Modulation of the light-harvesting chlorophyll antenna size in Chlamydomonas reinhardtii by TLA1 gene over-expression and RNA interference

The cadmium tolerance and bioaccumulation mechanism of Tetratostichococcus sp. P1: insight from transcriptomics analysis

Cadmium (Cd) has become a severe issue in relatively low concentration and attracts expert attention due to its toxicity, accumulation, and biomagnification in living organisms. Cd does not have a biological role and causes serious health issues. Therefore, Cd pollutants should be reduced and removed from the environment. Microalgae have great potential for Cd absorption for waste treatment since they are more environmentally friendly than existing treatment methods and have strong metal sorption selectivity. This study evaluated the tolerance and ability of the microalga Tetratostichococcus sp. P1 to remove Cd ions under acidic conditions and reveal mechanisms based on transcriptomics analysis. The results showed that Tetratostichococcus sp. P1 had a high Cd tolerance that survived under the presence of Cd up to 100 µM, and IC50, the half-maximal inhibitory concentration value, was 57.0 μM, calculated from the change in growth rate based on the chlorophyll content. Long-term Cd exposure affected the algal morphology and photosynthetic pigments of the alga. Tetratostichococcus sp. P1 removed Cd with a maximum uptake of 1.55 mg g-1 dry weight. Transcriptomic analysis revealed the upregulation of the expression of genes related to metal binding, such as metallothionein. Group A, Group B transporters and glutathione, were also found upregulated. While the downregulation of the genes were related to photosynthesis, mitochondria electron transport, ABC-2 transporter, polysaccharide metabolic process, and cell division. This research is the first study on heavy metal bioremediation using Tetratostichococcus sp. P1 and provides a new potential microalga strain for heavy metal removal in wastewater.[Figure: see text]Abbreviations:BP: Biological process; bZIP: Basic Leucine Zipper; CC: Cellular component; ccc1: Ca (II)-sensitive cross complementary 1; Cd: Cadmium; CDF: Cation diffusion facilitator; Chl: Chlorophyll; CTR: Cu TRansporter families; DAGs: Directed acyclic graphs; DEGs: Differentially expressed genes; DVR: Divinyl chlorophyllide, an 8-vinyl-reductase; FPN: FerroportinN; FTIR: Fourier transform infrared; FTR: Fe TRansporter; GO: Gene Ontology; IC50: Growth half maximal inhibitory concentration; ICP: Inductively coupled plasma; MF: molecular function; NRAMPs: Natural resistance-associated aacrophage proteins; OD: Optical density; RPKM: Reads Per Kilobase of Exon Per Million Reads Mapped; VIT1: Vacuolar iron transporter 1 families; ZIPs: Zrt-, Irt-like proteins.

Biomonitoring of heavy metals and their phytoremediation by duckweeds: Advances and prospects

Heavy metals (HMs) contamination of water bodies severely threatens human and ecosystem health. There is growing interest in the use of duckweeds for HMs biomonitoring and phytoremediation due to their fast growth, low cultivation costs, and excellent HM uptake efficiency. In this review, we summarize the current state of knowledge on duckweeds and their suitability for HM biomonitoring and phytoremediation. Duckweeds have been used for phytotoxicity assays since the 1930s. Some toxicity tests based on duckweeds have been listed in international guidelines. Duckweeds have also been recognized for their ability to facilitate HM phytoremediation in aquatic environments. Large-scale screening of duckweed germplasm optimized for HM biomonitoring and phytoremediation is still essential. We further discuss the morphological, physiological, and molecular effects of HMs on duckweeds. However, the existing data are clearly insufficient, especially in regard to dissection of the transcriptome, metabolome, proteome responses and molecular mechanisms of duckweeds under HM stresses. We also evaluate the influence of environmental factors, exogenous substances, duckweed community composition, and HM interactions on their HM sensitivity and HM accumulation, which need to be considered in practical application scenarios. Finally, we identify challenges and propose approaches for improving the effectiveness of duckweeds for bioremediation from the aspects of selection of duckweed strain, cultivation optimization, engineered duckweeds. We foresee great promise for duckweeds as phytoremediation agents, providing environmentally safe and economically efficient means for HM removal. However, the primary limiting issue is that so few researchers have recognized the outstanding advantages of duckweeds. We hope that this review can pique the interest and attention of more researchers.

Proteomic characterization of a lutein-hyperaccumulating Chlamydomonas reinhardtii mutant reveals photoprotection-related factors as targets for increasing cellular carotenoid content

BACKGROUND

Microalgae are emerging hosts for the sustainable production of lutein, a high-value carotenoid; however, to be commercially competitive with existing systems, their capacity for lutein sequestration must be augmented. Previous attempts to boost microalgal lutein production have focussed on upregulating carotenoid biosynthetic enzymes, in part due to a lack of metabolic engineering targets for expanding lutein storage.

RESULTS

Here, we isolated a lutein hyper-producing mutant of the model green microalga Chlamydomonas reinhardtii and characterized the metabolic mechanisms driving its enhanced lutein accumulation using label-free quantitative proteomics. Norflurazon- and high light-resistant C. reinhardtii mutants were screened to yield four mutant lines that produced significantly more lutein per cell compared to the CC-125 parental strain. Mutant 5 (Mut-5) exhibited a 5.4-fold increase in lutein content per cell, which to our knowledge is the highest fold increase of lutein in C. reinhardtii resulting from mutagenesis or metabolic engineering so far. Comparative proteomics of Mut-5 against its parental strain CC-125 revealed an increased abundance of light-harvesting complex-like proteins involved in photoprotection, among differences in pigment biosynthesis, central carbon metabolism, and translation. Further characterization of Mut-5 under varying light conditions revealed constitutive overexpression of the photoprotective proteins light-harvesting complex stress-related 1 (LHCSR1) and LHCSR3 and PSII subunit S regardless of light intensity, and increased accrual of total chlorophyll and carotenoids as light intensity increased. Although the photosynthetic efficiency of Mut-5 was comparatively lower than CC-125, the amplitude of non-photochemical quenching responses of Mut-5 was 4.5-fold higher than in CC-125 at low irradiance.

CONCLUSIONS

We used C. reinhardtii as a model green alga and identified light-harvesting complex-like proteins (among others) as potential metabolic engineering targets to enhance lutein accumulation in microalgae. These have the added value of imparting resistance to high light, although partially compromising photosynthetic efficiency. Further genetic characterization and engineering of Mut-5 could lead to the discovery of unknown players in photoprotective mechanisms and the development of a potent microalgal lutein production system.

Simultaneous CAS9 editing of cpSRP43, LHCA6, and LHCA7 in Picochlorum celeri lowers chlorophyll levels and improves biomass productivity

High cellular pigment levels in dense microalgal cultures contribute to excess light absorption. To improve photosynthetic yields in the marine microalga Picochlorum celeri, CAS9 gene editing was used to target the molecular chaperone cpSRP43. Depigmented strains (>50% lower chlorophyll) were generated, with proteomics showing attenuated levels of most light harvesting complex (LHC) proteins. Gene editing generated two types of cpSRP43 transformants with distinct lower pigment phenotypes: (i) a transformant (Δsrp43) with both cpSRP43 diploid alleles modified to encode non-functional polypeptides and (ii) a transformant (STR30309) with a 3 nt in-frame insertion in one allele at the CAS9 cut site (non-functional second allele), leading to expression of a modified cpSRP43. STR30309 has more chlorophyll than Δsrp43 but substantially less than wild type. To further decrease light absorption by photosystem I in STR30309, CAS9 editing was used to stack in disruptions of both LHCA6 and LHCA7 to generate STR30843, which has higher (5-24%) productivities relative to wild type in solar-simulating bioreactors. Maximal productivities required frequent partial harvests throughout the day. For STR30843, exemplary diel bioreactor yields of ~50 g m-2 day-1 were attained. Our results demonstrate diel productivity gains in P. celeri by lowering pigment levels.

Clustered regularly interspaced short palindromic repeats (CRISPR) technology and genetic engineering strategies for microalgae towards carbon neutrality: A critical review

Carbon dioxide is the major greenhouse gas and regards as the critical issue of global warming and climate changes. The inconspicuous microalgae are responsible for 40% of carbon fixation among all photosynthetic plants along with a higher photosynthetic efficiency and convert the carbon into lipids, protein, pigments, and bioactive compounds. Genetic approach and metabolic engineering are applied to accelerate the growth rate and biomass of microalgae, hence achieve the mission of carbon neutrality. Meanwhile, CRISPR/Cas9 is efficiently to enhance the productivity of high-value compounds in microalgae for it is easier operation, more affordable and is able to regulate multiple genes simultaneously. The genetic engineering strategies provide the multidisciplinary concept to evolute and increase the CO₂ fixation rate through Calvin-Benson-Bassham cycle. Therefore, the technologies, bioinformatics tools, systematic engineering approaches for carbon neutrality and circular economy are summarized and leading one step closer to the decarbonization society in this review.

Effect of heavy metals mixture on the growth and physiology of Tetraselmis sp.: Applications to lipid production and bioremediation

Phycoremediation of heavy metals from contaminated waters by oleaginous microalgae is an eco-friendly and emerging trend. Different concentrations of toxic metals such as nickel (Ni), chromium (Cr) and cobalt (Co) were added in Tetraselmis sp. culture media. Mixture Design was used to model the effect of these metals on cell growth, lipid production and heavy metals removal. Tetraselmis sp. was identified as an outstanding Ni, Cr and Co accumulator with bioconcentration factors of 675.17, 584.9 and 169.81 within binary mixtures (Ni × Cr), (Cr × Co) and (Cr × Co) at 6 mg/L, respectively. Optimization studies showed that the highest cell growth (9.22 × 10⁵ cells/mL), lipid content (31% Dry Weight) and metals removal (91%) were obtained with the optimum binary mixture Ni (54.45 %) and Cr (45.45 %). This work presents interesting results revealing the potential of Tetraselmis sp. for nickel removal up to 97 % combined to its potential for biodiesel production.

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.

Heavy metal detoxification mechanisms by microalgae: Insights from transcriptomics analysis

Heavy metal pollution in ecosystem is a global threat. The associated toxicity and carcinogenic nature of heavy metals/metalloids such as mercury, cadmium, lead, and arsenic are imposing a severe risk to both ecological diversity and human lives. Harnessing the adaptive feature of microalgae for remediating toxic heavy metal has reached a milestone in past few decades. Transcriptomics analyses have provided mechanistic insights to map the dynamics of cellular events under heavy metal stress, thus deciphering the strategic responses of microalgae. Here, the present review comprehensively addresses the elicited molecular responses of microalgae to detoxify the heavy metal stress. The review highlights the intricate role of biochemical components and signaling networks mediating stress responsive transitions of microalgae at physiological level. Furthermore, the differential gene expression signifying the transporters involved in uptake, distribution/sequestration, and efflux of heavy metal has also been reviewed. In a nutshell, this study provided a comprehensive understanding of the molecular mechanisms adopted by microalgae at transcriptome level to nullify the oxidative stress while detoxifying the heavy metals.

Variables Governing Photosynthesis and Growth in Microalgae Mass Cultures

Since the 1950s, microalgae have been grown commercially in man-made cultivation units and used for biomass production as a source of food and feed supplements, pharmaceuticals, cosmetics and lately biofuels, as well as a means for wastewater treatment and mitigation of atmospheric CO2 build-up. In this work, photosynthesis and growth affecting variables—light intensity, pH, CO2/O2 exchange, nutrient supply, culture turbulence, light/dark cell cycling, biomass density and culture depth (light path)—are reviewed as concerns in microalgae mass cultures. Various photosynthesis monitoring techniques were employed to study photosynthetic performance to optimize the growth of microalgae strains in outdoor cultivation units. The most operative and reliable techniques appeared to be fast-response ones based on chlorophyll fluorescence and oxygen production monitoring, which provide analogous results.

Light regulation of light-harvesting antenna size substantially enhances photosynthetic efficiency and biomass yield in green algae†

One of the major factors limiting biomass productivity in algae is the low thermodynamic efficiency of photosynthesis. The greatest thermodynamic inefficiencies in photosynthesis occur during the conversion of light into chemical energy. At full sunlight the light-harvesting antenna captures photons at a rate nearly 10 times faster than the rate-limiting step in photosynthetic electron transport. Excess captured energy is dissipated by non-productive pathways including the production of reactive oxygen species. Substantial improvements in photosynthetic efficiency have been achieved by reducing the optical cross-section of the light-harvesting antenna by selectively reducing chlorophyll b levels and peripheral light-harvesting complex subunits. Smaller light-harvesting antenna, however, may not exhibit optimal photosynthetic performance in low or fluctuating light environments. We describe a translational control system to dynamically adjust light-harvesting antenna sizes for enhanced photosynthetic performance. By expressing a chlorophyllide a oxygenase (CAO) gene having a 5' mRNA extension encoding a Nab1 translational repressor binding site in a CAO knockout line it was possible to continuously alter chlorophyll b levels and correspondingly light-harvesting antenna sizes by light-activated Nab1 repression of CAO expression as a function of growth light intensity. Significantly, algae having light-regulated antenna sizes had substantially higher photosynthetic rates and two-fold greater biomass productivity than the parental wild-type strains as well as near wild-type ability to carry out state transitions and non-photochemical quenching. These results have broad implications for enhanced algae and plant biomass productivity.

Potential and Challenges of Improving Photosynthesis in Algae

Sunlight energy largely exceeds the energy required by anthropic activities, and therefore its exploitation represents a major target in the field of renewable energies. The interest in the mass cultivation of green microalgae has grown in the last decades, as algal biomass could be employed to cover a significant portion of global energy demand. Advantages of microalgal vs. plant biomass production include higher light-use efficiency, efficient carbon capture and the valorization of marginal lands and wastewaters. Realization of this potential requires a decrease of the current production costs, which can be obtained by increasing the productivity of the most common industrial strains, by the identification of factors limiting biomass yield, and by removing bottlenecks, namely through domestication strategies aimed to fill the gap between the theoretical and real productivity of algal cultures. In particular, the light-to-biomass conversion efficiency represents one of the major constraints for achieving a significant improvement of algal cell lines. This review outlines the molecular events of photosynthesis, which regulate the conversion of light into biomass, and discusses how these can be targeted to enhance productivity through mutagenesis, strain selection or genetic engineering. This review highlights the most recent results in the manipulation of the fundamental mechanisms of algal photosynthesis, which revealed that a significant yield enhancement is feasible. Moreover, metabolic engineering of microalgae, focused upon the development of renewable fuel biorefineries, has also drawn attention and resulted in efforts for enhancing productivity of oil or isoprenoids.

Combined resistance to oxidative stress and reduced antenna size enhance light-to-biomass conversion efficiency in Chlorella vulgaris cultures

BACKGROUND

Microalgae are efficient producers of lipid-rich biomass, making them a key component in developing a sustainable energy source, and an alternative to fossil fuels. Chlorella species are of special interest because of their fast growth rate in photobioreactors. However, biological constraints still cast a significant gap between the high cost of biofuel and cheap oil, thus hampering perspective of producing CO₂-neutral biofuels. A key issue is the inefficient use of light caused by its uneven distribution in the culture that generates photoinhibition of the surface-exposed cells and darkening of the inner layers. Efficient biofuel production, thus, requires domestication, including traits which reduce optical density of cultures and enhance photoprotection.

RESULTS

We applied two steps of mutagenesis and phenotypic selection to the microalga Chlorella vulgaris. First, a pale-green mutant (PG-14) was selected, with a 50% reduction of both chlorophyll content per cell and LHCII complement per PSII, with respect to WT. PG-14 showed a 30% increased photon conversion into biomass efficiency vs. WT. A second step of mutagenesis of PG-14, followed by selection for higher tolerance to Rose Bengal, led to the isolation of pale-green genotypes, exhibiting higher resistance to singlet oxygen (strains SOR). Growth in photobioreactors under high light conditions showed an enhanced biomass production of SOR strains with respect to PG-14. When compared to WT strain, biomass yield of the pale green + sor genotype was enhanced by 68%.

CONCLUSIONS

Domestication of microalgae like Chlorella vulgaris, by optimizing both light distribution and ROS resistance, yielded an enhanced carbon assimilation rate in photobioreactor.

Biomass from microalgae: the potential of domestication towards sustainable biofactories

Interest in bulk biomass from microalgae, for the extraction of high-value nutraceuticals, bio-products, animal feed and as a source of renewable fuels, is high. Advantages of microalgal vs. plant biomass production include higher yield, use of non-arable land, recovery of nutrients from wastewater, efficient carbon capture and faster development of new domesticated strains. Moreover, adaptation to a wide range of environmental conditions evolved a great genetic diversity within this polyphyletic group, making microalgae a rich source of interesting and useful metabolites. Microalgae have the potential to satisfy many global demands; however, realization of this potential requires a decrease of the current production costs. Average productivity of the most common industrial strains is far lower than maximal theoretical estimations, suggesting that identification of factors limiting biomass yield and removing bottlenecks are pivotal in domestication strategies aimed to make algal-derived bio-products profitable on the industrial scale. In particular, the light-to-biomass conversion efficiency represents a major constraint to finally fill the gap between theoretical and industrial productivity. In this respect, recent results suggest that significant yield enhancement is feasible. Full realization of this potential requires further advances in cultivation techniques, together with genetic manipulation of both algal physiology and metabolic networks, to maximize the efficiency with which solar energy is converted into biomass and bio-products. In this review, we draft the molecular events of photosynthesis which regulate the conversion of light into biomass, and discuss how these can be targeted to enhance productivity through mutagenesis, strain selection or genetic engineering. We outline major successes reached, and promising strategies to achieving significant contributions to future microalgae-based biotechnology.

Leveraging algal omics to reveal potential targets for augmenting TAG accumulation

Ongoing global efforts to commercialize microalgal biofuels have expedited the use of multi-omics techniques to gain insights into lipid biosynthetic pathways. Functional genomics analyses have recently been employed to complement existing sequence-level omics studies, shedding light on the dynamics of lipid synthesis and its interplay with other cellular metabolic pathways, thus revealing possible targets for metabolic engineering. Here, we review the current status of algal omics studies to reveal potential targets to augment TAG accumulation in various microalgae. This review specifically aims to examine and catalog systems level data related to stress-induced TAG accumulation in oleaginous microalgae and inform future metabolic engineering strategies to develop strains with enhanced bioproductivity, which could pave a path for sustainable green energy.

Deletion of the chloroplast LTD protein impedes LHCI import and PSI-LHCI assembly in Chlamydomonas reinhardtii

Nuclear-encoded light-harvesting chlorophyll- and carotenoid-binding proteins (LHCPs) are imported into the chloroplast and transported across the stroma to thylakoid membrane assembly sites by the chloroplast signal recognition particle (CpSRP) pathway. The LHCP translocation defect (LTD) protein is essential for the delivery of imported LHCPs to the CpSRP pathway in Arabidopsis. However, the function of the LTD protein in Chlamydomonas reinhardtii has not been investigated. Here, we generated a C. reinhardtii ltd (Crltd) knockout mutant by using CRISPR-Cas9, a new target-specific knockout technology. The Crltd1 mutant showed a low chlorophyll content per cell with an unusual increase in appressed thylakoid membranes and enlarged cytosolic vacuoles. Profiling of thylakoid membrane proteins in the Crltd1 mutant showed a more severe reduction in the levels of photosystem I (PSI) core proteins and absence of functional LHCI compared with those of photosystem II, resulting in a much smaller PSI pool size and diminished chlorophyll antenna size. The lack of CrLTD did not prevent photoautotrophic growth of the cells. These results are substantially different from those for Arabidopsis ltd null mutant, indicating LTD function in LHCP delivery and PSI assembly may not be as stringent in C. reinhardtii as it is in higher plants.

The mechanisms whereby the green alga Chlorella ohadii, isolated from desert soil crust, exhibits unparalleled photodamage resistance

Excess illumination damages the photosynthetic apparatus with severe implications with regard to plant productivity. Unlike model organisms, the growth of Chlorella ohadii, isolated from desert soil crust, remains unchanged and photosynthetic O2 evolution increases, even when exposed to irradiation twice that of maximal sunlight. Spectroscopic, biochemical and molecular approaches were applied to uncover the mechanisms involved. D1 protein in photosystem II (PSII) is barely degraded, even when exposed to antibiotics that prevent its replenishment. Measurements of various PSII parameters indicate that this complex functions differently from that in model organisms and suggest that C. ohadii activates a nonradiative electron recombination route which minimizes singlet oxygen formation and the resulting photoinhibition. The light-harvesting antenna is very small and carotene composition is hardly affected by excess illumination. Instead of succumbing to photodamage, C. ohadii activates additional means to dissipate excess light energy. It undergoes major structural, compositional and physiological changes, leading to a large rise in photosynthetic rate, lipids and carbohydrate content and inorganic carbon cycling. The ability of C. ohadii to avoid photodamage relies on a modified function of PSII and the dissipation of excess reductants downstream of the photosynthetic reaction centers. The biotechnological potential as a gene source for crop plant improvement is self-evident.

Multi-Level Light Capture Control in Plants and Green Algae

Life on Earth relies on photosynthesis, and the ongoing depletion of fossil carbon fuels has renewed interest in phototrophic light-energy conversion processes as a blueprint for the conversion of atmospheric CO2 into various organic compounds. Light-harvesting systems have evolved in plants and green algae, which are adapted to the light intensity and spectral composition encountered in their habitats. These organisms are constantly challenged by a fluctuating light supply and other environmental cues affecting photosynthetic performance. Excess light can be especially harmful, but plants and microalgae are equipped with different acclimation mechanisms to control the processing of sunlight absorbed at both photosystems. We summarize the current knowledge and discuss the potential for optimization of phototrophic light-energy conversion.

In Metabolic Engineering of Eukaryotic Microalgae: Potential and Challenges Come with Great Diversity

The great phylogenetic diversity of microalgae is corresponded by a wide arrange of interesting and useful metabolites. Nonetheless metabolic engineering in microalgae has been limited, since specific transformation tools must be developed for each species for either the nuclear or chloroplast genomes. Microalgae as production platforms for metabolites offer several advantages over plants and other microorganisms, like the ability of GMO containment and reduced costs in culture media, respectively. Currently, microalgae have proved particularly well suited for the commercial production of omega-3 fatty acids and carotenoids. Therefore most metabolic engineering strategies have been developed for these metabolites. Microalgal biofuels have also drawn great attention recently, resulting in efforts for improving the production of hydrogen and photosynthates, particularly triacylglycerides. Metabolic pathways of microalgae have also been manipulated in order to improve photosynthetic growth under specific conditions and for achieving trophic conversion. Although these pathways are not strictly related to secondary metabolites, the synthetic biology approaches could potentially be translated to this field and will also be discussed.

Fast forward genetics to identify mutations causing a high light tolerant phenotype in Chlamydomonas reinhardtii by whole-genome-sequencing

BACKGROUND

High light tolerance of microalgae is a desired phenotype for efficient cultivation in large scale production systems under fluctuating outdoor conditions. Outdoor cultivation requires the use of either wild-type or non-GMO derived mutant strains due to safety concerns. The identification and molecular characterization of such mutants derived from untagged forward genetics approaches was limited previously by the tedious and time-consuming methods involving techniques such as classical meiotic mapping. The combination of mapping with next generation sequencing technologies offers alternative strategies to identify genes involved in high light adaptation in untagged mutants.

RESULTS

We used the model alga Chlamydomonas reinhardtii in a non-GMO mutation strategy without any preceding crossing step or pooled progeny to identify genes involved in the regulatory processes of high light adaptation. To generate high light tolerant mutants, wildtype cells were mutagenized only to a low extent, followed by a stringent selection. We performed whole-genome sequencing of two independent mutants hit1 and hit2 and the parental wildtype. The availability of a reference genome sequence and the removal of shared bakground variants between the wildtype strain and each mutant, enabled us to identify two single nucleotide polymorphisms within the same gene Cre02.g085050, hereafter called LRS1 (putative Light Response Signaling protein 1). These two independent single amino acid exchanges are both located in the putative WD40 propeller domain of the corresponding protein LRS1. Both mutants exhibited an increased rate of non-photochemical-quenching (NPQ) and an improved resistance against chemically induced reactive oxygen species. In silico analyses revealed homology of LRS1 to the photoregulatory protein COP1 in plants.

CONCLUSIONS

In this work we identified the nuclear encoded gene LRS1 as an essential factor for high light adaptation in C. reinhardtii. The causative random mutation within this gene was identified by a rapid and efficient method, avoiding any preceding crossing step, meiotic mapping, or pooled progeny. Our results open up new insights into mechanisms of high light adaptation in microalgae and at the same time provide a simplified strategy for non-GMO forward genetics, a crucial precondition that could result in the identification of key factors for economically relevant biological processes within algae.

Domestication of the green alga Chlorella sorokiniana: reduction of antenna size improves light-use efficiency in a photobioreactor

BACKGROUND

The utilization of biomass from microalgae for biofuel production is one of the key elements for the development of a sustainable and secure energy supply. Among the different microalgae, Chlorella species are of interest because of their high productivity, high lipid content, and resistance to the high light conditions typical of photobioreactors. However, the economic feasibility of growing algae at an industrial scale is yet to be realized, in part because of biological constraints that limit biomass yield. A key issue is the inefficient use of light due to uneven light distribution, and the dissipation of excess absorbed light as heat. The successful implementation of biofuel production facilities requires the development of algal strains with enhanced light use efficiency in photobioreactors. Such domestication strategies include decreasing the absorption cross section in order to enhance light penetration, increasing the size of metabolic sinks per chlorophyll and minimizing feedback energy dissipation.

RESULTS

In this work we applied random mutagenesis and phenotypic selection to the thermotolerant, fast-growing Chlorella species, C. sorokiniana. Truncated antenna mutants (TAMs) were selected that exhibited a lower fluorescence yield than the wild-type (WT) strain. Six putatively interesting mutants were selected by high throughput fluorescence video imaging, two of which, TAM-2 and TAM-4, were found to have approximately half the chlorophyll content per cell and LHCII complement per PSII with respect to the WT. In batch culture, TAM-2 showed an increased photon use efficiency, yielding a higher Pmax at saturating irradiances with respect to the WT. Cultivation of TAM-2 in both laboratory-scale and outdoor photobioreactors showed higher productivity than WT, with a 30% higher biomass yield in dense cell suspensions typical of industrial photobioreactors.

CONCLUSIONS

These results suggest that generation of mutants with low chlorophyll content can significantly improve the light-to-biomass conversion efficiency of C. sorokiniana under mass culture conditions. However, owing to the lack of sexual reproduction in this species, the presence of additional mutations might affect growth rate, suggesting that selection should include evaluation of multiple independent mutants for each desired phenotype.

Improving the sunlight-to-biomass conversion efficiency in microalgal biofactories

Microalgae represent promising organisms for the sustainable production of commodities, chemicals or fuels. Future use of such systems, however, requires increased productivity of microalgal mass cultures in order to reach an economic viability for microalgae-based production schemes. The efficiency of sunlight-to-biomass conversion that can be observed in bulk cultures is generally far lower (35-80%) than the theoretical maximum, because energy losses occur at multiple steps during the light-driven conversion of carbon dioxide to organic carbon. The light-harvesting system is a major source of energy losses and thus a prime target for strain engineering. Truncation of the light-harvesting antenna in the algal model organism Chlamydomonas reinhardtii was shown to be an effective way of increasing culture productivity at least under saturating light conditions. Furthermore engineering of the Calvin-Benson cycle or the creation of photorespiratory bypasses in A. thaliana proved to be successful in terms of achieving higher biomass productivities. An efficient generation of novel microalgal strains with improved sunlight conversion efficiencies by targeted engineering in the future will require an expanded molecular toolkit. In the meantime random mutagenesis coupled to high-throughput screening for desired phenotypes can be used to provide engineered microalgae.

Regulation and dynamics of the light-harvesting system

Photosynthetic organisms are continuously subjected to changes in light quantity and quality, and must adjust their photosynthetic machinery so that it maintains optimal performance under limiting light and minimizes photodamage under excess light. To achieve this goal, these organisms use two main strategies in which light-harvesting complex II (LHCII), the light-harvesting system of photosystem II (PSII), plays a key role both for the collection of light energy and for photoprotection. The first is energy-dependent nonphotochemical quenching, whereby the high-light-induced proton gradient across the thylakoid membrane triggers a process in which excess excitation energy is harmlessly dissipated as heat. The second involves a redistribution of the mobile LHCII between the two photosystems in response to changes in the redox poise of the electron transport chain sensed through a signaling chain. These two processes strongly diminish the production of damaging reactive oxygen species, but photodamage of PSII is unavoidable, and it is repaired efficiently.