGpp metabolism or its evolution in other photosynthetic eukaryotes. Here, we studied the function of ppGpp in the diatom Phaeodactylum tricornutum using transgenic lines containing an inducible system for ppGpp accumulation. We used these lines to investigate the effects of ppGpp on growth, photosynthesis, lipid metabolism and protein expression. We demonstrate that ppGpp accumulation reduces photosynthetic capacity and promotes a quiescent-like state with reduced proliferation and ageing. Strikingly, using nontargeted proteomics, we discovered that ppGpp accumulation also leads to the coordinated upregulation of a protein protection response in multiple cellular compartments. Our findings highlight the importance of ppGpp as a fundamental regulator of chloroplast function across different domains of life, and lead to new questions about the molecular mechanisms and roles of (p)ppGpp signalling in photosynthetic eukaryotes.

MiR8181 is involved in the cell growth regulation of Saccharina japonica

Aureochrome, a blue-light receptor specifically found in photosynthetic stramenopiles, plays an important role in algal growth and development. It holds a reversed effector-sensor topology for the reception of blue light, acting as a candidate of optogenetic tool in transcriptional regulation. However, the inner regulatory mechanism of aureochrome is still unclear. In this study, we explored the potential regulatory relationship between microRNAs (miRNAs) and mRNAs by small RNA, transcriptome and degradome sequencing in Saccharina japonica. Through screening miRNA-mRNA interaction networks at the whole-genome level, we found that 18 miRNAs perfectly paired with aureochrome. Among these screened miRNAs, miR8181 was negatively correlated with aureochrome5 with high credibility, exhibiting tissue-specific expression in sporophyte of S. japonica. Degradome analysis further revealed the exact cleavage site of miR8181 on aureochrome5, confirming their targeting relationship. For the 54 target genes of miR8181, nine genes that exhibited similar expression to that of aureochrome5 competed for the same binding site, thus establishing a competing endogenous RNA network. Functional enrichment of the target genes revealed that miR8181 was involved in the regulation of cell differentiation and development in S. japonica. Moreover, overexpression of miR8181 resulted in significant decreases in the cell growth rates of Phaeodactylum tricornutum, suggesting negative roles of miR8181 in regulating cell growth. Our study revealed that miR8181, the targeting miRNA of aureochrome5, played negative roles in cell growth and development.

New paradigm in diatom omics and genetic manipulation

Diatoms are one of the most heterogeneous eukaryotic plankton known for regulating earth's biogeochemical cycles and maintaining the marine ecosystems ever since the late Eocene epoch. The advent of multidisciplinary omics approach has both epitomized and revolutionized the nature of their chimeric genetic toolkit, ecophysiology, and metabolic adaptability as well as their interaction with other communities. In addition, advanced functional annotation of transcriptomic and proteomic data using cutting edge bioinformatics tools together with high-resolution genome-scale mathematical modeling has effectively proven as the catapult in solving genetic bottlenecks in microbial as well as diatom exploration. In this review, a corroborative summation of the robust work done in manipulating, engineering, and sequencing of the diatom genomes besides underpinning the holistic application of omics in transcription and translation has been discussed in order to shrewd their multifarious novel potential in the field of biotechnology and provide an insight into their dynamic evolutionary relevance.

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.

Genome wide natural variation of H3K27me3 selectively marks genes predicted to be important for cell differentiation in Phaeodactylum tricornutum

In multicellular organisms, Polycomb Repressive Complex2 (PRC2) is known to deposit tri-methylation of lysine 27 of histone H3 (H3K27me3) to establish and maintain gene silencing, critical for developmentally regulated processes. The PRC2 complex is absent in both widely studied model yeasts, which initially suggested that PRC2 arose with the emergence of multicellularity. However, its discovery in several unicellular species including microalgae questions its role in unicellular eukaryotes. Here, we use Phaeodactylum tricornutum enhancer of zeste E(z) knockouts and show that P. tricornutum E(z) is responsible for di- and tri-methylation of lysine 27 of histone H3. H3K27me3 depletion abolishes cell morphology in P. tricornutum providing evidence for its role in cell differentiation. Genome-wide profiling of H3K27me3 in fusiform and triradiate cells further revealed genes that may specify cell identity. These results suggest a role for PRC2 and its associated mark in cell differentiation in unicellular species, and highlight their ancestral function in a broader evolutionary context than currently is appreciated.

The heterologous expression of a plastocyanin in the diatom Phaeodactylum tricornutum improves cell growth under iron-deficient conditions

We have investigated if the heterologous expression of a functional green alga plastocyanin in the diatom Phaeodactylum tricornutum can improve photosynthetic activity and cell growth. Previous in vitro assays showed that a single-mutant of the plastocyanin from the green algae Chlamydomonas reinhardtii is effective in reducing P. tricornutum photosystem I. In this study, in vivo assays with P. tricornutum strains expressing this plastocyanin indicate that even the relatively low intracellular concentrations of holo-plastocyanin detected (≈4 μM) are enough to promote an increased growth (up to 60%) under iron-deficient conditions as compared with the WT strain, measured as higher cell densities, content in pigments and active photosystem I, global photosynthetic rates per cell, and even cell volume. In addition, the presence of plastocyanin as an additional photosynthetic electron carrier seems to decrease the over-reduction of the plastoquinone pool. Consequently, it promotes an improvement in the maximum quantum yield of both photosystem II and I, together with a decrease in the acceptor side photoinhibition of photosystem II-also associated to a reduced oxidative stress-a decrease in the peroxidation of membrane lipids in the choroplast, and a lower degree of limitation on the donor side of photosystem I. Thus the heterologous plastocyanin appears to act as a functional electron carrier, alternative to the native cytochrome c₆ , under iron-limiting conditions.

Enhancement of excitation-energy quenching in fucoxanthin chlorophyll a/c-binding proteins isolated from a diatom Phaeodactylum tricornutum upon excess-light illumination

Photosynthetic organisms regulate pigment composition and molecular oligomerization of light-harvesting complexes in response to solar light intensities, in order to improve light-harvesting efficiency. Here we report excitation-energy dynamics and relaxation of fucoxanthin chlorophyll a/c-binding protein (FCP) complexes isolated from a diatom Phaeodactylum tricornutum grown under high-light (HL) illumination. Two types of FCP complexes were prepared from this diatom under the HL condition, whereas one FCP complex was isolated from the cells grown under a low-light (LL) condition. The subunit composition and oligomeric states of FCP complexes under the HL condition are different from those under the LL condition. Absorption and fluorescence spectra at 77 K of the FCP complexes also vary between the two conditions, indicating modifications of the pigment composition and arrangement upon the HL illumination. Time-resolved fluorescence curves at 77 K of the FCP complexes under the HL condition showed shorter lifetime components compared with the LL condition. Fluorescence decay-associated spectra at 77 K showed distinct excitation-energy-quenching components and alterations of energy-transfer pathways in the FCP complexes under the HL condition. These findings provide insights into molecular and functional mechanisms of the dynamic regulation of FCPs in this diatom under excess-light conditions.

Mitochondrial fatty acid β-oxidation is required for storage-lipid catabolism in a marine diatom

Photoautotrophic growth in nature requires the accumulation of energy-containing molecules via photosynthesis during daylight to fuel nighttime catabolism. Many diatoms store photosynthate as the neutral lipid triacylglycerol (TAG). While the pathways of diatom fatty acid and TAG synthesis appear to be well conserved with plants, the pathways of TAG catabolism and downstream fatty acid β-oxidation have not been characterised in diatoms. We identified a putative mitochondria-targeted, bacterial-type acyl-CoA dehydrogenase (PtMACAD1) that is present in Stramenopile and Hacrobian eukaryotes, but not found in plants, animals or fungi. Gene knockout, protein-YFP tags and physiological assays were used to determine PtMACAD1's role in the diatom Phaeodactylum tricornutum. PtMACAD1 is located in the mitochondria. Absence of PtMACAD1 led to no consumption of TAG at night and slower growth in light : dark cycles compared with wild-type. Accumulation of transcripts encoding peroxisomal-based β-oxidation did not change in response to day : night cycles or to PtMACAD1 knockout. Mutants also hyperaccumulated TAG after the amelioration of N limitation. We conclude that diatoms utilise mitochondrial β-oxidation; this is in stark contrast to the peroxisomal-based pathways observed in plants and green algae. We infer that this pattern is caused by retention of catabolic pathways from the host during plastid secondary endosymbiosis.

Engineering the Unicellular Alga Phaeodactylum tricornutum for Enhancing Carotenoid Production

Microalgae represent a promising resource for the production of beneficial natural compounds due to their richness in secondary metabolites and easy cultivation. Carotenoids feature among distinctive compounds of many microalgae, including diatoms, which owe their golden color to the xanthophyll fucoxanthin. Carotenoids have antioxidant, anti-obesity and anti-inflammatory properties, and there is a considerable market demand for these compounds. Here, with the aim to increase the carotenoid content in the model diatom Phaeodactylum tricornutum, we exploited genetic transformation to overexpress genes involved in the carotenoid biosynthetic pathway. We produced transgenic lines over-expressing simultaneously one, two or three carotenoid biosynthetic genes, and evaluated changes in pigment content with high-performance liquid chromatography. Two triple transformants over-expressing the genes Violaxanthin de-epoxidase (Vde), Vde-related (Vdr) and Zeaxanthin epoxidase 3 (Zep3) showed an accumulation of carotenoids, with an increase in the fucoxanthin content up to four fold. Vde, Vdr and Zep3 mRNA and protein levels in the triple transformants were coherently increased. The exact role of these enzymes in the diatom carotenoid biosynthetic pathway is not completely elucidated nevertheless our strategy successfully modulated the carotenoid metabolism leading to an accumulation of valuable compounds, leading the way toward improved utilization of microalgae in the field of antioxidants.

Enhanced pyruvate metabolism in plastids by overexpression of putative plastidial pyruvate transporter in Phaeodactylum tricornutum

BACKGROUND

The development of microalgal strains for enhanced biomass and biofuel production has received increased attention. Moreover, strain development via metabolic engineering for commercial production is being considered as the most efficient strategy. Pyruvate is an essential metabolite in the cells and plays an essential role in amino acid biosynthesis and de novo fatty acid biosynthesis in plastids. Although pyruvate can be a valuable target for metabolic engineering, its transporters have rarely been studied in microalgae. In this study, we aimed to identify the plastidial pyruvate transporter of Phaeodactylum tricornutum and utilize it for strain development.

RESULTS

We identified putative pyruvate transporter localized in the plastid membrane of Phaeodactylum tricornutum. Transformants overexpressing the pyruvate transporter were generated to increase the influx of pyruvate into plastids. Overexpression of a plastidial pyruvate transporter in P. tricornutum resulted in enhanced biomass (13.6% to 21.9%), lipid contents (11% to 30%), and growth (3.3% to 8.0%) compared to those of wild type during one-stage cultivation.

CONCLUSION

To regulate the pyruvate influx and its metabolism in plastids, we generated transformants overexpressing the putative plastidial pyruvate transporter in P. tricornutum. They showed that its overexpression for compartmentalizing pyruvate in plastids could be an attractive strategy for the effective production of biomass and lipids with better growth, via enhanced pyruvate metabolism in plastids.

Sexual reproduction potential implied by functional analysis of SPO11 in Phaeodactylum tricornutum

Phaeodactylum tricornutum is a model microalgae that is widely used to study diatom physiology and ecology. Since the meiotic process and sexual cycle have never been observed directly, P. tricornutum has been considered to be an asexual species. However, phylogenetic analysis of the P. tricornutum genome has revealed a series of meiosis-specific gene homologues in this species. We identified two copies of differently transcribed SPO11 homologs that contain the conserved motifs of Winged-helix and Toprim domains. The homolog PtSPO11-3 interacts with TopoVIB in yeast two-hybrid analysis, whereas the homolog PtSPO11-2 could rescue the sporulation defect of a Spo11 yeast mutant strain. PtSPO11-2 was also found to be significantly up-regulated at low temperatures in P. tricornutum and its key catalytic residue was important to the homolog's function in sporulation. The results herein provide positive clue that meiosis and sexual reproduction could exist in this diatom.

The possibility of using marine diatom-infecting viral promoters for the engineering of marine diatoms

Marine diatoms constitute a major group of unicellular photosynthetic eukaryotes. Diatoms are widely applicable for both basic studies and applied studies. Molecular tools and techniques have been developed for diatom research. Among these tools, several endogenous gene promoters (e.g., the fucoxanthin chlorophyll a/c-binding protein gene promoter) have become available for expressing transgenes in diatoms. Gene promoters that drive transgene expression at a high level are very important for the metabolic engineering of diatoms. Various marine diatom-infecting viruses (DIVs), including both DNA viruses and RNA viruses, have recently been isolated, and their genome sequences have been characterized. Promoters from viruses that infect plants and mammals are widely used as constitutive promoters to achieve high expression of transgenes. Thus, we recently investigated the activity of promoters derived from marine DIVs in the marine diatom, Phaeodactylum tricornutum. We discuss novel viral promoters that will be useful for the future metabolic engineering of diatoms.

Metabolic Engineering Strategies in Diatoms Reveal Unique Phenotypes and Genetic Configurations With Implications for Algal Genetics and Synthetic Biology

Diatoms are photosynthetic microeukaryotes that dominate phytoplankton populations and have increasing applicability in biotechnology. Uncovering their complex biology and elevating strains to commercial standards depends heavily on robust genetic engineering tools. However, engineering microalgal genomes predominantly relies on random integration of transgenes into nuclear DNA, often resulting in detrimental “position-effects” such as transgene silencing, integration into transcriptionally-inactive regions, and endogenous sequence disruption. With the recent development of extrachromosomal transgene expression via independent episomes, it is timely to investigate both strategies at the phenotypic and genomic level. Here, we engineered the model diatom Phaeodactylum tricornutum to produce the high-value heterologous monoterpenoid geraniol, which, besides applications as fragrance and insect repellent, is a key intermediate of high-value pharmaceuticals. Using high-throughput phenotyping we confirmed the suitability of episomes for synthetic biology applications and identified superior geraniol-yielding strains following random integration. We used third generation long-read sequencing technology to generate a complete analysis of all transgene integration events including their genomic locations and arrangements associated with high-performing strains at a genome-wide scale with subchromosomal detail, never before reported in any microalga. This revealed very large, highly concatenated insertion islands, offering profound implications on diatom functional genetics and next generation genome editing technologies, and is key for developing more precise genome engineering approaches in diatoms, including possible genomic safe harbour locations to support high transgene expression for targeted integration approaches. Furthermore, we have demonstrated that exogenous DNA is not integrated inadvertently into the nuclear genome of extrachromosomal-expression clones, an important characterisation of this novel engineering approach that paves the road to synthetic biology applications.

Microalgae as a future food source

One of the key challenges that we face in the 21st century is the need to feed an ever-increasing human population with increasingly limited natural resources. Even today it is estimated that roughly 1 out of 9 people in the world are undernourished, of which the most important factor is protein-energy malnutrition. By establishing microalgae as a new food and feed platform, we have the opportunity to increase the supply of these essential products to address global demands in a more efficient and environmentally sustainable way. Many types of algae are nutritionally complete foods, their yields outperform most plant crops, and there is a growing set of tools to develop improved strains of algae. Similar improvements were achieved in traditional crops through thousands of years of breeding and strain selection, whereas with the newest genetic engineering tools and advanced strain selection techniques, similar changes can be implemented in microalgae in just a few years. Here we describe different strategies that could be used to enhance the nutritional content, productivity, and organoleptic traits of algae to help drive development of this new crop. Clearly developing more efficient, sustainable, and nutritious foods and feed would be an enormous benefit for the planet, and algae represents an opportunity to develop a new crop that would complement traditional agriculture, and one that could potential result in a more efficient means to meet the world's food and feed supply.

The Myb-like transcription factor phosphorus starvation response (PtPSR) controls conditional P acquisition and remodelling in marine microalgae

Phosphorus (P) is one of the limiting macronutrients for algal growth in marine environments. Microalgae have developed adaptation mechanisms to P limitation that involve remodelling of internal phosphate resources and accumulation of lipids. Here, we used in silico analyses to identify the P-stress regulator PtPSR (Phaeodactylum tricornutum phosphorus starvation response) in the diatom P. tricornutum. ptpsr mutant lines were generated using gene editing and characterised by various molecular, genetics and biochemical tools. PtPSR belongs to a clade of Myb transcription factors that are conserved in stramenopiles and distantly related to plant P-stress regulators. PtPSR bound specifically to a conserved cis-regulatory element found in the regulatory region of P-stress-induced genes. ptpsr knockout mutants showed reduction in cell growth under P limitation. P-stress responses were impaired in ptpsr mutants compared with wild-type, including reduced induction of P-stress response genes, near to complete loss of alkaline phosphatase activity and reduced phospholipid degradation. We conclude that PtPSR is a key transcription factor influencing P scavenging, phospholipid remodelling and cell growth in adaptation to P stress in diatoms.

A genomics approach reveals the global genetic polymorphism, structure, and functional diversity of ten accessions of the marine model diatom Phaeodactylum tricornutum

Diatoms emerged in the Mesozoic period and presently constitute one of the main primary producers in the world's ocean and are of a major economic importance. In the current study, using whole genome sequencing of ten accessions of the model diatom Phaeodactylum tricornutum, sampled at broad geospatial and temporal scales, we draw a comprehensive landscape of the genomic diversity within the species. We describe strong genetic subdivisions of the accessions into four genetic clades (A-D) with constituent populations of each clade possessing a conserved genetic and functional makeup, likely a consequence of the limited dispersal of P. tricornutum in the open ocean. We further suggest dominance of asexual reproduction across all the populations, as implied by high linkage disequilibrium. Finally, we show limited yet compelling signatures of genetic and functional convergence inducing changes in the selection pressure on many genes and metabolic pathways. We propose these findings to have significant implications for understanding the genetic structure of diatom populations in nature and provide a framework to assess the genomic underpinnings of their ecological success and impact on aquatic ecosystems where they play a major role. Our work provides valuable resources for functional genomics and for exploiting the biotechnological potential of this model diatom species.

Multiplexed Knockouts in the Model Diatom Phaeodactylum by Episomal Delivery of a Selectable Cas9

Marine diatoms are eukaryotic microalgae that play significant ecological and biogeochemical roles in oceans. They also have significant potential as organismal platforms for exploitation to address biotechnological and industrial goals. In order to address both modes of research, sophisticated molecular and genetic tools are required. We presented here new and improved methodologies for introducing CRISPR-Cas9 to the model diatom Phaeodactylum tricornutum cells and a streamlined protocol for genotyping mutant cell lines with previously unknown phenotypes. First, bacterial-conjugation was optimized for the delivery of Cas9 by transcriptionally fusing Cas9 to a selectable marker by the 2A peptide. An episome cloning strategy using both negative and positive selection was developed to streamline CRISPR-episome assembly. Next, cell line picking and genotyping strategies, that utilize manual sequencing curation, TIDE sequencing analysis, and a T7 endonuclease assay, were developed to shorten the time required to generate mutants. Following this new experimental pipeline, both single-gene and two-gene knockout cell lines were generated at mutagenesis efficiencies of 48% and 25%, respectively. Lastly, a protocol for precise gene insertions via CRISPR-Cas9 targeting was developed using particle-bombardment transformation methods. Overall, the novel Cas9 episome design and improved genotyping methods presented here allow for quick and easy genotyping and isolation of Phaeodactylum mutant cell lines (less than 3 weeks) without relying on a known phenotype to screen for mutants.

Evolution and regulation of nitrogen flux through compartmentalized metabolic networks in a marine diatom

Diatoms outcompete other phytoplankton for nitrate, yet little is known about the mechanisms underpinning this ability. Genomes and genome-enabled studies have shown that diatoms possess unique features of nitrogen metabolism however, the implications for nutrient utilization and growth are poorly understood. Using a combination of transcriptomics, proteomics, metabolomics, fluxomics, and flux balance analysis to examine short-term shifts in nitrogen utilization in the model pennate diatom in Phaeodactylum tricornutum, we obtained a systems-level understanding of assimilation and intracellular distribution of nitrogen. Chloroplasts and mitochondria are energetically integrated at the critical intersection of carbon and nitrogen metabolism in diatoms. Pathways involved in this integration are organelle-localized GS-GOGAT cycles, aspartate and alanine systems for amino moiety exchange, and a split-organelle arginine biosynthesis pathway that clarifies the role of the diatom urea cycle. This unique configuration allows diatoms to efficiently adjust to changing nitrogen status, conferring an ecological advantage over other phytoplankton taxa.

Here, using the diatom Phaeodactylum tricornutum as a model organism, the authors combine functional genomics, phylogenetics, and metabolic modeling to describe how diatoms might have functionally integrated nitrogen metabolism during evolution and how metabolic flux is regulated across cellular compartments

Electroporation Transformation Protocol for Phaeodactylum tricornutum

Genetic transformation system is very important for both basic biological research and commercial exploitation of diatoms. Here we describe a high-efficiency nuclear transformation method for the model diatom Phaeodactylum tricornutum using an electroporation system, and the maximum transformation frequency obtained is about 3 × 10^-5 cells. The described protocol also provides some clue for developing electroporation transformation system in other eukaryotic microalgae.

Verification of the Saccharina japonica Translocon Tic20 and its Localization in the Chloroplast Membrane in Diatoms

Tic20 is an important translocon protein that plays a role in protein transport in the chloroplast. The sequence of Tic20 was determined in the lower brown alga Saccharina japonica. Structural analysis of SjTic20 revealed a noncanonical structure consisting of an N-terminal non-cyanobacterium-originated EF-hand domain (a helix-loop-helix structural domain) and a C-terminal cyanobacterium-originated Tic20 domain. Subcellular localization and transmembrane analysis indicated that SjTic20 featured an "M"-type Nin-Cin-terminal orientation, with four transmembrane domains in the innermost membrane of the chloroplast in the microalga Phaeodactylum tricornutum, and the EF-hand domain was entirely extruded into the chloroplast stroma. Our study provides information on the structure, localization, and topological features of SjTic20, and further functional analysis of SjTic20 in S. japonica is needed.

A novel endogenous selection marker for the diatom Phaeodactylum tricornutum based on a unique mutation in phytoene desaturase 1

Phaeodactylum tricornutum is a well-developed model diatom for both marine ecology and microalgal biotechnology, which has been enabled by the sequenced genome and the availability of gene delivery tools, such as biolistic transformation and E. coli-mediated conjugation. Till now, these tools have mainly relied on two selectable markers of bacterial origin which confer resistance to antibiotics Zeocin and nourseothricin. An alternative cost-effective and preferably endogenous selectable marker would facilitate gene stacking efforts through successive transformation or conjugation. We performed UV-mutagenesis of P. tricornutum to obtain mutations in the phytoene desaturase (PDS) gene, conferring resistance to the bleaching herbicide norflurazon. Two mutants displaying high tolerance to norflurazon and carrying unique mutations in PtPDS1 (PHATRDRAFT_45735) were selected. These mutants revealed novel point mutations at a conserved residue Gly290 to Ser/Arg. Homology-based structural modeling of mutated PDS1, over a resolved crystallographic model of rice PDS1 complexed with norflurazon, suggests steric hindrance by bulkier residue substitution may confer herbicide resistance. We report the characterization of PtPDS1 mutants and the development of the first endogenous selectable marker in diatoms suitable for industrial strain development, with the added benefit of biocontainment. The plasmid carrying the mutated PDS1 as a selection marker and eGFP as a reporter was created. An optimized biolistic transformation system is reported which allowed the isolation of positive transgenic events at the rate of 96.7%. Additionally, the ease of in vivo UV-mutagenesis may be employed as a strategy to create PDS-norflurazon-based selectable markers for other diatoms.

Comparative characterization of putative chitin deacetylases from Phaeodactylum tricornutum and Thalassiosira pseudonana highlights the potential for distinct chitin-based metabolic processes in diatoms

Chitin is generally considered to be present in centric diatoms but not in pennate species. Many aspects of chitin biosynthetic pathways have not been explored in diatoms. We retrieved chitin metabolic genes from pennate (Phaeodactylum tricornutum) and centric (Thalassiosira pseudonana) diatom genomes. Chitin deacetylase (CDA) genes from each genome (PtCDA and TpCDA) were overexpressed in P. tricornutum. We performed comparative analysis of their sequence structure, phylogeny, transcriptional profiles, localization and enzymatic activities. The chitin relevant proteins show complex subcellular compartmentation. PtCDA was likely acquired by horizontal gene transfer from prokaryotes, whereas TpCDA has closer relationships with sequences in Opisthokonta. Using transgenic P. tricornutum lines expressing CDA-green fluorescent protein (GFP) fusion proteins, PtCDA predominantly localizes to Golgi apparatus whereas TpCDA localizes to endoplasmic reticulum/chloroplast endoplasmic reticulum membrane. CDA-GFP overexpression upregulated the transcription of chitin synthases and potentially enhanced the ability of chitin synthesis. Although both CDAs are active on GlcNAc₅ , TpCDA is more active on the highly acetylated chitin polymer DA60. We have addressed the ambiguous characters of CDAs from P. tricornutum and T. pseudonana. Differences in localization, evolution, expression and activities provide explanations underlying the greater potential of centric diatoms for chitin biosynthesis. This study paves the way for in vitro applications of novel CDAs.

Engineering the unicellular alga Phaeodactylum tricornutum for high-value plant triterpenoid production

Plant triterpenoids constitute a diverse class of organic compounds that play a major role in development, plant defence and environmental interaction. Several triterpenes have demonstrated potential as pharmaceuticals. One example is betulin, which has shown promise as a pharmaceutical precursor for the treatment of certain cancers and HIV. Major challenges for triterpenoid commercialization include their low production levels and their cost-effective purification from the complex mixtures present in their natural hosts. Therefore, attempts to produce these compounds in industrially relevant microbial systems such as bacteria and yeasts have attracted great interest. Here, we report the production of the triterpenes betulin and its precursor lupeol in the photosynthetic diatom Phaeodactylum tricornutum, a unicellular eukaryotic alga. This was achieved by introducing three plant enzymes in the microalga: a Lotus japonicus oxidosqualene cyclase and a Medicago truncatula cytochrome P450 along with its native reductase. The introduction of the L. japonicus oxidosqualene cyclase perturbed the mRNA expression levels of the native mevalonate and sterol biosynthesis pathway. The best performing strains were selected and grown in a 550-L pilot-scale photobioreactor facility. To our knowledge, this is the most extensive pathway engineering undertaken in a diatom and the first time that a sapogenin has been artificially produced in a microalga, demonstrating the feasibility of the photo-bio-production of more complex high-value, metabolites in microalgae.