Light-Harvesting Complex Protein LHCBM9 Is Critical for Photosystem II Activity and Hydrogen Production in Chlamydomonas reinhardtii

Effects of photoperiod on the growth and physiological responses in Ulva prolifera under constant and diurnal temperature difference conditions

Photoperiod and temperature are two main factors in the growth of macroalgae, and changes in photoperiod and diurnal temperature difference exist in natural condition. In order to study the effects of photoperiod and diurnal temperature difference on the growth of green algae Ulva prolifera, we cultured this species under three light/dark cycles (light: dark = 10:14, 12:12 and 16:08) with constant (22 °C for light and dark period, noted as 22-22 °C) and diurnal temperature difference (22 °C and 16 °C for light and dark period, respectively, noted as 22-16 °C) conditions. The results showed that: 1) Compared with 10:14 light/dark cycle, the growth of U. prolifera under 12:12 light/dark cycle was significantly enhanced by 39% and 16% for 22-22 °C and 22-16 °C treatments, respectively, while the increase proportion decreased when the daylength increase from 12 h to 16 h. 2) The enhancement in growth induced by diurnal temperature difference was observed under 10:14 light/dark cycle, but not for 12:12 and 16:08 light/dark cycle treatments. 3) The Chl a content and photosynthetic rate increased under short light period and 22-22 °C conditions, while under 22-16 °C conditions, higher photosynthetic rate was observed under 12:12 light/dark cycle and no significant difference in Chl a content was observed. 4) Under 22-22 °C conditions, compared with 10:14 (L:D) treatment, the expression levels of proteins in light-harvesting complexes, PSII and carbon fixation were down regulated, while the photorespiration and pentose phosphate pathway (PPP) were up regulated by 16:08 light dark cycle. Then we speculate that the higher photosynthetic rate may be one compensation mechanism in short photoperiod, and under long light period condition the up regulations of photorespiration and PPP can be in charge of the decrease in enhancement growth induced by longer daylength.

The role of the pigment-protein complex LHCBM1 in nonphotochemical quenching in Chlamydomonas reinhardtii

Nonphotochemical quenching (NPQ) is the process that protects photosynthetic organisms from photodamage by dissipating the energy absorbed in excess as heat. In the model green alga Chlamydomonas reinhardtii, NPQ is abolished in the knock-out mutants of the pigment-protein complexes LHCSR3 and LHCBM1. However, while LHCSR3 is a pH sensor and switches to a quenched conformation at low pH, the role of LHCBM1 in NPQ has not been elucidated yet. In this work, we combined biochemical and physiological measurements to study short-term high-light acclimation of npq5, the mutant lacking LHCBM1. In low light in the absence of this complex, the antenna size of PSII was smaller than in its presence; this effect was marginal in high light (HL), implying that a reduction of the antenna was not responsible for the low NPQ. The mutant expressed LHCSR3 at the wild-type level in HL, indicating that the absence of this complex is also not the reason. Finally, NPQ remained low in the mutant even when the pH was artificially lowered to values that can switch LHCSR3 to the quenched conformation. We concluded that both LHCSR3 and LHCBM1 are required for the induction of NPQ and that LHCBM1 is the interacting partner of LHCSR3. This interaction can either enhance the quenching capacity of LHCSR3 or connect this complex with the PSII supercomplex.

ROS-dependent cell death of Heterosigma akashiwo induced by algicidal bacterium Hahella sp. KA22

Marine algicidal bacteria and their metabolites are considered to be one of the most effective strategies to mitigate the harmful algal blooms (HABs). The bacterium Hahella sp. KA22 has previously been confirmed to have strong algicidal activity against the HABs causing microalgae, Heterosigma akashiwo. In this study, the molecular mechanism of microalgae cell death was detected. The results showed that the cell growth rate and photosynthetic efficiency were inhibited with addition of algicidal strain KA22, while the accumulation of reactive oxygen species (ROS) and oxidative damage in H. akashiwo cells increased. A total of 2056 unigenes were recognized to be differentially expressed in transcriptome sequences. In particular, the transcriptional levels of light-harvesting pigments and structural proteins in the oxygen-evolving-complex were continuously down-regulated, corresponding to the significant reduction of photosynthetic efficiency and the accumulation of ROS. Furthermore, glutamate dehydrogenase was significantly up-regulated in abundance. Meanwhile, calcium-dependent protein kinases were also detected with significant changes. Collectively, algicidal stress caused the suppressed electron transfer in chloroplast and impaired detoxification of intracellular oxidants by glutathione, which may subsequently result in multiple cell regulation and metabolic responses and ultimately lead to the ROS-dependent cell death of H. akashiwo.

Light Harvesting in Fluctuating Environments: Evolution and Function of Antenna Proteins across Photosynthetic Lineage

Photosynthesis is the major natural process that can harvest and harness solar energy into chemical energy. Photosynthesis is performed by a vast number of organisms from single cellular bacteria to higher plants and to make the process efficient, all photosynthetic organisms possess a special type of pigment protein complex(es) that is (are) capable of trapping light energy, known as photosynthetic light-harvesting antennae. From an evolutionary point of view, simpler (unicellular) organisms typically have a simple antenna, whereas higher plants possess complex antenna systems. The higher complexity of the antenna systems provides efficient fine tuning of photosynthesis. This relationship between the complexity of the antenna and the increasing complexity of the organism is mainly related to the remarkable acclimation capability of complex organisms under fluctuating environmental conditions. These antenna complexes not only harvest light, but also provide photoprotection under fluctuating light conditions. In this review, the evolution, structure, and function of different antenna complexes, from single cellular organisms to higher plants, are discussed in the context of the ability to acclimate and adapt to cope under fluctuating environmental conditions.

Assessment of cytotoxicity biomarkers on the microalga Chlamydomonas reinhardtii exposed to emerging and priority pollutants

Contamination of aquatic ecosystems linked to anthropogenic activity is currently a major concern; therefore, ecotoxicological studies are needed to assess its effect on organisms. The main objective of this study was to investigate the effects of different pollutants on microalgae in search of sensitive biomarkers that can promote a common cytotoxic response regardless of the contaminant. Cultures of the freshwater microalga Chlamydomonas reinhardtii were exposed for 24 h to four chemicals, three emerging pollutants (benzophenone-3, bisphenol A and oxytetracycline) and one priority substance (atrazine). A cytometric panel was carried out to assess toxicity biomarkers including cellular growth, inherent cell properties, viability, vitality, cytoplasmic membrane potential and ROS levels. Lipid peroxidation, photosynthetic efficiency and transcriptional responses of photosynthesis- and oxidative stress-related genes using RT-qPCR were also studied. Some toxicity responses showed a similar pattern; a decrease in growth rate, vitality and photosynthetic efficiency and an increase in autofluorescence and in the number of cells with depolarised cytoplasmic membrane and were found for all chemicals tested. However, ATZ and OTC provoked a decrease in cell size, whereas BP-3 and BPA caused an increase in cell size, intracellular complexity and ROS levels and a decrease in cell viability. Assayed pollutants generally promoted an overexpression of genes related to cellular antioxidant defence system and a subexpression of photosynthesis-related genes. In addition to the traditional growth endpoint, cell vitality, autofluorescence and gene expression of catalase, glutathione peroxidase and Fe-superoxide dismutase were significantly affected for all chemicals tested, showing a common cytotoxic response. Among the tested substances, BP-3 provoked the strongest cytotoxic alterations on this microalga, pointing out that some emerging contaminants could be more harmful to organisms than priority pollutants.

Chlamydomonas reinhardtii LHCSR1 and LHCSR3 proteins involved in photoprotective non-photochemical quenching have different quenching efficiency and different carotenoid affinity

Microalgae are unicellular photosynthetic organisms considered as potential alternative sources for biomass, biofuels or high value products. However, their limited biomass productivity represents a bottleneck that needs to be overcome to meet the applicative potential of these organisms. One of the domestication targets for improving their productivity is the proper balance between photoprotection and light conversion for carbon fixation. In the model organism for green algae, Chlamydomonas reinhardtii, a photoprotective mechanism inducing thermal dissipation of absorbed light energy, called Non-photochemical quenching (NPQ), is activated even at relatively low irradiances, resulting in reduced photosynthetic efficiency. Two pigment binding proteins, LHCSR1 and LHCSR3, were previously reported as the main actors during NPQ induction in C. reinhardtii. While previous work characterized in detail the functional properties of LHCSR3, few information is available for the LHCSR1 subunit. Here, we investigated in vitro the functional properties of LHCSR1 and LHCSR3 subunits: despite high sequence identity, the latter resulted as a stronger quencher compared to the former, explaining its predominant role observed in vivo. Pigment analysis, deconvolution of absorption spectra and structural models of LHCSR1 and LHCR3 suggest that different quenching efficiency is related to a different occupancy of L2 carotenoid binding site.

Frenkel excitons in heat-stressed supramolecular nanocomposites enabled by tunable cage-like scaffolding

Delocalized Frenkel excitons-coherently shared excitations among chromophores-are responsible for the remarkable efficiency of supramolecular light-harvesting assemblies within photosynthetic organisms. The translation of nature's design principles to applications in optoelectronic devices has been limited by the fragility of the supramolecular structures used and the delicate nature of Frenkel excitons, particularly under mildly changing solvent conditions and elevated temperatures and upon deposition onto solid substrates. Here, we overcome those functionalization barriers through composition of stable supramolecular light-harvesting nanotubes enabled by tunable (~4.3-4.9 nm), uniform (±0.3 nm) cage-like scaffolds. High-resolution cryogenic electron microscopy, combined with scanning electron microscopy, broadband femtosecond transient absorption spectroscopy and near-field scanning optical microscopy revealed that excitons within the cage-like scaffolds are robust, even under extreme heat stress, and control over nanocomposite dimensions is maintained on solid substrates. Our bio-inspired nanocomposites provide a general framework for the development of next-generation organic devices made from stable supramolecular materials.

Comparative physiological and transcriptomic analysis of pear leaves under distinct training systems

Canopy architecture is critical in determining the light interception and distribution, and subsequently the photosynthetic efficiency and productivity. However, the physiological responses and molecular mechanisms by which pear canopy architectural traits impact on photosynthesis remain poorly understood. Here, physiological investigations coupled with comparative transcriptomic analyses were performed in pear leaves under distinct training systems. Compared with traditional freestanding system, flat-type trellis system (DP) showed higher net photosynthetic rate (P_N) levels at the most time points throughout the entire monitored period, especially for the interior of the canopy in sunny side. Gene ontology analysis revealed that photosynthesis, carbohydrate derivative catabolic process and fatty acid metabolic process were over-represented in leaves of DP system with open-canopy characteristics. Weighted gene co-expression network analysis uncovered a significant network module positive correlated with P_N value. The hub genes (PpFKF1 and PpPRR5) of the module were enriched in circadian rhythm pathway, suggesting a functional role for circadian clock genes in mediating photosynthetic performance under distinct training systems. These results draw a link between pear photosynthetic response and specific canopy architectural traits, and highlight light harvesting and circadian clock network as potential targets for the input signals from the fluctuating light availability under distinct training systems.

Photoprotective Capabilities of Light-Harvesting Complex II Trimers in the Green Alga Chlamydomonas reinhardtii

Major light-harvesting complex (LHCII) trimers in plants induce the thermal dissipation of absorbed excitation energy against photooxidative damage under excess light conditions. LHCII trimers in green algae have been thought to be incapable of energy dissipation without additional quencher proteins, although LHCIIs in plants and green algae are homologous. In this study, we investigated the energy-dissipative capabilities of four distinct types of LHCII trimers isolated from the model green alga Chlamydomonas reinhardtii using spectroscopic analysis. Our results revealed that the LHCII trimers possessing LHCII type II (LHCBM5) and LHCII type IV (LHCBM1) had efficient energy-dissipative capabilities, whereas LHCII type I (LHCBM3/4/6/8/9) and type III (LHCBM2/7) did not. On the basis of the amino acid sequences of LHCBM5 and LHCBM1 compared with the other LHCBMs, we propose that positively charged extra N-terminal amino acid residues mediate the interactions between LHCII trimers to form energy-dissipative states.

Regulation of Light Harvesting in Chlamydomonas reinhardtii Two Protein Phosphatases Are Involved in State Transitions

Protein phosphorylation plays important roles in short-term regulation of photosynthetic electron transfer, and during state transitions, the kinase STATE TRANSITION7 (STT7) of Chlamydomonas reinhardtii phosphorylates components of light-harvesting antenna complex II (LHCII). This reversible phosphorylation governs the dynamic allocation of a part of LHCII to PSI or PSII, depending on light conditions and metabolic demands, but counteracting phosphatase(s) remain unknown in C. reinhardtii Here we analyzed state transitions in C. reinhardtii mutants of two phosphatases, PROTEIN PHOSPHATASE1 and PHOTOSYSTEM II PHOSPHATASE, which are homologous to proteins that antagonize the state transition kinases (STN7 and STN8) in Arabidopsis (Arabidopsis thaliana). The transition from state 2 to state 1 was retarded in pph1, and surprisingly also in pbcp However, both mutants eventually returned to state 1. In contrast, the double mutant pph1;pbcp appeared strongly locked in state 2. The complex phosphorylation patterns of the LHCII trimers and of the monomeric subunits were affected in the phosphatase mutants. Their analysis indicated that the two phosphatases have different yet overlapping sets of protein targets. The dual control of thylakoid protein dephosphorylation and the more complex antenna phosphorylation patterns in C. reinhardtii compared to Arabidopsis are discussed in the context of the stronger amplitude of state transitions and the more diverse LHCII isoforms in the alga.

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.

Structural insight into light harvesting for photosystem II in green algae

Green algae and plants rely on light-harvesting complex II (LHCII) to collect photon energy for oxygenic photosynthesis. In Chlamydomonas reinhardtii, LHCII molecules associate with photosystem II (PSII) to form various supercomplexes, including the C₂S₂M₂L₂ type, which is the largest PSII-LHCII supercomplex in algae and plants that is presently known. Here, we report high-resolution cryo-electron microscopy (cryo-EM) maps and structural models of the C₂S₂M₂L₂ and C₂S₂ supercomplexes from C. reinhardtii. The C₂S₂ supercomplex contains an LhcbM1-LhcbM2/7-LhcbM3 heterotrimer in the strongly associated LHCII, and the LhcbM1 subunit assembles with CP43 through two interfacial galactolipid molecules. The loosely and moderately associated LHCII trimers interact closely with the minor antenna complex CP29 to form an intricate subcomplex bound to CP47 in the C₂S₂M₂L₂ supercomplex. A notable direct pathway is established for energy transfer from the loosely associated LHCII to the PSII reaction centre, as well as several indirect routes. Structure-based computational analysis on the excitation energy transfer within the two supercomplexes provides detailed mechanistic insights into the light-harvesting process in green algae.

Four distinct trimeric forms of light-harvesting complex II isolated from the green alga Chlamydomonas reinhardtii

Light-harvesting complex II (LHCII) absorbs light energy and transfers it primarily to photosystem II in green algae and land plants. Although the trimeric structure of LHCII is conserved between the two lineages, its subunit composition and function are believed to differ significantly. In this study, we purified four LHCII trimers from the green alga Chlamydomonas reinhardtii and analyzed their biochemical properties. We used several preparation methods to obtain four distinct fractions (fractions 1-4), each of which contained an LHCII trimer with different contents of Type I, III, and IV proteins. The pigment compositions of the LHCIIs in the four fractions were similar. The absorption and fluorescence spectra were also similar, although the peak positions differed slightly. These results indicate that this green alga contains four types of LHCII trimer with different biochemical and spectroscopic features. Based on these findings, we discuss the function and structural organization of green algal LHCII antennae.

LHC-like proteins involved in stress responses and biogenesis/repair of the photosynthetic apparatus

LHC (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.

A Stepwise NaHSO3 Addition Mode Greatly Improves H2 Photoproduction in Chlamydomonas reinhardtii

NaHSO₃ addition greatly increases the yield of H₂ photoproduction in a unicellular green alga Chlamydomonas reinhardtii through removing O₂ and activating hydrogenase but significantly impairs the activity of PSII, an electron source for H₂ photoproduction. Here, a stepwise addition mode of total 13 mM NaHSO₃, an optimal concentration for H₂ photoproduction of C. reinhardtii identified in a previous one step addition method, significantly improved H₂ photoproduction. Such improvement was believed to be the result of increased residual PSII activity in an anaerobic background, but was at least independent of two alternative electron sinks for H₂ photoproduction, cyclic electron transport around PSI and CO₂ assimilation. Based on the above results, we propose that increased residual PSII activity in an anaerobic environment is an efficient strategy to enhance H₂ photoproduction in C. reinhardtii, and the stepwise NaHSO₃ addition mode is a case study in the strategy.

The mechanism of photosystem-II inactivation during sulphur deprivation-induced H2 production in Chlamydomonas reinhardtii

Sulphur limitation may restrain cell growth and viability. In the green alga Chlamydomonas reinhardtii, sulphur limitation may induce H₂ production lasting for several days, which can be exploited as a renewable energy source. Sulphur limitation causes a large number of physiological changes, including the inactivation of photosystem II (PSII), leading to the establishment of hypoxia, essential for the increase in hydrogenase expression and activity. The inactivation of PSII has long been assumed to be caused by the sulphur-limited turnover of its reaction center protein PsbA. Here we reinvestigated this issue in detail and show that: (i) upon transferring Chlamydomonas cells to sulphur-free media, the cellular sulphur content decreases only by about 25%; (ii) as demonstrated by lincomycin treatments, PsbA has a significant turnover, and other photosynthetic subunits, namely RbcL and CP43, are degraded more rapidly than PsbA. On the other hand, sulphur limitation imposes oxidative stress early on, most probably involving the formation of singlet oxygen in PSII, which leads to an increase in the expression of GDP-L-galactose phosphorylase, playing an essential role in ascorbate biosynthesis. When accumulated to the millimolar concentration range, ascorbate may inactivate the oxygen-evolving complex and provide electrons to PSII, albeit at a low rate. In the absence of a functional donor side and sufficient electron transport, PSII reaction centers are inactivated and degraded. We therefore demonstrate that the inactivation of PSII is a complex and multistep process, which may serve to mitigate the damaging effects of sulphur limitation.

Functional analysis of photosynthetic pigment binding complexes in the green alga Haematococcus pluvialis reveals distribution of astaxanthin in Photosystems

Astaxanthin is a ketocarotenoid produced by photosynthetic microalgae. It is a pigment of high industrial interest in acquaculture, cosmetics, and nutraceutics due to its strong antioxidant power. Haematococcus pluvialis, a fresh-water microalga, accumulates high levels of astaxanthin upon oxidative stress, reaching values up to 5% per dry weight. H. pluvialis accumulates astaxanthin in oil droplets in the cytoplasm, while the chloroplast volume is reduced. In this work, we investigate the biochemical and spectroscopic properties of the H. pluvialis pigment binding complexes responsible for light harvesting and energy conversion. Our findings demonstrate that the main features of chlorophyll and carotenoid binding complexes previously reported for higher plants or Chlamydomonas reinhardtii are preserved under control conditions. Transition to astaxanthin rich cysts however leads to destabilization of the Photosystems. Surprisingly, astaxanthin was found to be bound to both Photosystem I and II, partially substituting β-carotene, and thus demonstrating possible astaxanthin biosynthesis in the plastids or transport from the cytoplasm to the chloroplast. Astaxanthin binding to Photosystems does not however improve their photoprotection, but rather reduces the efficiency of excitation energy transfer to the reaction centers. We thus propose that astaxanthin binding partially destabilizes Photosystem I and II.

Nutrient scavenging and energy management: acclimation responses in nitrogen and sulfur deprived Chlamydomonas

Photosynthetic organisms have evolved to modulate their metabolism to accommodate the highly dynamic light and nutrient conditions in nature. In this review we discuss ways in which the green alga Chlamydomonas reinhardtii acclimates to nitrogen and sulfur deprivation, conditions that would limit the anabolic use of excitation energy because of a markedly reduced capacity for cell growth and division. Major aspects of this acclimation process are stringently regulated and involve scavenging the limited nutrient from internal and external sources, and the redirection of fixed carbon toward energy storage (e.g. starch, oil). However, photosynthetic organisms have also evolved mechanisms to dissipate excess absorbed light energy, and to eliminate potentially dangerous energetic electrons through the reduction of O₂ and H⁺ to H₂O; this reduction can occur both through photosynthetic electron transport (e.g. Mehler reaction, chlororespiration) and mitochondrial respiration. Furthermore, algal cells likely exploit other energy management pathways that are currently not linked to nutrient limitation responses or that remain to be identified.

Functional modulation of LHCSR1 protein from Physcomitrella patens by zeaxanthin binding and low pH

Light harvesting for oxygenic photosynthesis is regulated to prevent the formation of harmful photoproducts by activation of photoprotective mechanisms safely dissipating the energy absorbed in excess. Lumen acidification is the trigger for the formation of quenching states in pigment binding complexes. With the aim to uncover the photoprotective functional states responsible for excess energy dissipation in green algae and mosses, we compared the fluorescence dynamic properties of the light-harvesting complex stress-related (LHCSR1) protein, which is essential for fast and reversible regulation of light use efficiency in lower plants, as compared to the major LHCII antenna protein, which mainly fulfills light harvesting function. Both LHCII and LHCSR1 had a chlorophyll fluorescence yield and lifetime strongly dependent on detergent concentration but the transition from long- to short-living states was far more complete and fast in the latter. Low pH and zeaxanthin binding enhanced the relative amplitude of quenched states in LHCSR1, which were characterized by the presence of 80 ps fluorescence decay components with a red-shifted emission spectrum. We suggest that energy dissipation occurs in the chloroplast by the activation of 80 ps quenching sites in LHCSR1 which spill over excitons from the photosystem II antenna system.

Calcium mediates the cellular response of Chlamydomonas reinhardtii to the emerging aquatic pollutant Triclosan

The present study was aimed at investigating the role of intracellular free calcium, [Ca²⁺]_c, in the early cellular response of the green alga Chlamydomonas reinhardtii to the emergent pollutant Triclosan (13.8μM; 24h of exposure). There is a growing concern about the persistence and toxicity of this antimicrobial in aquatic environments, where non-target organisms such as C. reinhardtii, a primary producer of ecological relevance, might be severely impacted. A mechanistic study was undertaken which combined flow cytometry protocols, physiological as well as gene expression analysis. As an early response, Triclosan strongly altered [Ca²⁺]_c homeostasis which could be prevented by prechelation with the intracellular calcium chelator BAPTA-AM. Triclosan induced ROS overproduction which ultimately leads to oxidative stress with loss of membrane integrity, membrane depolarization, photosynthesis inhibition and mitochondrial membrane depolarization; within this context, Triclosan also induced an increase in caspase 3/7 activity and altered the expression of metacaspase genes which are indicative of apoptosis. All these adverse outcomes were dependent on [Ca²⁺]_c. Interestingly, an interconnection between [Ca²⁺]_c alterations and increased ROS formation by Triclosan was found. Taken altogether these results shed light on the mechanisms behind Triclosan toxicity in the green alga Chlamydomonas reinhardtii and demonstrate the role of [Ca²⁺]_c in mediating the observed toxicity.

Light Intensity is Important for Hydrogen Production in NaHSO3-Treated Chlamydomonas reinhardtii

Chlamydomonas reinhardtii is a unicellular green alga that can use light energy to produce H2 from H2O in the background of NaHSO3 treatment. However, the role of light intensity in such H2 production remains elusive. Here, light intensity significantly affected the yield of H2 production in NaHSO3-treated C. reinhardtii, which was consistent with its effects on the content of O2 and the expression and activity of hydrogenase. Further, NaHSO3 was found to be able to remove O2 via a reaction of bisulfite with superoxide anion produced at the acceptor side of PSI, and light intensity affected the reaction rate significantly. Accordingly, high light and strong light but not low light can create an anaerobic environment, which is important to activate hydrogenase and produce H2. Based on the above results, we conclude that light intensity plays an important role in removing O2 and consequently activating hydrogenase and producing H2 in NaHSO3-treated C. reinhardtii.

The function of LHCBM4/6/8 antenna proteins in Chlamydomonas reinhardtii

In eukaryotic autotrophs, photosystems are composed of a core moiety, hosting charge separation and electron transport reactions, and an antenna system, enhancing light harvesting and photoprotection. In Chlamydomonas reinhardtii, the major antenna of PSII is a heterogeneous trimeric complex made up of LHCBM1-LHCBM9 subunits. Despite high similarity, specific functions have been reported for several members including LHCBM1, 2, 7, and 9. In this work, we analyzed the function of LHCBM4 and LHCBM6 gene products in vitro by synthesizing recombinant apoproteins from individual sequences and refolding them with pigments. Additionally, we characterized knock-down strains in vivo for LHCBM4/6/8 genes. We show that LHCBM4/6/8 subunits could be found as a component of PSII supercomplexes with different sizes, although the largest pool was free in the membranes and poorly connected to PSII. Impaired accumulation of LHCBM4/6/8 caused a decreased LHCII content per PSII and a reduction in the amplitude of state 1-state 2 transitions. In addition, the reduction of LHCBM4/6/8 subunits caused a significant reduction of the Non-photochemical quenching activity and in the level of photoprotection.

Highly efficient methane generation from untreated microalgae biomass

BACKGROUND

The fact that microalgae perform very efficiently photosynthetic conversion of sunlight into chemical energy has moved them into the focus of regenerative fuel research. Especially, biogas generation via anaerobic digestion is economically attractive due to the comparably simple apparative process technology and the theoretical possibility of converting the entire algal biomass to biogas/methane. In the last 60 years, intensive research on biogas production from microalgae biomass has revealed the microalgae as a rather challenging substrate for anaerobic digestion due to its high cell wall recalcitrance and unfavorable protein content, which requires additional pretreatment and co-fermentation strategies for sufficient fermentation. However, sustainable fuel generation requires the avoidance of cost/energy intensive biomass pretreatments to achieve positive net-energy process balance.

RESULTS

Cultivation of microalgae in replete and limited nitrogen culture media conditions has led to the formation of protein-rich and low protein biomass, respectively, with the last being especially optimal for continuous fermentation. Anaerobic digestion of nitrogen limited biomass (low-N BM) was characterized by a stable process with low levels of inhibitory substances and resulted in extraordinary high biogas, and subsequently methane productivity [750 ± 15 and 462 ± 9 mL_N g^-1 volatile solids (VS) day^-1, respectively], thus corresponding to biomass-to-methane energy conversion efficiency of up to 84%. The microbial community structure within this highly efficient digester revealed a clear predominance of the phyla Bacteroidetes and the family Methanosaetaceae among the Bacteria and Archaea, respectively. The fermentation of replete nitrogen biomass (replete-N BM), on the contrary, was demonstrated to be less productive (131 ± 33 mL_N CH₄ g^-1VS day^-1) and failed completely due to acidosis, caused through high ammonia/ammonium concentrations. The organization of the microbial community of the failed (replete-N) digester differed greatly compared to the stable low-N digester, presenting a clear shift to the phyla Firmicutes and Thermotogae, and the archaeal population shifted from acetoclastic to hydrogenotrophic methanogenesis.

CONCLUSIONS

The present study underlines the importance of cultivation conditions and shows the practicability of microalgae biomass usage as mono-substrate for highly efficient continuous fermentation to methane without any pretreatment with almost maximum practically achievable energy conversion efficiency (biomass to methane).Graphical abstractGrowth condition dependence of anaerobic conversion efficiency of microalgae biomass to methane.

Hydrogen photoproduction in green algae Chlamydomonas reinhardtii sustainable over 2 weeks with the original cell culture without supply of fresh cells nor exchange of the whole culture medium

Unicellular green algae Chlamydomonas reinhardtii are known to make hydrogen photoproduction under the anaerobic condition with water molecules as the hydrogen source. Since the hydrogen photoproduction occurs for a cell to circumvent crisis of its survival, it is only temporary. It is a challenge to realize persistent hydrogen production because the cells must withstand stressful conditions to survive with alternation of generations in the cell culture. In this paper, we have found a simple and cost-effective method to sustain the hydrogen production over 14 days in the original culture, without supply of fresh cells nor exchange of the culture medium. This is achieved for the cells under hydrogen production in a sulfur-deprived culture solution on the {anaerobic, intense light} condition in a desiccator, by periodically providing a short period of the recovery time (2 h) with a small amount of TAP(+S) supplied outside of the desiccator. As this operation is repeated, the response time of transition into hydrogen production (preparation time) is shortened and the rate of hydrogen production (build up time) is increased. The optimum states of these properties favorable to the hydrogen production are attained in a few days and stably sustained for more than 10 days. Since generations are alternated during this consecutive hydrogen production experiment, it is suggested that the improved hydrogen production properties are inherited to next generations without genetic mutation. The properties are reset only when the cells are placed on the {sulfur-sufficient, aerobic, moderate light} conditions for a long time (more than 1 day at least).

Ascorbate accumulation during sulphur deprivation and its effects on photosystem II activity and H2 production of the green alga Chlamydomonas reinhardtii

In nature, H2 production in Chlamydomonas reinhardtii serves as a safety valve during the induction of photosynthesis in anoxia, and it prevents the over-reduction of the photosynthetic electron transport chain. Sulphur deprivation of C. reinhardtii also triggers a complex metabolic response resulting in the induction of various stress-related genes, down-regulation of photosynthesis, the establishment of anaerobiosis and expression of active hydrogenase. Photosystem II (PSII) plays dual role in H2 production because it supplies electrons but the evolved O2 inhibits the hydrogenase. Here, we show that upon sulphur deprivation, the ascorbate content in C. reinhardtii increases about 50-fold, reaching the mM range; at this concentration, ascorbate inactivates the Mn-cluster of PSII, and afterwards, it can donate electrons to tyrozin Z(+) at a slow rate. This stage is followed by donor-side-induced photoinhibition, leading to the loss of charge separation activity in PSII and reaction centre degradation. The time point at which maximum ascorbate concentration is reached in the cell is critical for the establishment of anaerobiosis and initiation of H2 production. We also show that ascorbate influenced H2 evolution via altering the photosynthetic electron transport rather than hydrogenase activity and starch degradation.

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.

Omics in Chlamydomonas for Biofuel Production

In response to demands for sustainable domestic fuel sources, research into biofuels has become increasingly important. Many challenges face biofuels in their effort to replace petroleum fuels, but rational strain engineering of algae and photosynthetic organisms offers a great deal of promise. For decades, mutations and stress responses in photosynthetic microbiota were seen to result in production of exciting high-energy fuel molecules, giving hope but minor capability for design. However, '-omics' techniques for visualizing entire cell processing has clarified biosynthesis and regulatory networks. Investigation into the promising production behaviors of the model organism C. reinhardtii and its mutants with these powerful techniques has improved predictability and understanding of the diverse, complex interactions within photosynthetic organisms. This new equipment has created an exciting new frontier for high-throughput, predictable engineering of photosynthetically produced carbon-neutral biofuels.

Relevance of nutrient media composition for hydrogen production in Chlamydomonas

Microalgae are capable of biological H2 photoproduction from water, solar energy, and a variety of organic substrates. Acclimation responses to different nutrient regimes finely control photosynthetic activity and can influence H2 production. Hence, nutrient stresses are an interesting scenario to study H2 production in photosynthetic organisms. In this review, we mainly focus on the H2-production mechanisms in Chlamydomonas reinhardtii and the physiological relevance of the nutrient media composition when producing H2.

Current Understanding of the Interplay between Phytohormones and Photosynthesis under Environmental Stress

Abiotic stress accounts for huge crop losses every year across the globe. In plants, the photosynthetic machinery gets severely damaged at various levels due to adverse environmental conditions. Moreover, the reactive oxygen species (ROS) generated as a result of stress further promote the photosynthetic damage by inhibiting the repair system of photosystem II. Earlier studies have suggested that phytohormones are not only required for plant growth and development, but they also play a pivotal role in regulating plants’ responses to different abiotic stress conditions. Although, phytohormones have been studied in great detail in the past, their influence on the photosynthetic machinery under abiotic stress has not been studied. One of the major factors that limits researchers fromelucidating the precise roles of phytohormones is the highly complex nature of hormonal crosstalk in plants. Another factor that needs to be elucidated is the method used for assessing photosynthetic damage in plants that are subjected to abiotic stress. Here, we review the current understanding on the role of phytohormones in the photosynthetic machinery under various abiotic stress conditions and discuss the potential areas for further research.

Sulphur responsiveness of the Chlamydomonas reinhardtii LHCBM9 promoter

A 44-base-pair region in the Chlamydomonas reinhardtii LHCBM9 promoter is essential for sulphur responsiveness. The photosynthetic light-harvesting complex (LHC) proteins play essential roles both in light capture, the first step of photosynthesis, and in photoprotective mechanisms. In contrast to the other LHC proteins and the majority of photosynthesis proteins, the Chlamydomonas reinhardtii photosystem II-associated LHC protein, LHCBM9, was recently reported to be up-regulated under sulphur deprivation conditions, which also induce hydrogen production. Here, we examined the sulphur responsiveness of the LHCBM9 gene at the transcriptional level, through promoter deletion analysis. The LHCBM9 promoter was found to be responsive to sulphur deprivation, with a 44-base-pair region between nucleotide positions -136 and -180 relative to the translation start site identified as essential for this response. Anaerobiosis was found to enhance promoter activity under sulphur deprivation conditions, however, alone was unable to induce promoter activity. The study of LHCBM9 is of biological and biotechnological importance, as its expression is linked to photobiological hydrogen production, theoretically the most efficient process for biofuel production, while the simplicity of using an S-deprivation trigger enables the development of a novel C. reinhardtii-inducible promoter system based on LHCBM9.

Characterization of the major light-harvesting complexes (LHCBM) of the green alga Chlamydomonas reinhardtii

Nine genes (LHCBM1-9) encode the major light-harvesting system of Chlamydomonas reinhardtii. Transcriptomic and proteomic analyses have shown that those genes are all expressed albeit in different amounts and some of them only in certain conditions. However, little is known about the properties and specific functions of the individual gene products because they have never been isolated. Here we have purified several complexes from native membranes and/or we have reconstituted them in vitro with pigments extracted from C. reinhardtii. It is shown that LHCBM1 and -M2/7 represent more than half of the LHCBM population in the membrane. LHCBM2/7 forms homotrimers while LHCBM1 seems to be present in heterotrimers. Trimers containing only type I LHCBM (M3/4/6/8/9) were also observed. Despite their different roles, all complexes have very similar properties in terms of pigment content, organization, stability, absorption, fluorescence and excited-state lifetimes. Thus the involvement of LHCBM1 in non-photochemical quenching is suggested to be due to specific interactions with other components of the membrane and not to the inherent quenching properties of the complex. Similarly, the overexpression of LHCBM9 during sulfur deprivation can be explained by its low sulfur content as compared with the other LHCBMs. Considering the highly conserved biochemical and spectroscopic properties, the major difference between the complexes may be in their capacity to interact with other components of the thylakoid membrane.

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.