1,721,294 research outputs found
Algae as a “new” biomass resource – Possibilities and Constraints
No full textAlgae are oxygenic photoautotrophs, offering a very high level of biodiversity and thus suitable for
different practical applications. Today they are mainly cultivated for human/animal food or to
extract high-value chemicals and pharmaceuticals. However, their exploitation could be extended.
Algae are attractive as high yield biomass producers, because of the short life cycle, the ability to
grow up to very high cell densities and the easy large-scale cultivation that doesn’t compete with
other demands such as those of conventional crops agriculture. Algae can be a resource of
renewable, sustainable biofuels. In addition, they can be transformed into ‘cell factories’ to produce
recombinant proteins of interest for pharmaceutical companies
Lhc proteins and the regulation of photosynthetic light harvesting function by xanthophylls
No full textPhotoprotection of the chloroplast is an important component of abiotic stress resistance in plants. Carotenoids have a central role in photoprotection. We review here the recent evidence, derived mainly from in vitro reconstitution of recombinant Lhc proteins with different carotenoids and from carotenoid biosynthesis mutants, for the existence of different mechanisms of photoprotection and regulation based on xanthophyll binding to Lhc proteins into multiple sites and the exchange of chromophores between different Lhc proteins during exposure of plants to high light stress and the operation of the xanthophyll cycle. The use of recombinant Lhc proteins has revealed up to four binding sites in members of Lhc families with distinct selectivity for xanthophyll species which are here hypothesised to have different functions. Site L1 is selective for lutein and is here proposed to be essential for catalysing the protection from singlet oxygen by quenching chlorophyll triplets. Site L2 and N1 are here proposed to act as allosteric sites involved in the regulation of chlorophyll singlet excited states by exchanging ligand during the operation of the xanthophyll cycle. Site V1 of the major antenna complex LHC II is here hypothesised to be a deposit for readily available substrate for violaxanthin de-epoxidase rather than a light harvesting pigment. Moreover, xanthophylls bound to Lhc proteins can be released into the lipid bilayer where they contribute to the scavenging of reactive oxygen species produced in excess ligh
Dissipation of Light Energy Absorbed in Excess: The Molecular Mechanisms
No full text: Light is essential for photosynthesis. Nevertheless, its intensity widely changes depending on time of day, weather, season, and localization of individual leaves within canopies. This variability means that light collected by the light-harvesting system is often in excess with respect to photon fluence or spectral quality in the context of the capacity of photosynthetic metabolism to use ATP and reductants produced from the light reactions. Absorption of excess light can lead to increased production of excited, highly reactive intermediates, which expose photosynthetic organisms to serious risks of oxidative damage. Prevention and management of such stress are performed by photoprotective mechanisms, which operate by cutting down light absorption, limiting the generation of redox-active molecules, or scavenging reactive oxygen species that are released despite the operation of preventive mechanisms. Here, we describe the major physiological and molecular mechanisms of photoprotection involved in the harmless removal of the excess light energy absorbed by green algae and land plants. In vivo analyses of mutants targeting photosynthetic components and the enhanced resolution of spectroscopic techniques have highlighted specific mechanisms protecting the photosynthetic apparatus from overexcitation. Recent findings unveil a network of multiple interacting elements, the reaction times of which vary from a millisecond to weeks, that continuously maintain photosynthetic organisms within the narrow safety range between efficient light harvesting and photoprotection
LHC-like proteins involved in stress responses and biogenesis/repair of the photosynthetic apparatus
Full text linkLHC (light-harvesting complex) proteins of plants and algae are known to be involved both in collecting light energy for driving the primary photochemical reactions of photosynthesis and in photoprotection when the absorbed light energy exceeds the capacity of the photosynthetic apparatus. These proteins usually contain three transmembrane (TM) helices which span the thylakoid membranes and bind several chlorophyll, carotenoid and lipid molecules. In addition, the LHC protein family includes LHC-like proteins containing one, two, three or even four TM domains. One-helix proteins are not only present in eukaryotic photosynthetic organisms but also in cyanobacteria where they have been named high light-inducible proteins. These small proteins are probably the ancestors of the members of the extant LHC protein family which arouse through gene duplications, deletions and fusions. During evolution, some of these proteins have diverged and acquired novel functions. In most cases, LHC-like proteins are induced in response to various stress conditions including high light, high salinity, elevated temperature and nutrient limitation. Many of these proteins play key roles in photoprotection, notably in non-photochemical quenching of absorbed light energy. Moreover, some of these proteins appear to be involved in the regulation of chlorophyll synthesis and in the assembly and repair of Photosystem II and also of Photosystem I possibly by mediating the insertion of newly synthesized pigments into the photosynthetic reaction centers
Multi-Level Light Capture Control in Plants and Green Algae
No full textLife 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
A mechanism of non-photochemical energy dissipation, independent from PsbS, revealed by a conformational change in the antenna protein CP26
No full textThe regulation of light harvesting in higher plant photosynthesis, defined as stress-dependent modulation of the ratio of energy transfer to the reaction centers versus heat dissipation, was studied by means of carotenoid biosynthesis mutants and recombinant light harvesting complexes (LHCs) with modified chromophore binding. The npq2 mutant of Arabidopsis thaliana, blocked in the biosynthesis of violaxanthin and thus accumulating zeaxanthin, was shown to have a lower fluorescence yield of chlorophyll in vivo and, correspondingly, a higher level of energy dissipation, with respect to the wild-type strain and npq1 mutant, the latter of which is incapable of zeaxanthin accumulation. Experiments on purified thylakoid membranes from all three mutants showed that the major source of the difference between the npq2 and wild-type preparations was a change in pigment to protein interactions, which can explain the lower chlorophyll fluorescence yield in the npq2 samples. Analysis of the xanthophyll binding LHC proteins showed that the Lhcb5 photosystem II subunit (also called CP26) undergoes a change in its pI upon binding of zeaxanthin. The same effect was observed in wild-type CP26 upon treatment that leads to the accumulation of zeaxanthin in the membrane and was interpreted as the consequence of a conformational change. This hypothesis was confirmed by the analysis of two recombinant proteins obtained by overexpression of the Lhcb5 apoprotein in Escherichia coli and reconstitution in vitro with either violaxanthin or zeaxanthin. The V and Z containing pigment-protein complexes obtained by this procedure showed different pIs and high and low fluorescence yields, respectively. These results confirm that LHC proteins exist in multiple conformations, an idea suggested by previous spectroscopic measurements (Moya et al., 2001), and imply that the switch between the different LHC protein conformations is activated by the binding of zeaxanthin to the allosteric site L2. The results suggest that the quenching process induced by the accumulation of zeaxanthin contributes to qI, a component of NPQ whose origin was previously poorly understood
Engineering Microalgae: Novel Tools for Genetic Manipulation of Chlamydomonas and Chlorella sp., from Light Harvesting to Cell Wall Remodelling
No full textMicroalgae are microscopic photosynthetic organisms living in aquatic environments widely recognized as promising “green bio-factories” due to their metabolic flexibility and capacity to convert solar energy into biomass. Their extensive biodiversity, which comprises approximately 72,500 species, and adaptability provide a robust foundation for biotechnological exploitation. Among green microalgae, species such as Chlamydomonas reinhardtii and Chlorella vulgaris have emerged as key models for both fundamental research and industrial-scale cultivation, owing to their genetic knowledge and metabolic potential. Nonetheless, industrial cultivation of microalgae remains largely limited to the production of value-added products, primarily due to high cultivation and processing costs.
To address these limitations, this thesis explores advanced genetic engineering strategies, with a focus on CRISPR-Cas9 genome editing, to enhance the domestication and productivity of microalgae. The work is structured around the development and application of genome editing tools in C. reinhardtii and C. vulgaris, aiming to dissect photosynthetic pathways, improve light-harvesting efficiency, and facilitate downstream bioprocessing.
In particular, photosynthesis in microalgae occurs through oxygenic mechanisms in the thylakoid membranes and is mediated by pigment–protein complexes. The absorption and utilization of light energy are influenced by the structure and composition of light-harvesting complexes (LHCs), which have evolved in aquatic species to optimize energy capture and photoprotection. Indeed, modifications in LHC composition and pigment-binding properties can significantly impact photosynthetic performance, particularly under variable light conditions.
To investigate those physiological processes, we developed a DNA-free CRISPR-Cas9 system in C. reinhardtii to enhance genome-editing efficiency while avoiding the drawbacks of DNA integration and antibiotic marker dependence. This enabled functional analyses of LHC-related genes, including targeted knock-outs of LHCBM1, a key antenna protein implicated in non-photochemical quenching (NPQ), a photoprotective mechanism activated under excess light conditions. Complementation with mutated variants of LHCBM1 further allowed investigation into its interaction with stress-responsive protein LHCSR3. Additionally, we used CRISPR-Cas9 in C. reinhardtii to produce gene knock-outs of specific monomeric antenna proteins (LHCB4 and LHCB5) and applied site-directed mutagenesis aimed at enhancing far-red light absorption. To do this, we targeted chlorophyll-binding residues in light-harvesting proteins to shift light absorption towards the far-red spectrum—a range not naturally absorbed by algal PSII– by mutating a key histidine residue to asparagine to induce red-shifted chlorophyll forms, similar to those observed in higher plants. This red shift could improve growth under low light and increase productivity in photobioreactors affected by self-shading.
Parallel efforts focused on C. vulgaris, a robust non-model species limited by a highly recalcitrant cell wall that hinders genetic transformation and metabolite extraction. We developed a domestication pipeline combining random mutagenesis, flow cytometry-based phenotypic screening, and ultrastructural analysis to isolate cell wall mutants with enhanced permeability. These advances provide a viable route to facilitate genetic manipulation and downstream processing in industrial settings.
In conclusion, this thesis contributes to the advancement of microalgal biotechnology by developing precise, efficient gene-editing tools and applying them to key physiological pathways and industrial constraints. Through an integrative approach encompassing both model and non-model species, this work lays the groundwork for optimizing microalgae-based production systems
Increased biomass productivity in green algae by tuning non-photochemical quenching
Full text linkPhotosynthetic microalgae have a high potential for the production of biofuels and highly valued metabolites. However, their current industrial exploitation is limited by a productivity in photobioreactors that is low compared to potential productivity. The high cell density and pigment content of the surface layers of photosynthetic microalgae result in absorption of excess photons and energy dissipation through non-photochemical quenching (NPQ). NPQ prevents photoinhibition, but its activation reduces the efficiency of photosynthetic energy conversion. In Chlamydomonas reinhardtii, NPQ is catalyzed by protein subunits encoded by three lhcsr (light harvesting complex stress related) genes. Here, we show that heat dissipation and biomass productivity depends on LHCSR protein accumulation. Indeed, algal strains lacking two lhcsr genes can grow in a wide range of light growth conditions without suffering from photoinhibition and are more productive than wild-type. Thus, the down-regulation of NPQ appears to be a suitable strategy for improving light use efficiency for biomass and biofuel production in microalgae
Zeaxanthin has enhanced antioxidant capacity with respect to all other xanthophylls in arabidopsis leaves and functions independent of binding to PSII antennae
No full textThe ch1 mutant of Arabidopsis thaliana lacks chlorophyll b. Leaves of this mutant are devoid
of PSII chlorophyll-protein antenna complexes (LHCII) and have a very low capacity of nonphotochemical
energy quenching (NPQ). Lhcb5 was the only PSII antenna protein that
accumulated to significant level in ch1 mutant leaves, but the apoprotein did not assemble in
vivo with chlorophylls to form a functional antenna. The abundance of Lhca proteins was also
reduced, to ~20% of wild-type level. Ch1 was crossed with various xanthophyll mutants to
analyze the antioxidant activity of carotenoids unbound to PSII antenna. Suppression of
zeaxanthin by crossing ch1 with npq1 resulted in oxidative stress in high light, while
removing other xanthophylls or the PSII protein PsbS had no such effect. The tocopheroldeficient
ch1 vte1 double mutant was as sensitive to high hight as ch1 npq1, and the triple
mutant ch1 npq1 vte1 exhibited an extreme sensitivity to photooxidative stress, indicating that
zeaxanthin and tocopherols have cumulative effects. Conversely, constitutive accumulation of
zeaxanthin in the ch1 npq2 double mutant led to an increased phototolerance relative to ch1.
Comparison of ch1 npq2 with another zeaxanthin-accumulating mutant (ch1 lut2) that lacks
lutein suggests that protection of polyunsaturated lipids by zeaxanthin is enhanced when
lutein is also present. During photooxidative stress, α-tocopherol noticeably decreased in ch1
npq1 and increased in ch1 npq2 relative to ch1, suggesting protection of vitamin E by high
zeaxanthin levels. Our results indicate that the antioxidant activity of zeaxanthin, distinct from
NPQ, can occur in the absence of LHCII. The capacity of zeaxanthin to protect thylakoid
membrane lipids is comparable to that of vitamin E, but noticeably higher than that of all
other xanthophylls of Arabidopsis leaves
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