Introduction of exogenous DNA into Chlamydomonas reinhardtii by electroporation

A potential paradigm in CRISPR/Cas systems delivery: at the crossroad of microalgal gene editing and algal-mediated nanoparticles

Microalgae as the photosynthetic organisms offer enormous promise in a variety of industries, such as the generation of high-value byproducts, biofuels, pharmaceuticals, environmental remediation, and others. With the rapid advancement of gene editing technology, CRISPR/Cas system has evolved into an effective tool that revolutionised the genetic engineering of microalgae due to its robustness, high target specificity, and programmability. However, due to the lack of robust delivery system, the efficacy of gene editing is significantly impaired, limiting its application in microalgae. Nanomaterials have become a potential delivery platform for CRISPR/Cas systems due to their advantages of precise targeting, high stability, safety, and improved immune system. Notably, algal-mediated nanoparticles (AMNPs), especially the microalgae-derived nanoparticles, are appealing as a sustainable delivery platform because of their biocompatibility and low toxicity in a homologous relationship. In addition, living microalgae demonstrated effective and regulated distribution into specified areas as the biohybrid microrobots. This review extensively summarised the uses of CRISPR/Cas systems in microalgae and the recent developments of nanoparticle-based CRISPR/Cas delivery systems. A systematic description of the properties and uses of AMNPs, microalgae-derived nanoparticles, and microalgae microrobots has also been discussed. Finally, this review highlights the challenges and future research directions for the development of gene-edited microalgae.

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.

In Vivo Monitoring of Intracellular Metabolite in a Microalgal Cell Using an Aptamer/Graphene Oxide Nanosheet Complex

Real-time sensing and imaging of intracellular metabolites in living cells are crucial tools for the characterization of complex biological processes, including the dynamic fluctuation of metabolites. Therefore, additional efforts are required to develop in vivo detection strategies for the visualization and quantification of specific target metabolites, particularly in microalgae. In this study, we developed a strategy to monitor a specific microalgal metabolite in living cells using an aptamer/graphene oxide nanosheet (GOnS) complex. As a proof-of-concept, β-carotene, an antioxidant pigment that accumulates in most microalgal species, was chosen as a target metabolite. To achieve this, a β-carotene-specific aptamer was selected through graphene oxide-assisted systematic evolution of ligands by exponential enrichment (GO-SELEX) and characterized thereafter. The aptamer could sensitively sense the changes in the concentration of β-carotene (i.e., the target metabolite) and more specifically bind to β-carotene than to nontargets. The selected aptamer was labeled with a fluorophore (fluorescein; FAM) and allowed to form an aptamer/GOnS complex that protected the aptamer from nucleic cleavages. The aptamer/GOnS complex was delivered into the cells via electroporation, thus enabling the sensitive monitoring of β-carotene in the cell by quantifying the aptamer fluorescence intensity. The results suggest that our biocompatible strategy could be employed to visualize and semiquantify intracellular microalgae metabolites in vivo, which holds a great potential in diverse fields such as metabolite analysis and mutant screening.

Pulse Electric Field Technology for Wastewater and Biomass Residues’ Improved Valorization

Development and adoption of more efficient and robust technologies for reuse of wastewater embedded resources, in particular materials and energy, is becoming an unavoidable necessity. Among many emerging technologies in the sector of wastewater treatment residuals valorization, Pulsed Electric Field (PEF) processes have shown interesting potential, although they have not yet entered the sector’s mainstream as a consolidated commercial technology, as in other industrial applications, such as the food, medical, and bio-based industries. PEF is a non-thermal technology suitable to biological applications, involving gentle cell disintegration and enhanced cell membrane permeability and as such applicable to disinfection, sterilization, and to those processes that benefit from an enhanced extraction of organic compounds from biological matter, such as anaerobic digestion, biological processes for recovery of nutrients, and biorefinery of cell-embedded compounds. PEF technology applications in wastewater/biomass residues management are reported and advantages, drawbacks, and barriers of the technology are discussed in this paper.

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.

Nuclear transformation of Chlamydomonas reinhardtii: A review

Chlamydomonas reinhardtii is a model organism with three sequenced genomes capable of genetic transformation. C. reinhardtii has the advantages of being low cost, non-toxic, and having a post-translational modification system that ensures the recombinant proteins have the same activity as natural proteins, thus making it a great platform for application in molecular biology and other fields. In this review, we summarize the existing methods for nuclear transformation of C. reinhardtii, genes for selection, examples of foreign protein expression, and factors affecting transformation efficiency, to provide insights into effective strategies for the nuclear transformation of C. reinhardtii.

Genome Editing by CRISPR-Cas: A Game Change in the Genetic Manipulation of Chlamydomonas

Microalgae are promising photosynthetic unicellular eukaryotes among the most abundant on the planet and are considered as alternative sustainable resources for various industrial applications. Chlamydomonas is an emerging model for microalgae to be manipulated by multiple biotechnological tools in order to produce high-value bioproducts such as biofuels, bioactive peptides, pigments, nutraceuticals, and medicines. Specifically, Chlamydomonas reinhardtii has become a subject of different genetic-editing techniques adapted to modulate the production of microalgal metabolites. The main nuclear genome-editing tools available today include zinc finger nucleases (ZFNs), transcriptional activator-like effector nucleases (TALENs), and more recently discovered the clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR associated protein (Cas) nuclease system. The latter, shown to have an interesting editing capacity, has become an essential tool for genome editing. In this review, we highlight the available literature on the methods and the applications of CRISPR-Cas for C. reinhardtii genetic engineering, including recent transformation methods, most used bioinformatic tools, best strategies for the expression of Cas protein and sgRNA, the CRISPR-Cas mediated gene knock-in/knock-out strategies, and finally the literature related to CRISPR expression and modification approaches.

Stable transformation of unicellular green alga Coccomyxa subellipsoidea C-169 via electroporation

In this study, we have shown the applicability of electroporation and hygromycin B as a convenient selectable marker for stable nuclear transformation of Coccomyxa subellipsoidea C-169. Since it is the first sequenced eukaryotic microorganism from polar environment, this offers unique opportunities to study adaptation mechanisms to cold.

A review on edible vaccines and their prospects

For a long time, vaccines have been the main mode of defense and protection against several bacterial, viral, and parasitic diseases. However, the process of production and purification makes them expensive and unaffordable to many developing nations. An edible vaccine is when the antigen is expressed in the edible part of the plant. This reduces the cost of production of the vaccine because of ease of culturing. In this article, various types of edible vaccines that include algal and probiotics in addition to plants are discussed. Various diseases against which research has been carried out are also reviewed. This article focused on the conception of edible vaccines highlighting the various ways by which vaccines can be delivered.

Electroporation Procedures for Genetic Modification of Green Algae (Chlorella spp.)

The green algae of Chlorella spp. are usually very small (about 3-6 μm), typically have solid and thick cell wall tissue; thus, neither the gene-gun method based on particle carrier nor the glass-bead transformation method is suitable enough. Selecting the proper, effective strategy has always attracted researcher's attention. Electroporation is currently the most widely used method for the transformation of algal species. The principle of electroporation is that the cell membrane produces tiny holes by high-voltage pulses, which lead to the introduction of exogenous DNA into cells. The method was proved by simple in principle and effective in introducing foreign genes in several Chlorella species.

Agrobacterium-mediated transformation of Ceratocystis albifundus

Functional association between genomic loci and specific biological traits remains lacking in many fungi, including the African tree pathogen Ceratocystis albifundus. This is mainly because of the absence of suitable transformation systems for allowing genetic manipulation of this and other fungi. Here, we present an optimized protocol for Agrobacterium tumefaciens-mediated transformation of C. albifundus. Strain AGL-1 of A. tumefaciens and four binary T-DNA vectors (conferring hygromycin B or geneticin resistance and/or expressing the green fluorescent protein [GFP]) were used for transforming germinated conidia of three isolates of C. albifundus. Stable expression of these T-DNA-encoded traits was confirmed through sequential sub-culturing of fungal transformants on selective and non-selective media and by using PCR and sequence analysis. Single-copy integration of the respective T-DNAs into the genomes of these fungi was confirmed using Southern hybridization analysis. The range of experimental parameters determined and optimised included: (i) concentrations of hygromycin B and geneticin required for inhibiting growth of the wild type fungus and (ii) the dependence of transformation on acetosyringone for inducing the bacterium's virulence genes, as well as (iii) the duration of fungus-bacterium co-cultivation periods and (iv) the concentrations of fungal conidia and bacterial cells used for the latter. The system developed in this study is stable with a high-efficiency, yielding up to 400 transformants per 10⁶ conidia. This is the first report of a transformation protocol for C. albifundus and its availability will be invaluable for functional studies in this important fungus.

Rapid and high efficiency transformation of Chlamydomonas reinhardtii by square-wave electroporation

Chlamydomonas reinhardtii, the unicellular green algae, is the model organism for studies in various physiological processes and for bioindustrial applications. To explore the molecular mechanisms underlying physiological processes or to establish engineered cell lines, the exogenous DNA needs to be integrated into the genome for the insertional mutagenesis or transgene expression. However, the amount of selected marker DNA is not seriously considered in the existing electroporation methods for mutants library construction. Here, we reported a rapid-and-high-efficiency transformation technique for cell-walled strains using square-wave electroporation system. The final yield with this electroporation method was 2-6 × 10³ transformants per μg exogenous DNA for cell-walled strains in a strain-dependent manner. In general, this electroporation technique was the easy and applicable way to build a mutant library for screening phenotypes of interest.

Intraflagellar transport protein IFT52 recruits IFT46 to the basal body and flagella

Cilia are microtubule-based organelles and perform motile, sensing and signaling functions. The assembly and maintenance of cilia depend on intraflagellar transport (IFT). Besides ciliary localization, most IFT proteins accumulate at basal bodies. However, little is known about the molecular mechanism of basal body targeting of IFT proteins. We first identified the possible basal body-targeting sequence in IFT46 by expressing IFT46 truncation constructs in an ift46-1 mutant. The C-terminal sequence between residues 246-321, termed BBTS3, was sufficient to target YFP to basal bodies in the ift46-1 strain. Interestingly, BBTS3 is also responsible for the ciliary targeting of IFT46. BBTS3::YFP moves bidirectionally in flagella and interacts with other IFT complex B (IFT-B) proteins. Using IFT and motor mutants, we show that the basal body localization of IFT46 depends on IFT52, but not on IFT81, IFT88, IFT122, FLA10 or DHC1b. IFT52 interacts with IFT46 through residues L285 and L286 of IFT46 and recruits it to basal bodies. Ectopic expression of the C-terminal domain of IFT52 in the nucleus resulted in accumulation of IFT46 in nuclei. These data suggest that IFT52 and IFT46 can preassemble as a complex in the cytoplasm, which is then targeted to basal bodies.

Energy-efficient biomass processing with pulsed electric fields for bioeconomy and sustainable development

Fossil resources-free sustainable development can be achieved through a transition to bioeconomy, an economy based on sustainable biomass-derived food, feed, chemicals, materials, and fuels. However, the transition to bioeconomy requires development of new energy-efficient technologies and processes to manipulate biomass feed stocks and their conversion into useful products, a collective term for which is biorefinery. One of the technological platforms that will enable various pathways of biomass conversion is based on pulsed electric fields applications (PEF). Energy efficiency of PEF treatment is achieved by specific increase of cell membrane permeability, a phenomenon known as membrane electroporation. Here, we review the opportunities that PEF and electroporation provide for the development of sustainable biorefineries. We describe the use of PEF treatment in biomass engineering, drying, deconstruction, extraction of phytochemicals, improvement of fermentations, and biogas production. These applications show the potential of PEF and consequent membrane electroporation to enable the bioeconomy and sustainable development.

Transgene Expression in Microalgae-From Tools to Applications

Microalgae comprise a biodiverse group of photosynthetic organisms that reside in water sources and sediments. The green microalgae Chlamydomonas reinhardtii was adopted as a useful model organism for studying various physiological systems. Its ability to grow under both photosynthetic and heterotrophic conditions allows efficient growth of non-photosynthetic mutants, making Chlamydomonas a useful genetic tool to study photosynthesis. In addition, this green alga can grow as haploid or diploid cells, similar to yeast, providing a powerful genetic system. As a result, easy and efficient transformation systems have been developed for Chlamydomonas, targeting both the chloroplast and nuclear genomes. Since microalgae comprise a rich repertoire of species that offer variable advantages for biotech and biomed industries, gene transfer technologies were further developed for many microalgae to allow for the expression of foreign proteins of interest. Expressing foreign genes in the chloroplast enables the targeting of foreign DNA to specific sites by homologous recombination. Chloroplast transformation also allows for the introduction of genes encoding several enzymes from a complex pathway, possibly as an operon. Expressing foreign proteins in the chloroplast can also be achieved by introducing the target gene into the nuclear genome, with the protein product bearing a targeting signal that directs import of the transgene-product into the chloroplast, like other endogenous chloroplast proteins. Integration of foreign genes into the nuclear genome is mostly random, resulting in large variability between different clones, such that extensive screening is required. The use of different selection modalities is also described, with special emphasis on the use of herbicides and metabolic markers which are considered to be friendly to the environment, as compared to drug-resistance genes that are commonly used. Finally, despite the development of a wide range of transformation tools and approaches, expression of foreign genes in microalgae suffers from low efficiency. Thus, novel tools have appeared in recent years to deal with this problem. Finally, while C. reinhardtii was traditionally used as a model organism for the development of transformation systems and their subsequent improvement, similar technologies can be adapted for other microalgae that may have higher biotechnological value.

Exogenous Gene Integration for Microalgal Cell Transformation Using a Nanowire-Incorporated Microdevice

Superior green algal cells showing high lipid production and rapid growth rate are considered as an alternative for the next generation green energy resources. To achieve the biomass based energy generation, transformed microalgae with superlative properties should be developed through genetic engineering. Contrary to the normal cells, microalgae have rigid cell walls, so that target gene delivery into cells is challengeable. In this study, we report a ZnO nanowire-incorporated microdevice for a high throughput microalgal transformation. The proposed microdevice was equipped with not only a ZnO nanowire in the microchannel for gene delivery into cells but also a pneumatic polydimethylsiloxane (PDMS) microvalve to modulate the cellular attachment and detachment from the nanowire. As a model, hygromycin B resistance gene cassette (Hyg3) was functionalized on the hydrothermally grown ZnO nanowires through a disulfide bond and released into green algal cells, Chlamydomonas reinhardtii, by reductive cleavage. During Hyg3 gene delivery, a monolithic PDMS membrane was bent down, so that algal cells were pushed down toward ZnO nanowires. The supply of vacuum in the pneumatic line made the PDMS membrane bend up, enabling the gene delivered algal cells to be recovered from the outlet of the microchannel. We successfully confirmed Hyg3 gene integrated in microalgae by amplifying the inserted gene through polymerase chain reaction (PCR) and DNA sequencing. The efficiency of the gene delivery to algal cells using the ZnO nanowire-incorporated microdevice was 6.52 × 10(4)- and 9.66 × 10(4)-fold higher than that of a traditional glass bead beating and electroporation.

Biotechnological exploitation of microalgae

Microalgae are a diverse group of single-cell photosynthetic organisms that include cyanobacteria and a wide range of eukaryotic algae. A number of microalgae contain high-value compounds such as oils, colorants, and polysaccharides, which are used by the food additive, oil, and cosmetic industries, among others. They offer the potential for rapid growth under photoautotrophic conditions, and they can grow in a wide range of habitats. More recently, the development of genetic tools means that a number of species can be transformed and hence used as cell factories for the production of high-value chemicals or recombinant proteins. In this article, we review exploitation use of microalgae with a special emphasis on genetic engineering approaches to develop cell factories, and the use of synthetic ecology approaches to maximize productivity. We discuss the success stories in these areas, the hurdles that need to be overcome, and the potential for expanding the industry in general.

Genetic tools and techniques for Chlamydomonas reinhardtii

The development of tools has always been a major driving force for the advancement of science. Optical microscopes were the first instruments that allowed discovery and descriptive studies of the subcellular features of microorganisms. Although optical and electron microscopes remained at the forefront of microbiological research tools since their inventions, the advent of molecular genetics brought about questions which had to be addressed with new "genetic tools". The unicellular green microalgal genus Chlamydomonas, especially the most prominent species C. reinhardtii, has become a frequently used model organism for many diverse fields of research and molecular genetic analyses of C. reinhardtii, as well as the available genetic tools and techniques, have become increasingly sophisticated throughout the last decades. The aim of this review is to provide an overview of the molecular key features of C. reinhardtii and summarize the progress related to the development of tools and techniques for genetic engineering of this organism, from pioneering DNA transformation experiments to state-of-the-art techniques for targeted nuclear genome editing and high-throughput screening approaches.

Molecular techniques to interrogate and edit the Chlamydomonas nuclear genome

The success of the green alga Chlamydomonas reinhardtii as a model organism is to a large extent due to the wide range of molecular techniques that are available for its characterization. Here, we review some of the techniques currently used to modify and interrogate the C. reinhardtii nuclear genome and explore several technologies under development. Nuclear mutants can be generated with ultraviolet (UV) light and chemical mutagens, or by insertional mutagenesis. Nuclear transformation methods include biolistic delivery, agitation with glass beads, and electroporation. Transforming DNA integrates into the genome at random sites, and multiple strategies exist for mapping insertion sites. A limited number of studies have demonstrated targeted modification of the nuclear genome by approaches such as zinc-finger nucleases and homologous recombination. RNA interference is widely used to knock down expression levels of nuclear genes. A wide assortment of transgenes has been successfully expressed in the Chlamydomonas nuclear genome, including transformation markers, fluorescent proteins, reporter genes, epitope tagged proteins, and even therapeutic proteins. Optimized expression constructs and strains help transgene expression. Emerging technologies such as the CRISPR/Cas9 system, high-throughput mutant identification, and a whole-genome knockout library are being developed for this organism. We discuss how these advances will propel future investigations.

Construction of novel chloroplast expression vector and development of an efficient transformation system for the diatom Phaeodactylum tricornutum

Plastids are ideal subcellular hosts for the expression of transgenes and have been successfully used for the production of different biopolymers, therapeutic proteins and industrial enzymes. Phaeodactylum tricornutum is a widely used aquatic feed species. In this study, we focused on developing a high-efficiency plastid expression system for P. tricornutum. In the plastid transformation vector, the site selected for integration was the transcriptionally active intergenic region present between the trnI and trnA genes, located in the IR (inverted repeat) regions of the plastid genome. Initially, a CAT reporter gene (encoding chloramphenicol acetyltransferase) was integrated at this site in the plastid genome. The expression of CAT in the transformed microalgae conferred resistance to the antibiotic chloramphenicol, which enabled growth in the selection media. Overall, the plastid transformation efficiency was found to be approximately one transplastomic colony per 1,000 microalgae cells. Subsequently, a heterologous gene expression cassette for high-level expression of the target gene was created and cloned between the homologous recombination elements. A TA cloning strategy based on the designed XcmI-XcmI sites could conveniently clone the heterologous gene. An eGFP (green fluorescent protein) reporter gene was used to test the expression level in the plastid system. The relatively high-level expression of eGFP without codon optimisation in stably transformed microalgae was determined to account for 0.12 % of the total soluble protein. Thus, this study presents the first and convenient plastid gene expression system for diatoms and represents an interesting tool to study diatom plastids.

The potential of transgenic green microalgae; a robust photobioreactor to produce recombinant therapeutic proteins

Microalgae have been used in food, cosmetic, and biofuel industries as a natural source of lipids, vitamins, pigments and antioxidants for a long time. Green microalgae, as potent photobioreactors, can be considered as an economical expression system to produce recombinant therapeutical proteins at large-scale due to low cost of production and scaling-up capitalization owning to the inexpensive medium requirement, fast growth rate, and the ease of manipulation. These microalgae possess all benefit eukaryotic expression systems including the ability of post-translational modifications required for proper folding and stability of active proteins. Among the many items regarded as recombinant protein production, this review compares the different expression systems with green microalgae like Dunaliella by viewing the nuclear/chloroplast transformation challenges/benefits, related selection markers/reporter genes, and crucial factors/strategies affecting the increase of foreign protein expression in microalgae transformants. Some important factors were discussed regarding the increase of protein yielding in microalgae transformants including: transformation-associated genotypic modifications, endogenous regulatory factors, promoters, codon optimization, enhancer elements, and milking of recombinant protein.

Stable nuclear transformation of Pandorina morum

Background

Volvocine green algae like Pandorina morum represent one of the most recent inventions of multicellularity diverged from their unicellular relatives. The 8–16 celled P. morum alga and its close multicellular relatives constitute a model lineage for research into cellular differentiation, morphogenesis and epithelial folding, sexual reproduction and evolution of multicellularity. Pandorina is the largest and most complex organism in the volvocine lineage that still exhibits isogamous sexual reproduction. So far, molecular-biological investigations in P. morum were constricted due to the absence of methods for transformation of this species, which is a prerequisite for introduction of reporter genes and (modified) genes of interest.

Results

Stable nuclear transformation of P. morum was achieved using chimeric constructs with a selectable marker, a reporter gene, promoters and upstream and downstream flanking sequences from heterologous sources. DNA was introduced into the cells by particle bombardment with plasmid-coated gold particles. The aminoglycoside 3′-phosphotransferase VIII (aphVIII) gene of Streptomyces rimosus under control of an artificial, heterologous promoter was used as the selectable marker. The artificial promoter contained a tandem arrangement of the promoter of both the heat shock protein 70A (hsp70A) and the ribulose-1,5-bisphosphat-carboxylase/-oxygenase S3 (rbcS3) gene of Volvox carteri. Due to the expression of aphVIII, transformants gained up to 333-fold higher resistance to paromomycin in comparison to the parent wild-type strain.

The heterologous luciferase (gluc) gene of Gaussia princeps, which was previously genetically engineered to match the nuclear codon usage of Chlamydomonas reinhardtii, was used as a co-transformed, unselectable reporter gene. The expression of the co-bombarded gluc gene in transformants and the induction of gluc by heat shock were demonstrated through bioluminescence assays.

Conclusion

Stable nuclear transformation of P. morum using the particle bombardment technique is now feasible. Functional expression of heterologous genes is achieved using heterologous flanking sequences from Volvox carteri and Chlamydomonas reinhardtii. The aphVIII gene of the actinobacterium S. rimosus can be used as a selectable marker for transformation experiments in the green alga P. morum. The gluc gene of the marine copepod G. princeps, expressed under control of heterologous promoter elements, represents a suitable reporter gene for monitoring gene expression or for other applications in P. morum.

Stable nuclear transformation of Eudorina elegans

Background

A fundamental step in evolution was the transition from unicellular to differentiated, multicellular organisms. Volvocine algae have been used for several decades as a model lineage to investigate the evolutionary aspects of multicellularity and cellular differentiation. There are two well-studied volvocine species, a unicellular alga (Chlamydomonas reinhardtii) and a multicellular alga with differentiated cell types (Volvox carteri). Species with intermediate characteristics also exist, which blur the boundaries between unicellularity and differentiated multicellularity. These species include the globular alga Eudorina elegans, which is composed of 16–32 cells. However, detailed molecular analyses of E. elegans require genetic manipulation. Unfortunately, genetic engineering has not yet been established for Eudorina, and only limited DNA and/or protein sequence information is available.

Results

Here, we describe the stable nuclear transformation of E. elegans by particle bombardment using both a chimeric selectable marker and reporter genes from different heterologous sources. Transgenic algae resistant to paromomycin were achieved using the aminoglycoside 3^′-phosphotransferase VIII (aphVIII) gene of Streptomyces rimosus, an actinobacterium, under the control of an artificial promoter consisting of two V. carteri promoters in tandem. Transformants exhibited an increase in resistance to paromomycin by up to 333-fold. Co-transformation with non-selectable plasmids was achieved with a rate of 50 - 100%. The luciferase (gluc) gene from the marine copepod Gaussia princeps, which previously was engineered to match the codon usage of C. reinhardtii, was used as a reporter gene. The expression of gluc was mediated by promoters from C. reinhardtii and V. carteri. Heterologous heat shock promoters induced an increase in luciferase activity (up to 600-fold) at elevated temperatures. Long-term stability and both constitutive and inducible expression of the co-bombarded gluc gene was demonstrated by transcription analysis and bioluminescence assays.

Conclusions

Heterologous flanking sequences, including promoters, work in E. elegans and permit both constitutive and inducible expression of heterologous genes. Stable nuclear transformation of E. elegans is now routine. Thus, we show that genetic engineering of a species is possible even without the resources of endogenous genes and promoters.

Droplet electroporation in microfluidics for efficient cell transformation with or without cell wall removal

An efficient cell transformation method is presented that utilizes droplet electroporation on a microfluidic chip. Two types of green microalgae, a wall-less mutant and a wild type of Chlamydomonas reinhardtii, are used as model cells. The PDMS-glass electroporation chip is simply composed of a flow-focusing microstructure for generating cell-encapsulating droplets and a serpentine channel for better mixing of the content in the droplet, and five pairs of parallel microelectrodes on the glass slide, without involving any expensive electrical equipment. The transformation efficiency via the microfluidic electroporation is shown to be more than three orders of magnitude higher for the wall-less mutant, and more than two orders of magnitude higher for the wild type, which has its cell wall intact, than bulk phase electroporation under identical conditions. Furthermore, the microfluidic transformation is remarkably efficient even at a low DNA/cell ratio, facilitating ways of controlling the transgenic copy number, which is important for the stability of the transgene expression.

Transformation of diatom Phaeodactylum tricornutum by electroporation and establishment of inducible selection marker

Diatoms are important primary producers in the marine ecosystem. Currently it is difficult to genetically transform diatoms due to the technical limitations of existing methods. The promoter/terminator of the nitrate reductase gene of the model diatom Phaeodactylum tricornutum was cloned and used to drive chloramphenicol acetyltransferase (CAT) reporter gene expression. The construct was transferred by electroporation into P. tricornutum grown in medium lacking silicon. CAT expression was induced in transformed diatoms in the presence of nitrate, enabling growth in selective medium, and was repressed when ammonium was the only nitrogen source. Expression of CAT transcript and protein were demonstrated by RT-PCR and Western blot analysis, respectively. Our study is the first to report a successful genetic transformation of diatom by electroporation in an economical and efficient manner and provides a tightly regulated inducible gene expression system for diatom.

Advances in genetic engineering of marine algae

Algae are a component of bait sources for animal aquaculture, and they produce abundant valuable compounds for the chemical industry and human health. With today's fast growing demand for algae biofuels and the profitable market for cosmetics and pharmaceuticals made from algal natural products, the genetic engineering of marine algae has been attracting increasing attention as a crucial systemic technology to address the challenge of the biomass feedstock supply for sustainable industrial applications and to modify the metabolic pathway for the more efficient production of high-value products. Nevertheless, to date, only a few marine algae species can be genetically manipulated. In this article, an updated account of the research progress in marine algal genomics is presented along with methods for transformation. In addition, vector construction and gene selection strategies are reviewed. Meanwhile, a review on the progress of bioreactor technologies for marine algae culture is also revisited.

Microalgae as platforms for production of recombinant proteins and valuable compounds: progress and prospects

Over the last few years microalgae have gained increasing interest as a natural source of valuable compounds and as bioreactors for recombinant protein production. Natural high-value compounds including pigments, long-chain polyunsaturated fatty acids, and polysaccharides, which have a wide range of applications in the food, feed, cosmetics, and pharmaceutical industries, are currently produced with nontransgenic microalgae. However, transgenic microalgae can be used as bioreactors for the production of therapeutic and industrially relevant recombinant proteins. This technology shows great promise to simplify the production process and significantly decrease the production costs. To date, a variety of recombinant proteins have been produced experimentally from the nuclear or chloroplast genome of transgenic Chlamydomonas reinhardtii. These include monoclonal antibodies, vaccines, hormones, pharmaceutical proteins, and others. In this review, we outline recent progress in the production of recombinant proteins with transgenic microalgae as bioreactors, methods for genetic transformation of microalgae, and strategies for highly efficient expression of heterologous genes. In particular, we highlight the importance of maximizing the value of transgenic microalgae through producing recombinant proteins together with recovery of natural high-value compounds. Finally, we outline some important issues that need to be addressed before commercial-scale production of high-value recombinant proteins and compounds from transgenic microalgae can be realized.

Solar-driven hydrogen production in green algae

The twin problems of energy security and global warming make hydrogen an attractive alternative to traditional fossil fuels with its combustion resulting only in the release of water vapor. Biological hydrogen production represents a renewable source of the gas and can be performed by a diverse range of microorganisms from strict anaerobic bacteria to eukaryotic green algae. Compared to conventional methods for generating H(2), biological systems can operate at ambient temperatures and pressures without the need for rare metals and could potentially be coupled to a variety of biotechnological processes ranging from desalination and waste water treatment to pharmaceutical production. Photobiological hydrogen production by microalgae is particularly attractive as the main inputs for the process (water and solar energy) are plentiful. This chapter focuses on recent developments in solar-driven H(2) production in green algae with emphasis on the model organism Chlamydomonas reinhardtii. We review the current methods used to achieve sustained H(2) evolution and discuss possible approaches to improve H(2) yields, including the optimization of culturing conditions, reducing light-harvesting antennae and targeting auxiliary electron transport and fermentative pathways that compete with the hydrogenase for reductant. Finally, industrial scale-up is discussed in the context of photobioreactor design and the future prospects of the field are considered within the broader context of a biorefinery concept.

Biofuels from algae: challenges and potential

Algae biofuels may provide a viable alternative to fossil fuels; however, this technology must overcome a number of hurdles before it can compete in the fuel market and be broadly deployed. These challenges include strain identification and improvement, both in terms of oil productivity and crop protection, nutrient and resource allocation and use, and the production of co-products to improve the economics of the entire system. Although there is much excitement about the potential of algae biofuels, much work is still required in the field. In this article, we attempt to elucidate the major challenges to economic algal biofuels at scale, and improve the focus of the scientific community to address these challenges and move algal biofuels from promise to reality.

Protein electroporation into Chlamydomonas for mutant rescue

Electroporation can be used to deliver proteins into Chlamydomonas cells. This technique is particularly useful to demonstrate rescue of paralyzed-flagella mutants with recombinant proteins that complement the mutations. The appearance of motile cells immediately provides assurance that the recombinant proteins have their native structures and are functional. It can be also used for the introduction of artificially modified proteins or other substances into live cells.

Influence of the cell wall on intracellular delivery to algal cells by electroporation and sonication

To assess the cell wall's role as a barrier to intracellular delivery, wild-type Chlamydomonas reinhardtii algal cells and mutant cells lacking a cell wall were exposed to electroporation or sonication. Flow cytometry determined intracellular uptake of calcein and bovine serum albumin (BSA) and loss of cell viability as functions of electroporation transmembrane potential and acoustic energy. Electroporation of wild-type cells increased calcein uptake with increasing transmembrane potential, but delivered much less BSA. Electroporation of wall-deficient cells had similar effects on calcein uptake, but increased BSA uptake as much as 7.5-fold relative to wild-type cells, which indicated that the cell wall was a significant barrier to BSA delivery during electroporation. Sonication of wild-type cells caused calcein and BSA uptake at similar levels. This suggests that the cell wall barrier to BSA delivery can be overcome by sonication. Increased electroporation transmembrane potential or acoustic energy also caused increased loss of cell viability, where wall-deficient cells were especially susceptible to lysis. Overall, we believe this is the first study to compare directly the effects of electroporation and sonication in any cell type. Specifically, these findings suggest that electroporation primarily transports molecules across the plasma membrane because its mechanism is specific to lipid bilayer disruption, whereas sonication transports molecules across both the plasma membrane and cell wall, because it nonspecifically disrupts cell-surface barriers.

Alternative methods for genetic transformation of Pseudozyma antarctica, a basidiomycetous yeast-like fungus

Electroporation and Agrobacterium tumefaciens-mediated transformation (ATMT) were adapted and optimized for genetic transformation of the basidiomycetous yeast-like fungus Pseudozyma antarctica as alternatives to the cumbersome PEG/CaCl(2)-mediated transformation of protoplasts. Electroporation yielded 100-200 transformants per mug of DNA per 10(8) cells after 3 days on selective medium. For its part, ATMT yielded 60-160 transformants per 10(6) input cfu after 5-10 days on a selective medium. Transformants obtained from both methods showed stable hygromycin resistance and strong expression of green fluorescent protein. Analysis of integration events revealed a limited number of predominantly tandem insertions in the genome of transformants, an improvement over PEG/CaCl(2)-mediated transformation. Both protocols relied on intact conidia of P. antarctica as starting material and thus eliminated the need for cell wall-degrading or weakening agents such as lytic enzymes or chemicals. Other advantages over protoplast transformation included higher yield of transformants and shorter recovery time of transformed colonies on selective medium.

Microalgae as bioreactors

Microalgae already serve as a major natural source of valuable macromolecules including carotenoids, long-chain polyunsaturated fatty acids and phycocolloids. As photoautotrophs, their simple growth requirements make these primitive plants potentially attractive bioreactor systems for the production of high-value heterologous proteins. The difficulty of producing stable transformants has meant that the field of transgenic microalgae is still in its infancy. Nonetheless, several species can now be routinely transformed and algal biotechnology companies have begun to explore the possibilities of synthesizing recombinant therapeutic proteins in microalgae and the engineering of metabolic pathways to produce increased levels of desirable compounds. In this review, we compare the current commercially viable bioreactor systems, outline recent progress in microalgal biotechnology and transformation, and discuss the potential of microalgae as bioreactors for the production of heterologous proteins.

Microalgae as bioreactors

Microalgae already serve as a major natural source of valuable macromolecules including carotenoids, long-chain polyunsaturated fatty acids and phycocolloids. As photoautotrophs, their simple growth requirements make these primitive plants potentially attractive bioreactor systems for the production of high-value heterologous proteins. The difficulty of producing stable transformants has meant that the field of transgenic microalgae is still in its infancy. Nonetheless, several species can now be routinely transformed and algal biotechnology companies have begun to explore the possibilities of synthesizing recombinant therapeutic proteins in microalgae and the engineering of metabolic pathways to produce increased levels of desirable compounds. In this review, we compare the current commercially viable bioreactor systems, outline recent progress in microalgal biotechnology and transformation, and discuss the potential of microalgae as bioreactors for the production of heterologous proteins.

Chlamydomonas reinhardtii in the landscape of pigments

This review focuses on the biosynthesis of pigments in the unicellular alga Chlamydomonas reinhardtii and their physiological and regulatory functions in the context of information gathered from studies of other photosynthetic organisms. C. reinhardtii is serving as an important model organism for studies of photosynthesis and the pigments associated with the photosynthetic apparatus. Despite extensive information pertaining to the biosynthetic pathways critical for making chlorophylls and carotenoids, we are just beginning to understand the control of these pathways, the coordination between pigment and apoprotein synthesis, and the interactions between the activities of these pathways and those for other important cellular metabolites branching from these pathways. Other exciting areas relating to pigment function are also emerging: the role of intermediates of pigment biosynthesis as messengers that coordinate metabolism in the chloroplast with nuclear gene activity, and the identification of photoreceptors and their participation in critical cellular processes including phototaxis, gametogenesis, and the biogenesis of the photosynthetic machinery. These areas of research have become especially attractive for intensive development with the application of potent molecular and genomic tools currently being applied to studies of C. reinhardtii.