Sustained hydrogen photoproduction by Chlamydomonas reinhardtii: Effects of culture parameters

Biohydrogen production from Euglena acus microalgae available in Bangladesh

Hydrogen is generally considered as an ideal non-polluting future energy carrier because it releases energy and water as a byproduct on combustion. Besides, hydrogen possesses the highest energy density on mass basis compared to any other fuel. However, hydrogen production in a sustainable and environmentally friendly way still remains a challenge. Recently, biohydrogen production from green microalgae has gained significant attention due to availability of the feedstock, which are environmentally friendly and renewable. Biohydrogen production from photosynthetic microalgae is attractive, however in the current context, it has a low yield, and an optimization of the affecting parameters including algae concentration, light intensity, culture medium, etc. is critical. In this study, biohydrogen was produced in laboratory from Euglena acus microalgae as it was locally available in Bangladesh.•The effect of two different culture mediums (i.e. sulfur-rich and sulfur-deprived TAP mediums) for microalgae cultivation and biohydrogen yield were studied.•Depending on the concentration of microalgae (50% and 75% by weight) in the medium solution ∼3 ml to 5 ml biohydrogen was obtained.

Scaling-up and proteomic analysis reveals photosynthetic and metabolic insights toward prolonged H2 photoproduction in Chlamydomonas hpm91 mutant lacking proton gradient regulation 5 (PGR5)

Clean and sustainable H₂ production is crucial to a carbon-neutral world. H₂ generation by Chlamydomonas reinhardtii is an attractive approach for solar-H₂ from H₂O. However, it is currently not large-scalable because of lacking desirable strains with both optimal H₂ productivity and sufficient knowledge of underlying molecular mechanism. We hereby carried out extensive and in-depth investigations of H₂ photoproduction of hpm91 mutant lacking PGR5 (Proton Gradient Regulation 5) toward its up-scaling and fundamental mechanism issues. We show that hpm91 is at least 100-fold scalable (up to 10 L) with continuous H₂ collection of 7287 ml H₂/10L-HPBR in averagely 26 days under sulfur deprivation. Also, we show that hpm91 is robust and active during sustained H₂ photoproduction, most likely due to decreased intracellular ROS relative to wild type. Moreover, we obtained quantitative proteomic profiles of wild type and hpm91 at four representing time points of H₂ evolution, leading to 2229 and 1350 differentially expressed proteins, respectively. Compared to wild type, major proteome alterations of hpm91 include not only core subunits of photosystems and those related to anti-oxidative responses but also essential proteins in photosynthetic antenna, C/N metabolic balance, and sulfur assimilation toward both cysteine biosynthesis and sulfation of metabolites during sulfur-deprived H₂ production. These results reveal not only new insights of cellular and molecular basis of enhanced H₂ production in hpm91 but also provide additional candidate gene targets and modules for further genetic modifications and/or in artificial photosynthesis mimics toward basic and applied research aiming at advancing solar-H₂ technology.

Sustainable production of algae-bacteria granular consortia based biological hydrogen: New insights

Microbes recycling nutrient and detoxifying ecosystems are capable to fulfil the future energy need by producing biohydrogen by due to the coupling of autotrophic and heterotrophic microbes. In granules microbes mutualy exchanging nutrients and electrons for hydrogen production. The consortial biohydrogen production depend upon constituent microbes, their interdependence, competition for resources, and other operating parameters while remediating a waste material in nature or bioreactor. The present review deals with development of granular algae-bacteria consortia, hydrogen yield in coculture, important enzymes and possible engineering for improved hydrogen production.

Chemical Flocculation-Based Green Algae Materials for Photobiological Hydrogen Production

Photobiological hydrogen production is among the most promising ways toward the mass production of hydrogen energy. The use of green algal aggregates to produce photobiological hydrogen has attracted much attention because it overcomes the limitations of sulfur deprivation and oxygen scavengers. However, the current preparation of green algal aggregates that are capable of hydrogen production is time-consuming and laborious, leading to a difficulty in large-scale applications. Here, we demonstrated that the chemical flocculation of green algae is able to generate aggregates for photobiological hydrogen production. We find that Chlorella pyrenoidosa can directly form aggregates in the original liquid cultures by a commercial chemical flocculant, cationic etherified starch, thereby achieving sustainable hydrogen production for 11 days under continuous light irradiation, and the average rate of photobiological production reaches 0.37 μmol H₂ (mg chlorophyll·h)^-1. This research provides a feasible approach for preparing a low-cost photobiological hydrogen production system helping to realize carbon neutrality.

The Potential of Sustainable Biomass Producer Gas as a Waste-to-Energy Alternative in Malaysia

It has been widely accepted worldwide, that the greenhouse effect is by far the most challenging threat in the new century. Renewable energy has been adopted to prevent excessive greenhouse effects, and to enhance sustainable development. Malaysia has a large amount of biomass residue, which provides the country with the much needed support the foreseeable future. This investigation aims to analyze potentials biomass gases from major biomass residues in Malaysia. The potential biomass gasses can be obtained using biomass conversion technologies, including biological and thermo-chemical technologies. The thermo-chemical conversion technology includes four major biomass conversion technologies such as gasification, combustion, pyrolysis, and liquefaction. Biomass wastes can be attained through solid biomass technologies to obtain syngas which includes carbon monoxide, carbon dioxide, oxygen, hydrogen, and nitrogen. The formation of tar occurs during the main of biomass conversion reaction such as gasification and pyrolysis. The formation of tar hinders equipment or infrastructure from catalytic aspects, which will be applied to prevent the formation of tar. The emission, combustion, and produced gas reactions were investigated. It will help to contribute the potential challenges and strategies, due to sustainable biomass, to harness resources management systems in Malaysia to reduce the problem of biomass residues and waste.

Biohydrogen from Microalgae: Production and Applications

The need to safeguard our planet by reducing carbon dioxide emissions has led to a significant development of research in the field of alternative energy sources. Hydrogen has proved to be the most promising molecule, as a fuel, due to its low environmental impact. Even if various methods already exist for producing hydrogen, most of them are not sustainable. Thus, research focuses on the biological sector, studying microalgae, and other microorganisms’ ability to produce this precious molecule in a natural way. In this review, we provide a description of the biochemical and molecular processes for the production of biohydrogen and give a general overview of one of the most interesting technologies in which hydrogen finds application for electricity production: fuel cells.

Advanced microalgae-based renewable biohydrogen production systems: A review

The reliance of fossil fuel for industrial and energy sectors has resulted in its depletion. Therefore, enormous efforts have been considered to move-out from fossil fuels to renewable energy sources based industrial process developments. Recently, biohydrogen (bio-H₂) has been recognised as a clean source of fuel with high-energy efficiency, which can be produced via different routes. Among them, biological fermentation processes are highly recommended due to eco-friendly and economically viable approaches compared to that of thermochemical processes. However, the low H₂ yield and high production cost are major bottlenecks for commercial scale operations. Thus, this review proposed an integrated microalgae-based H₂ production process, which will provides a possible route for commercialization in near future. Furthermore, process integration to improve efficiency and implementation of advanced strategies for the enhancement of bio-H₂ production, economic viability, and future research needs are discussed.

Enhancement of biofuel production by microalgae using cement flue gas as substrate

The cement industry generates a substantial amount of gaseous pollutants that cannot be treated efficiently and economically using standard techniques. Microalgae, a promising bioremediation and biodegradation agent used as feedstock for biofuel production, can be used for the biotreatment of cement flue gas. In specific, components of cement flue gas such as carbon dioxide, nitrogen, and sulfur oxides are shown to serve as nutrients for microalgae. Microalgae also have the capacity to sequestrate heavy metals present in cement kiln dust, adding further benefits. This work provides an extensive overview of multiple approaches taken in the inclusion of microalgae biofuel production in the cement sector. In addition, factors influencing the production of microalgal biomass are also described in such an integrated plant. In addition, process limitations such as the adverse impact of flue gas on medium pH, exhaust gas toxicity, and efficient delivery of carbon dioxide to media are also discussed. Finally, the article concludes by proposing the future potential for incorporating the microalgae biofuel plant into the cement sector.

Photoautotrophic cultures of Chlamydomonas reinhardtii: sulfur deficiency, anoxia, and hydrogen production

The aim of this work was a comparative study of S-repleted and S-depleted photoautotrophic cultures of Chlamydomonas reinhardtii under aerobic and anoxic conditions with the main focus on PSII activity. For that we used photobioreactor with short light path connected on-line to PAM fluorometer and cultivated microalgae in twice concentrated HS medium to avoid any uncontrolled limitation by mineral elements. Photoautotrophic cultures grown under Ar + CO₂ gas mixture did not reach the same Chl (a + b) concentration as control culture (grown under air + CO₂). At pO₂ 40% of air saturation (96 µM O₂), the actual quantum yield of PSII started to decrease. Under microaerobic conditions when cultures stopped growing, the most significant changes in PSII function were observed. Maximum quantum yield F_v/F_m decreased significantly along with performance index, PIabs. It was accompanied by increase of fluorescence at J point, V_j. Results indicate that microaerobic conditions are stressful for photoautotrophic cultures. Photoautotrophic cultures of microalgae under S-deprivation in aerobic or anaerobic conditions showed similar behavior as photoheterotrophic ones described earlier. However, photoautotrophic cultures during anaerobiosis establishment did not show sharp "switch off" effect of actual quantum yield. We show also that S-deprivation under air or argon as well as the growth under Ar + CO₂ cause significant increase of initial rise of fluorescence, which indicates that PSII and oxygen-evolving complex might be disintegrated.

Characterization of the transient fluorescence wave phenomenon that occurs during H2 production in Chlamydomonas reinhardtii

The redox state of the plastoquinone (PQ) pool in sulfur-deprived, H2-producing Chlamydomonas reinhardtii cells was studied using single flash-induced variable fluorescence decay kinetics. During H2 production, the fluorescence decay kinetics exhibited an unusual post-illumination rise of variable fluorescence, giving a wave-like appearance. The wave showed the transient fluorescence minimum at ~60 ms after the flash, followed by a rise, reaching the transient fluorescence maximum at ~1 s after the flash, before decaying back to the initial fluorescence level. Similar wave-like fluorescence decay kinetics have been reported previously in anaerobically incubated cyanobacteria but not in green algae. From several different electron and proton transfer inhibitors used, polymyxin B, an inhibitor of type II NAD(P)H dehydrogenase (NDA2), had the effect of eliminating the fluorescence wave feature, indicating involvement of NDA2 in this phenomenon. This was further confirmed by the absence of the fluorescence wave in the Δnda2 mutant lacking NDA2. Additionally, Δnda2 mutants have also shown delayed and diminished H2 production (only 23% if compared with the wild type). Our results show that the fluorescence wave phenomenon in C. reinhardtii is observed under highly reducing conditions and is induced by the NDA2-mediated electron flow from the reduced stromal components to the PQ pool. Therefore, the fluorescence wave phenomenon is a sensitive probe for the complex network of redox reactions at the PQ pool level in the thylakoid membrane. It could be used in further characterization and improvement of the electron transfer pathways leading to H2 production in C. reinhardtii.

Exogenic glucose as an electron donor for algal hydrogenases to promote hydrogen photoproduction by Chlorella pyrenoidosa

In this work, glucose addition (0.7 g l^-1) almost doubled hydrogen yield of Chlorella pyrenoidosa (121.1 ml l^-1 vs 65.5 ml l^-1), with a positive correlation between hydrogen production and glucose consumption (-0.977, P < 0.01). When the electrons transport from water photolysis to algal hydrogenase was inhibited, the hydrogen productivity declined by 21.1%; whereas it dramatically decreased by 70.9% when the algal nicotinamide adeninedinucleotide dehydrogenase (NADH) was inhibited. Therefore, in the presence of glucose, the electrons for algae based hydrogen production would be mainly from glucose glycolysis rather than water photolysis. Further deuterated-glucose trial indicated that the glucose might serve as an electron donor for algal hydrogenases. Finally, a tentative electron transport route from glucose to algal hydrogenase was proposed, hoping to provide more scientific direction for further algae-based hydrogen production improvement.

Photosystem I fluorescence as a physiological indicator of hydrogen production in Chlamydomonas reinhardtii

This study investigated the interrelations between hydrogen synthesis and Photosystem I electron transport rate in Chlamydomonas reinhardtii. The fluorescence of both photosystems (PS I and PS II) was monitored using a Dual Pulse Amplitude Modulated (PAM) Fluorometer. Hydrogen synthesis was induced by eliminating sulphur from the growth media (TAP-S). Multiple physiological parameters [rETR, Y (I), Y (II), NPQ, α, F_v/F_m and YI:YII] were recorded using the Dual PAM and correlated to hydrogen produced. There was a 66% increase in Photosystem I rETR_max during hydrogen production. A significant direct correlation existed between PS 1 rETR_max and hydrogen evolution values over the ten-day period (r = 0.895, p < 0.01) indicating that PS I can be considered as a driver of H₂ production. Significant correlations between rETR_max of PS I and H₂ evolution suggest a novel physiological indicator to monitor H₂ production during the three critical phases identified in this study.

Elimination of the flavodiiron electron sink facilitates long-term H2 photoproduction in green algae

BACKGROUND

The development of renewable and sustainable biofuels to cover the future energy demand is one of the most challenging issues of our time. Biohydrogen, produced by photosynthetic microorganisms, has the potential to become a green biofuel and energy carrier for the future sustainable world, since it provides energy without CO₂ emission. The recent development of two alternative protocols to induce hydrogen photoproduction in green algae enables the function of the O₂-sensitive [FeFe]-hydrogenases, located at the acceptor side of photosystem I, to produce H₂ for several days. These protocols prevent carbon fixation and redirect electrons toward H₂ production. In the present work, we employed these protocols to a knockout Chlamydomonas reinhardtii mutant lacking flavodiiron proteins (FDPs), thus removing another possible electron competitor with H₂ production.

RESULTS

The deletion of the FDP electron sink resulted in the enhancement of H₂ photoproduction relative to wild-type C. reinhardtii. Additionally, the lack of FDPs leads to a more effective obstruction of carbon fixation even under elongated light pulses.

CONCLUSIONS

We demonstrated that the rather simple adjustment of cultivation conditions together with genetic manipulation of alternative electron pathways of photosynthesis results in efficient re-routing of electrons toward H₂ photoproduction. Furthermore, the introduction of a short recovery phase by regular switching from H₂ photoproduction to biomass accumulation phase allows to maintain cell fitness and use photosynthetic cells as long-term H₂-producing biocatalysts.

Regulation of sulfur deprivation-induced expression of the ferredoxin-encoding FDX5 gene Chlamydomonas reinhardtii in aerobic conditions

The unicellular green alga Chlamydomonas reinhardtii reacts to sulfur (S) starvation with the increased expression of numerous genes. One gene which is induced in illuminated anaerobic S-deprived cells is the ferredoxin-5 gene (FDX5). To test FDX5 transcriptional regulation in aerobic cultures, we used a real-time PCR analysis and an artificial microRNA approach. We demonstrated that FDX5 gene is controlled by S deprivation independently of anoxia-treatment. The Ser/Thr kinase SNRK2.1 is necessary for expression of FDX5 during deprivation to S. Copper response regulator 1 (CRR1) is not involved in FDX5 up-regulation in S-deficient cells under aerobic conditions. Furthermore, expression of FDX5 is negatively regulated by nitric oxide (NO). Moreover, truncated hemoglobin 1 (THB1) underexpression resulted in the decrease in FDX5 transcript abundance in S-deficient cells under aerobic conditions. Together, our results imply that the FDX5 gene is controlled by NO in THB1-dependent pathway under conditions of depleted S supply.

Water-splitting-based, sustainable and efficient H2 production in green algae as achieved by substrate limitation of the Calvin-Benson-Bassham cycle

BACKGROUND

Photobiological H₂ production has the potential of becoming a carbon-free renewable energy source, because upon the combustion of H₂, only water is produced. The [Fe-Fe]-type hydrogenases of green algae are highly active, although extremely O₂-sensitive. Sulphur deprivation is a common way to induce H₂ production, which, however, relies substantially on organic substrates and imposes a severe stress effect resulting in the degradation of the photosynthetic apparatus.

RESULTS

We report on the establishment of an alternative H₂ production method by green algae that is based on a short anaerobic induction, keeping the Calvin-Benson-Bassham cycle inactive by substrate limitation and preserving hydrogenase activity by applying a simple catalyst to remove the evolved O₂. Cultures remain photosynthetically active for several days, with the electrons feeding the hydrogenases mostly derived from water. The amount of H₂ produced is higher as compared to the sulphur-deprivation procedure and the process is photoautotrophic.

CONCLUSION

Our protocol demonstrates that it is possible to sustainably use algal cells as whole-cell catalysts for H₂ production, which enables industrial application of algal biohydrogen production.

Comparative analyses of H2 photoproduction in magnesium- and sulfur-starved Chlamydomonas reinhardtii cultures

Magnesium (Mg)-deprived Chlamydomonas reinhardtii cells are capable to sustain hydrogen (H₂ ) photoproduction at relatively high photosystem II (PSII) activity levels for an extended time period as compared with sulfur (S)-deprived cells. Herein, we present a comparative study of H₂ photoproduction induced by Mg and S shortage to unravel the specific rearrangements of the photosynthetic machinery and cell metabolism occurring under the two deprivation protocols. The exhaustive analysis of photosynthetic activity and regulatory pathways, respiration and starch metabolism revealed the specific rearrangements of the photosynthetic machinery and cellular metabolism, which occur under the two deprivation conditions. The obtained results allowed us to conclude that the expanded time period of H₂ production upon Mg-deprivation is due to the less harmful effects that Mg-depletion has on viability and metabolic performance of the cells. Unlike S-deprivation, the photosynthetic light and dark reactions in Mg-deprived cells remained active over the whole H₂ production period. However, the elevated PSII activity in Mg-deprived cells was counteracted by the operation of pathways for O₂ consumption that maintain anaerobic conditions in the presence of active water splitting.

Microalgal cultivation for value-added products: a critical enviro-economical assessment

The present review focuses on the cultivation of algal biomass for generating value-added products (VAP) and to assess their economic benefits and harmful environmental impact. Additionally, the impact of bioreactor designs on the yield of microalgal biomass for VAP is also considered. All these factors are discussed in relation to the impact of microalgae production on the bio-economy sector of commercial biotechnology.

Hydrogen Production by a Chlamydomonas reinhardtii Strain with Inducible Expression of Photosystem II

Chlamydomonas reinhardtii cy6Nac2.49 is a genetically modified algal strain that activates photosynthesis in a cyclical manner, so that photosynthesis is not active constitutively in the presence of oxygen, but is turned on only in response to a metabolic trigger (anaerobiosis). Here, we further investigated hydrogen production by this strain comparing it with the parental wild-type strain under photoheterotrophic conditions in regular tris-acetate-phosphate (TAP) medium with a 10-h:14-h light/dark regime. Unlike the wild-type, whose level of H₂ production remained low during illumination, H₂ production in the mutant strain increased gradually with each subsequent light period, and by the final light period was significantly higher than the wild-type. The relatively low Photosystem II (PSII) activity of the mutant culture was shown by low fluorescence yield both in the dark (Fv/Fm) and in the light (δF/Fm’) periods. Measurement of oxygen evolution confirmed the low photosynthetic activity of the mutant cells, which gradually accumulated O₂ to a lesser extent than the wild-type, thus allowing the mutant strain to maintain hydrogenase activity over a longer time period and to gradually accumulate H₂ during periods of illumination. Therefore, controllable expression of PSII can be used to increase hydrogen production under nutrient replete conditions, thus avoiding many of the limitations associated with nutrient deprivation approaches sometimes used to promote hydrogen production.

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.

Temperature-sensitive PSII: a novel approach for sustained photosynthetic hydrogen production

The need for energy and the associated burden are ever growing. It is crucial to develop new technologies for generating clean and efficient energy for society to avoid upcoming energetic and environmental crises. Sunlight is the most abundant source of energy on the planet. Consequently, it has captured our interest. Certain microalgae possess the ability to capture solar energy and transfer it to the energy carrier, H2. H2 is a valuable fuel, because its combustion produces only one by-product: water. However, the establishment of an efficient biophotolytic H2 production system is hindered by three main obstacles: (1) the hydrogen-evolving enzyme, [FeFe]-hydrogenase, is highly sensitive to oxygen; (2) energy conversion efficiencies are not economically viable; and (3) hydrogen-producing organisms are sensitive to stressful conditions in large-scale production systems. This study aimed to circumvent the oxygen sensitivity of this process with a cyclic hydrogen production system. This approach required a mutant that responded to high temperatures by reducing oxygen evolution. To that end, we randomly mutagenized the green microalgae, Chlamydomonas reinhardtii, to generate mutants that exhibited temperature-sensitive photoautotrophic growth. The selected mutants were further characterized by their ability to evolve oxygen and hydrogen at 25 and 37 °C. We identified four candidate mutants for this project. We characterized these mutants with PSII fluorescence, P700 absorbance, and immunoblotting analyses. Finally, we demonstrated that these mutants could function in a prototype hydrogen-producing bioreactor. These mutant microalgae represent a novel approach for sustained hydrogen production.

Design and construction of a photobioreactor for hydrogen production, including status in the field

Several species of microalgae and phototrophic bacteria are able to produce hydrogen under certain conditions. A range of different photobioreactor systems have been used by different research groups for lab-scale hydrogen production experiments, and some few attempts have been made to upscale the hydrogen production process. Even though a photobioreactor system for hydrogen production does require special construction properties (e.g., hydrogen tight, mixing by other means than bubbling with air), only very few attempts have been made to design photobioreactors specifically for the purpose of hydrogen production. We have constructed a flat panel photobioreactor system that can be used in two modes: either for the cultivation of phototrophic microorganisms (upright and bubbling) or for the production of hydrogen or other anaerobic products (mixing by "rocking motion"). Special emphasis has been taken to avoid any hydrogen leakages, both by means of constructional and material choices. The flat plate photobioreactor system is controlled by a custom-built control system that can log and control temperature, pH, and optical density and additionally log the amount of produced gas and dissolved oxygen concentration. This paper summarizes the status in the field of photobioreactors for hydrogen production and describes in detail the design and construction of a purpose-built flat panel photobioreactor system, optimized for hydrogen production in terms of structural functionality, durability, performance, and selection of materials. The motivations for the choices made during the design process and advantages/disadvantages of previous designs are discussed.

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.

Chlamydomonas Flavodiiron Proteins Facilitate Acclimation to Anoxia During Sulfur Deprivation

The flavodiiron proteins (FDPs) are involved in the detoxification of oxidative compounds, such as nitric oxide (NO) or O(2) in Archaea and Bacteria. In cyanobacteria, the FDPs Flv1 and Flv3 are essential in the light-dependent reduction of O(2) downstream of PSI. Phylogenetic analysis revealed that two genes (flvA and flvB) in the genome of Chlamydomonas reinhardtii show high homology to flv1 and flv3 genes of the cyanobacterium Synechocystis sp. PCC 6803. The physiological role of these FDPs in eukaryotic green algae is not known, but it is of a special interest since these phototrophic organisms perform oxygenic photosynthesis similar to higher plants, which do not possess FDP homologs. We have analyzed the levels of flvA and flvB transcripts in C. reinhardtii cells under various environmental conditions and showed that these genes are highly expressed under ambient CO(2) levels and during the early phase of acclimation to sulfur deprivation, just before the onset of anaerobiosis and the induction of efficient H(2) photoproduction. Importantly, the increase in transcript levels of the flvA and flvB genes was also corroborated by protein levels. These results strongly suggest the involvement of FLVA and FLVB proteins in alternative electron transport.

Characterization of Hydrogen Metabolism in the Multicellular Green Alga Volvox carteri

Hydrogen gas functions as a key component in the metabolism of a wide variety of microorganisms, often acting as either a fermentative end-product or an energy source. The number of organisms reported to utilize hydrogen continues to grow, contributing to and expanding our knowledge of biological hydrogen processes. Here we demonstrate that Volvox carteri f. nagariensis, a multicellular green alga with differentiated cells, evolves H₂ both when supplied with an abiotic electron donor and under physiological conditions. The genome of Volvox carteri contains two genes encoding putative [FeFe]-hydrogenases (HYDA1 and HYDA2), and the transcripts for these genes accumulate under anaerobic conditions. The HYDA1 and HYDA2 gene products were cloned, expressed, and purified, and both are functional [FeFe]-hydrogenases. Additionally, within the genome the HYDA1 and HYDA2 genes cluster with two putative genes which encode hydrogenase maturation proteins. This gene cluster resembles operon-like structures found within bacterial genomes and may provide further insight into evolutionary relationships between bacterial and algal [FeFe]-hydrogenase genes.

Prospective technology on bioethanol production from photofermentation

The most important global demand is the energy supply from alternative source. Ethanol may be considered an environmental friendly fuel that has been produced by feedstock. The production of ethanol by microalgae represent a process with reduced environmental impact with efficient CO2 fixation and requiring less arable land. This work studied the production of ethanol from green alga Chlamydomonas reinhardtii through the cellular metabolism in a light/dark cycle at 25 °C in a TAP medium with sulfur depletion. The parameters evaluated were inoculum concentration and the medium supplementation with mixotrophic carbon sources. The combination of C.reinhardtii and Rhodobacter capsulatus through a hybrid or co-culture systems was also investigated as well. C.reinhardtii maintained in TAP-S produced 19.25±4.16 g/L (ethanol). In addition, in a hybrid system, with medium initially supplemented with milk whey permeated and the algal effluent used by R. capsulatus, the ethanol production achieved 19.94±2.67 g/L.

Low oxygen levels contribute to improve photohydrogen production in mixotrophic non-stressed Chlamydomonas cultures

BACKGROUND

Currently, hydrogen fuel is derived mainly from fossil fuels, but there is an increasing interest in clean and sustainable technologies for hydrogen production. In this context, the ability of some photosynthetic microorganisms, particularly cyanobacteria and microalgae, to produce hydrogen is a promising alternative for renewable, clean-energy production. Among a diverse array of photosynthetic microorganisms able to produce hydrogen, the green algae Chlamydomonas reinhardtii is the model organism widely used to study hydrogen production. Despite the well-known fact that acetate-containing medium enhances hydrogen production in this algae, little is known about the precise role of acetate during this process.

RESULTS

We have examined several physiological aspects related to acetate assimilation in the context of hydrogen production metabolism. Measurements of oxygen and CO2 levels, acetate uptake, and cell growth were performed under different light conditions, and oxygenic regimes. We show that oxygen and light intensity levels control acetate assimilation and modulate hydrogen production. We also demonstrate that the determination of the contribution of the PSII-dependent hydrogen production pathway in mixotrophic cultures, using the photosynthetic inhibitor DCMU, can lead to dissimilar results when used under various oxygenic regimes. The level of inhibition of DCMU in hydrogen production under low light seems to be linked to the acetate uptake rates. Moreover, we highlight the importance of releasing the hydrogen partial pressure to avoid an inherent inhibitory factor on the hydrogen production.

CONCLUSION

Low levels of oxygen allow for low acetate uptake rates, and paradoxically, lead to efficient and sustained production of hydrogen. Our data suggest that acetate plays an important role in the hydrogen production process, during non-stressed conditions, other than establishing anaerobiosis, and independent of starch accumulation. Potential metabolic pathways involved in hydrogen production in mixotrophic cultures are discussed. Mixotrophic nutrient-replete cultures under low light are shown to be an alternative for the simultaneous production of hydrogen and biomass.

Acclimation to hypoxia in Chlamydomonas reinhardtii: can biophotolysis be the major trigger for long-term H2 production?

In anaerobiosis, the microalga Chlamydomonas reinhardtii is able to produce H2 gas. Electrons mainly derive from mobilization of internal reserves or from water through biophotolysis. However, the exact mechanisms triggering this process are still unclear. Our hypothesis was that, once a proper redox state has been achieved, H2 production is eventually observed. To avoid nutrient depletion, which would result in enhanced fermentative pathways, we aimed to induce long-lasting H2 production solely through a photosynthesis : respiration equilibrium. Thus, growing cells were incubated in Tris Acetate Phosphate (TAP) medium under low light and high chlorophyll content. After a 250-h acclimation phase, a 350-h H2 production phase was observed. The light-to-H2 conversion efficiency was comparable to that given in some reports operating under sulphur starvation. Electron sources were found to be water, through biophotolysis, and proteins, particularly through photofermentation. Nonetheless, a substantial contribution from acetate could not be ruled out. In addition, photosystem II (PSII) inhibition by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) showed that it actively contributed to maintaining a redox balance during cell acclimation. In appropriate conditions, PSII may represent the major source of reducing power to feed the H2 evolution process, by inducing and maintaining an ideal excess of reducing power.

Profiling Chlamydomonas metabolism under dark, anoxic H2-producing conditions using a combined proteomic, transcriptomic, and metabolomic approach

Chlamydomonas reinhardtii is well adapted to survive under different environmental conditions due to the unique flexibility of its metabolism. Here we report metabolic pathways that are active during acclimation to anoxia, but were previously not thoroughly studied under dark, anoxic H2-producing conditions in this model green alga. Proteomic analyses, using 2D-differential in-gel electrophoresis in combination with shotgun mass fingerprinting, revealed increased levels of proteins involved in the glycolytic pathway downstream of 3-phosphoglycerate, the glyoxylate pathway, and steps of the tricarboxylic acid (TCA) reactions. Upregulation of the enzyme, isocitrate lyase (ICL), was observed, which was accompanied by increased intracellular succinate levels, suggesting the functioning of glyoxylate pathway reactions. The ICL-inhibitor study revealed presence of reverse TCA reactions under these conditions. Contributions of the serine-isocitrate lyase pathway, glycine cleavage system, and c1-THF/serine hydroxymethyltransferase pathway in the acclimation to dark anoxia were found. We also observed increased levels of amino acids (AAs) suggesting nitrogen reorganization in the form of de novo AA biosynthesis during anoxia. Overall, novel routes for reductant utilization, in combination with redistribution of carbon and nitrogen, are used by this alga during acclimation to O2 deprivation in the dark.

Advances in the biotechnology of hydrogen production with the microalga Chlamydomonas reinhardtii

Biological hydrogen production is being evaluated for use as a fuel, since it is a promising substitute for carbonaceous fuels owing to its high conversion efficiency and high specific energy content. The basic advantages of biological hydrogen production over other "green" energy sources are that it does not compete for agricultural land use, and it does not pollute, as water is the only by-product of the combustion. These characteristics make hydrogen a suitable fuel for the future. Among several biotechnological approaches, photobiological hydrogen production carried out by green microalgae has been intensively investigated in recent years. A select group of photosynthetic organisms has evolved the ability to harness light energy to drive hydrogen gas production from water. Of these, the microalga Chlamydomonas reinhardtii is considered one of the most promising eukaryotic H2 producers. In this model microorganism, light energy, H2O and H2 are linked by two excellent catalysts, the photosystem 2 (PSII) and the [FeFe]-hydrogenase, in a pathway usually referred to as direct biophotolysis. This review summarizes the main advances made over the past decade as an outcome of the discovery of the sulfur-deprivation process. Both the scientific and technical barriers that need to be overcome before H2 photoproduction can be scaled up to an industrial level are examined. Actual and theoretical limits of the efficiency of the process are also discussed. Particular emphasis is placed on algal biohydrogen production outdoors, and guidelines for an optimal photobioreactor design are suggested.

Sensing the dynamics of oxidative stress using enhanced absorption in protein-loaded random media

Reactive oxygen species play a key role in cell signalling and oxidative stress mechanisms, therefore, sensing their production by living organisms is of fundamental interest. Here we describe a novel biosensing method for extracellular detection of endogenous hydrogen peroxide (H2O2). The method is based on the enhancement of the optical absorption spectrum of the hemoprotein cytochrome c when loaded into a highly scattering random medium. Such a configuration enables, in contrast to existing techniques, non-invasive and dynamic detection of the oxidation of cyt c in the presence of H2O2 with unprecedented sensitivity. Dynamic information on the modification of the cell oxidative status of Chlamydomonas reinhardtii, an aquatic green algae, was obtained under oxidative stress conditions induced by the presence of trace concentrations of Cd(II). Furthermore, the dynamics of H2O2 production was investigated under different lighting conditions confirming the impact of Cd(II) on the photosynthetic activity of those phytoplanktonic cells.

Effects of fluid and light dynamics on H2 production in a mechanically stirred photobioreactor

Hydrogen productions through biophotolysis by microalgae in photobioreactors (PBRs) were studied using a computational model integrated with fluid dynamics, particle tracking technique, light attenuation dynamics, biochemical kinetics, and mass transport. The trajectories of microalgae entrained in the flow fields within these PBRs were traced by the particle tracking technique and were used to determine the dynamics of light attenuation subjected by the cells, which were analyzed and compared with those obtained from the unstirred PBR under different incident light illuminations. The results show an improvement on the light penetration depth in the mechanically stirred cultures. The dynamics of light attenuation was incorporated into the kinetics equations for the analysis of the inhomogeneous biochemical process for hydrogen production by microalgae. Hydrogen production in the unstirred and the impeller-stirred PBRs were determined under different light illumination conditions and the results show an improvement on hydrogen production in the impeller-stirred PBRs.

Mechanistic modeling of sulfur-deprived photosynthesis and hydrogen production in suspensions of Chlamydomonas reinhardtii

The ability of unicellular green algal species such as Chlamydomonas reinhardtii to produce hydrogen gas via iron-hydrogenase is well known. However, the oxygen-sensitive hydrogenase is closely linked to the photosynthetic chain in such a way that hydrogen and oxygen production need to be separated temporally for sustained photo-production. Under illumination, sulfur-deprivation has been shown to accommodate the production of hydrogen gas by partially-deactivating O₂ evolution activity, leading to anaerobiosis in a sealed culture. As these facets are coupled, and the system complex, mathematical approaches potentially are of significant value since they may reveal improved or even optimal schemes for maximizing hydrogen production. Here, a mechanistic model of the system is constructed from consideration of the essential pathways and processes. The role of sulfur in photosynthesis (via PSII) and the storage and catabolism of endogenous substrate, and thus growth and decay of culture density, are explicitly modeled in order to describe and explore the complex interactions that lead to H₂ production during sulfur-deprivation. As far as possible, functional forms and parameter values are determined or estimated from experimental data. The model is compared with published experimental studies and, encouragingly, qualitative agreement for trends in hydrogen yield and initiation time are found. It is then employed to probe optimal external sulfur and illumination conditions for hydrogen production, which are found to differ depending on whether a maximum yield of gas or initial production rate is required. The model constitutes a powerful theoretical tool for investigating novel sulfur cycling regimes that may ultimately be used to improve the commercial viability of hydrogen gas production from microorganisms. Biotechnol. Bioeng. 2014;111: 320–335. © 2013 The Authors. Biotechnology and Bioengineering Published by Wiley Periodicals, Inc.