Hydrogen production by microalgae

The production of H₂ gas from water and sunlightusing microalgae, `biophotolysis', has been a subjectof applied research since the early 1970s. A numberof approaches have been investigated, but most provedto have fundamental limitations or requireunpredictable research breakthroughs. Examples areprocesses based on nitrogen-fixing microalgae andthose producing H<sub>2</sub> and O<sub>2</sub> simultaneously fromwater (`direct biophotolysis'). The most plausibleprocesses for future applied R & D are those whichcouple separate stages of microalgal photosynthesisand fermentations (`indirect biophotolysis'). Theseinvolve fixation of CO₂ into storagecarbohydrates followed by their conversion to H₂by the reversible hydrogenase, both in dark andpossibly light-driven anaerobic metabolic processes. Based on a preliminary engineering and economicanalysis, biophotolysis processes must achieve closeto an overall 10% solar energy conversion efficiencyto be competitive with alternatives sources ofrenewable H₂, such as photovoltaic-electrolysisprocesses. Such high solar conversion efficiencies inphotosynthetic CO₂ fixation could be reached bygenetically reducing the number of light harvesting(antenna) chlorophylls and other pigments inmicroalgae. Similarly, greatly increased yields ofH₂ from dark fermentation by microalgae could beobtained through application of the techniques ofmetabolic engineering. Another challenge is toscale-up biohydrogen processes with economicallyviable bioreactors.Solar energy driven microalgae processes forbiohydrogen production are potentially large-scale,but also involve long-term and economically high-riskR&D. In the nearer-term, it may be possible tocombine microalgal H₂ production with wastewatertreatment.     

Bioengineering aspects of biophotolysis

The practical aspects of producing hydrogen by photosynthetic microorganisms are reviewed. Various alternative concepts for hydrogen production are discussed, both single and two-stage systems. The best developed process currently is based on nitrogen-fixing heterocystous blue-green algae which can produce hydrogen and oxygen simultaneously. Solar energy conversion to hydrogen efficiencies of 0.2% averaged over several weeks have been obtained with outdoor systems. Practical systems would require a ten-fold increase in conversion efficiencies. Also, systems which produce pure hydrogen are preferred. Photosynthetic bacteria are of near-term applications. A general design for a biophotolysis system is proposed consisting of vertically arranged, thin-walled glass tubes with an inert gas recirculated through the cultures for mixing and removal of hydrogen. Gas mass transfer considerations, energy utilization, and economics favour such a system.     

Modeling microalgae cultivation productivities in different geographic locations - estimation method for idealized photobioreactors

Microalgae can be used to produce versatile high-value fuels, such as methane, biodiesel, ethanol, or hydrogen gas. One of the most important factors that influence the economics of microalgae cultivation is the primary production of biomass per unit area. This is determined by productivity rates during cultivation, which are influenced by the local climate conditions (solar irradiation, temperature). To compare locations in different climate regions for microalgae cultivation, a mathematical model for an idealized closed photobioreactor was developed. The applied growth kinetics were based on theoretical maximum photon-conversion efficiencies (for the conversion of solar energy to chemical energy in the form of biomass). Known or estimated temperature effects for different algal strains were incorporated. The model was used to calculate hourly average areal productivity rates as well as annual primary production values under local conditions at seven example locations. Here, hourly weather data (solar irradiance and air temperature) were taken into account. According to these model calculations, maximum annual yields were achieved in regions with high irradiation and temperature patterns in or near the optimum range of the specific algal strain (here, desert and equatorial humid climates). The developed model can be used as a tool to assess and compare individual locations for microalgae cultivation.     

Solar energy conversion efficiencies in photosynthesis: Minimizing the chlorophyll antennae to maximize efficiency

The theoretical maxima of solar energy conversion efficiencies and productivities in oxygenic photosynthesis are evaluated. These are contrasted with actual measurements in a variety of photosynthetic organisms, including green microalgae, cyanobacteria, C4 and C3 plants. Minimizing, or truncating, the chlorophyll antenna size of the photosystems can improve photosynthetic solar energy conversion efficiency and productivity up to 3-fold. Generation of truncated light-harvesting chlorophyll antenna size (tla) strains, in all classes of photosynthetic organisms would help to alleviate excess absorption of sunlight and the ensuing wasteful dissipation of excitation energy, and to maximize solar-to-product energy conversion efficiency and photosynthetic productivity in high-density mass cultivations. The tla concept may find application in the commercial exploitation of microalgae and plants for the generation of biomass, biofuels, chemical feedstocks, as well as nutraceuticals and pharmaceuticals.     

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.     

A simple algal production system designed to utilize the flashing light effect

Arrays of foils similar in design to airplane wings have been placed in an algal culture flume to create systematic mixing. Vortices are produced in the culture due to the pressure differential created as water flows over and under the foils. In a flume having a flow rate of 30 cm/s, the foil arrays produced vortices with rotation rates of ca. 0.5-1.0 Hz. This rotation rate is satisfactory to take advantage of the flashing light effect if the culture is sufficiently dense. Solar energy conversion efficiencies in an experimental culture of P. tricornutum increased 2.2-2.4 fold with the foil arrays in place versus controls with no foil arrays and solar energy conversion efficiencies averaged 3.7% over a three-month period. Five-day running means of solar energy conversion efficiencies reached as high as 10% during the three-month period. The use of foil arrays appears to be an effective and inexpensive way to utilize the flashing light effect in a dense algal culture system.     

Characterization of energy conversion of Synechococcus sp. PCC7942 under photoautotrophic conditions based on metabolic flux and chlorophyll fluorescence analysis

Energy conversion efficiency of photoautotrophic microalgae plays an important role in the utilization of light energy for cell growth and production of metabolites. To understand the utilization of light energy, Synechococcus sp. PCC7942 was cultivated at different incident light intensities of 15.8, 47.3, and 94.6 μmol/m²/sec in continuous culture. The influence of light on the carbon and energy metabolism of microalgae was investigated by combining metabolic flux analysis (MFA) and chlorophyll fluorescence analysis (CFA). Results showed that the yields of biomass based on ATP (Y _ATP) and total light energy (Y _E) both declined with increasing light, and the maximal values of Y _ATP and Y _E were estimated to be 4.73 g/mol-ATP, and 17.10 × 10^?3 g/kJ respectively, at the examined conditions. The overall efficiency of energy conversion against total absorbed energy changed with the varying irradiances. However, the actual conversion efficiency of total energy based on CFA was almost constant, regardless of the different irradiances used in the present study.     

Biohydrogen production from microalgae—Major bottlenecks and future research perspectives

The imprudent use of fossil fuels has resulted in high greenhouse gas (GHG) emissions, leading to climate change and global warming. Reduction in GHG emissions and energy insecurity imposed by the depleting fossil fuel reserves led to the search for alternative sustainable fuels. Hydrogen is a potential alternative energy carrier and is of particular interest because hydrogen combustion releases only water. Hydrogen is also an important industrial feedstock. As an alternative energy carrier, hydrogen can be used in fuel cells for power generation. Current hydrogen production mainly relies on fossil fuels and is usually energy and CO₂-emission intensive, thus the use of fossil fuel-derived hydrogen as a carbon-free fuel source is fallacious. Biohydrogen production can be achieved via microbial methods, and the use of microalgae for hydrogen production is outstanding due to the carbon mitigating effects and the utilization of solar energy as an energy source by microalgae. This review provides comprehensive information on the mechanisms of hydrogen production by microalgae and the enzymes involved. The major challenges in the commercialization of microalgae-based photobiological hydrogen production are critically analyzed and future research perspectives are discussed. Life cycle analysis and economic assessment of hydrogen production by microalgae are also presented.     

Solar energy conversion by green microalgae: a photosystem for hydrogen peroxide production

A photosystem for solar energy conversion, comprised of a culture of green microalgae supplemented with methyl viologen, is proposed. The capture of solar energy is based on the Mehler reaction. The reduction of methyl viologen by the photosynthetic apparatus and its subsequent reoxidation by oxygen produces hydrogen peroxide. This is a rich-energy compound that can be used as a nonpollutant and efficient fuel. Four different species of green microalgae, Chlamydomonas reinhardtii (21gr) C. reinhardtii (CW15), Chlorella fusca, and Monoraphidium braunii, were tested as a possible biocatalyst. Each species presented a different efficiency level in the transformation of energy. Azide was an efficient inhibitor of the hydrogen peroxide scavenging system while maintaining photosynthetic activity of the microalgae, and thus significantly increasing the production of the photosystem. The strain C. reinhardtii (21gr), among the species studied, was the most efficient with an initial production rate of 185 micromol H(2)O(2)/h x mg Chl and reaching a maximum of 42.5 micromol H(2)O(2)/mg Chl when assayed in the presence of azide inhibitor.     

A state of the art of metabolic networks of unicellular microalgae and cyanobacteria for biofuel production

The most promising and yet challenging application of microalgae and cyanobacteria is the production of renewable energy: biodiesel from microalgae triacylglycerols and bioethanol from cyanobacteria carbohydrates. A thorough understanding of microalgal and cyanobacterial metabolism is necessary to master and optimize biofuel production yields. To this end, systems biology and metabolic modeling have proven to be very efficient tools if supported by an accurate knowledge of the metabolic network. However, unlike heterotrophic microorganisms that utilize the same substrate for energy and as carbon source, microalgae and cyanobacteria require light for energy and inorganic carbon (CO₂ or bicarbonate) as carbon source. This double specificity, together with the complex mechanisms of light capture, makes the representation of metabolic network nonstandard. Here, we review the existing metabolic networks of photoautotrophic microalgae and cyanobacteria. We highlight how these networks have been useful for gaining insight on photoautotrophic metabolism.     

Steam–air fluidized bed gasification of distillers grains: Effects of steam to biomass ratio,equivalence ratio and gasification temperature

In this study, thermochemical biomass gasification was performed on a bench-scale fluidized-bed gasifier with steam and air as fluidizing and oxidizing agents. Distillers grains, a non-fermentable byproduct of ethanol production, were used as the biomass feedstock for the gasification. The goal was to investigate the effects of furnace temperature, steam to biomass ratio and equivalence ratio on gas composition, carbon conversion efficiency and energy conversion efficiency of the product gas. The experiments were conducted using a 3 × 3 × 3 full factorial design with temperatures of 650, 750 and 850 °C, steam to biomass ratios of 0, 7.30 and 14.29 and equivalence ratios of 0.07, 0.15 and 0.29. Gasification temperature was found to be the most influential factor. Increasing the temperature resulted in increases in hydrogen and methane contents, carbon conversion and energy efficiencies. Increasing equivalence ratio decreased the hydrogen content but increased carbon conversion and energy efficiencies. The steam to biomass ratio was optimal in the intermediate levels for maximal carbon conversion and energy efficiencies.     

The chloroplast signal recognition particle (CpSRP) pathway as a tool to minimize chlorophyll antenna size and maximize photosynthetic productivity

The concept of the Truncated Light-harvesting chlorophyll Antenna (TLA) size, as a tool by which to maximize sunlight utilization and photosynthetic productivity in microalgal mass cultures or high-density plant canopies, is discussed. TLA technology is known to improve sunlight-to-product energy conversion efficiencies and is hereby exemplified by photosynthetic productivity estimates of wild type and a TLA strain under simulated mass culture conditions. Recent advances in the generation of TLA-type mutants by targeting genes of the chloroplast signal-recognition particle (CpSRP) pathway, affecting the thylakoid membrane assembly of light-harvesting proteins, are also summarized. Two distinct CpSRP assembly pathways are recognized, one entailing post-translational, the other a co-translational mechanism. Differences between the post-translational and co-translational integration mechanisms are outlined, as these pertain to the CpSRP-mediated assembly of thylakoid membrane protein complexes in higher plants and green microalgae. The applicability of the CpSRP pathway genes in efforts to generate TLA-type strains with enhanced solar energy conversion efficiency in photosynthesis is evaluated.     

Energetics and carbon metabolism during growth of microalgal cells under photoautotrophic, mixotrophic and cyclic light-autotrophic/dark-heterotrophic conditions

Chlorella pyrenoidosa was cultivated under photoautotrophic, mixotrophic and cyclic light-autotrophic/dark-heterotrophic conditions. The influence of light on the carbon and energy metabolism of microalgae was investigated by the use of metabolic flux analysis. The respiratory activity of microalgae in the light was assessed from the autotrophic flux distribution. Results showed that the glycolytic pathway, tricarboxylic acid cycle and mitochondrial oxidative phosphorylation maintained high activities during illumination, indicating little effect of light on these pathways, while the flux through the pentose phosphate pathway during illumination was very small due to the light-mediated regulation. The theoretical yields of biomass on ATP decreased in the following order: heterotrophic culture>mixotrophic culture>autotrophic culture, and a significant amount of the available ATP was required for maintenance processes in microalgal cells. The energy conversion efficiency between the supplied energy to culture, the absorbed energy by cells and the free energy conserved in ATP were analyzed for the different cultures. Analysis showed that the heterotrophic culture generated more ATP from the supplied energy than the autotrophic and mixotrophic cultures. The maximum thermodynamic efficiency of ATP production from the absorbed energy, which was calculated from the metabolic fluxes at zero growth rate, was the highest in the heterotrophic culture and as low as 16% in the autotrophic culture. By evaluating the energy economy through the energy utilization efficiency, it was found that the biomass yield on the supplied energy was the lowest in the autotrophic cultivation, and the cyclic culture gave the most efficient utilization of energy for biomass production.     

Progress in physicochemical parameters of microalgae cultivation for biofuel production

Microalgae have been exploited for biofuel generation in the current era due to its enormous energy content, fast cellular growth rate, inexpensive culture approaches, accumulation of inorganic compounds, and CO₂ sequestration. Currently, research is ongoing towards the advancement of the microalgae cultivation parameters to enhance the biomass yield. The main objective of this study was to delineate the progress of physicochemical parameters for microalgae cultivation such as gaseous transfer, mixing, light demand, temperature, pH, nutrients and the culture period. This review demonstrates the latest research trends on mass transfer coefficient of different microalgae culturing reactors, gas velocity optimization, light intensity, retention time, and radiance effects on microalgae cellular growth, temperature impact on chlorophyll production, and nutrient dosage ratios for cellulosic metabolism to avoid nutrient deprivation. Besides that, cultivation approaches for microalgae associated with mathematical modeling for different parameters, mechanisms of microalgal growth rate and doubling time have been elaborately described. Along with that, this review also documents potential lipid-carbohydrate-protein enriched microalgae candidates for biofuel, biomass productivity, and different cultivation conditions including open-pond cultivation, closed-loop cultivation, and photobioreactors. Various photobioreactor types, the microalgae strain, productivity, advantages, and limitations were tabulated. In line with microalgae cultivation, this study also outlines in detail numerous biofuels from microalgae.     

Microalgal carbohydrates: an overview of the factors influencing carbohydrates production, and of main bioconversion technologies for production of biofuels

Microalgal biomass seems to be a promising feedstock for biofuel generation. Microalgae have relative high photosynthetic efficiencies, high growth rates, and some species can thrive in brackish water or seawater and wastewater from the food- and agro-industrial sector. Today, the main interest in research is the cultivation of microalgae for lipids production to generate biodiesel. However, there are several other biological or thermochemical conversion technologies, in which microalgal biomass could be used as substrate. However, the high protein content or the low carbohydrate content of the majority of the microalgal species might be a constraint for their possible use in these technologies. Moreover, in the majority of biomass conversion technologies, carbohydrates are the main substrate for production of biofuels. Nevertheless, microalgae biomass composition could be manipulated by several cultivation techniques, such as nutrient starvation or other stressed environmental conditions, which cause the microalgae to accumulate carbohydrates. This paper attempts to give a general overview of techniques that can be used for increasing the microalgal biomass carbohydrate content. In addition, biomass conversion technologies, related to the conversion of carbohydrates into biofuels are discussed.     

Microalgae: a robust “green bio-bridge” between energy and environment

Microalgae are a potential candidate for biofuel production and environmental treatment because of their specific characteristics (e.g. fast growth, carbon neutral, and rich lipid accumulations). However, several primary bottlenecks still exist in current technologies, including low biomass conversion efficiency, bio-invasion from the external environment, limited or costly nutrient sources, and high energy and capital input for harvest, and stalling its industrial progression. Coupling biofuel production with environmental treatment renders microalgae a more feasible feedstock. This review focuses on microalgae biotechnologies for both bioenergy generation and environmental treatment (e.g. CO₂ sequestration and wastewater reclamation). Different intelligent technologies have been developed, especially during the last decade, to eliminate the bottlenecks, including mixotrophic/heterotrophic cultivation, immobilization, and co-cultivation. It has been realized that any single purpose for the cultivation of microalgae is not an economically feasible option. Combinations of applications in biorefineries are gradually reckoned to be necessary as it provides more economically feasible and environmentally sustainable operations. This presents microalgae as a special niche occupier linking the fields of energy and environmental sciences and technologies. The integrated application of microalgae is also proven by most of the life-cycle analysis studies. This study summarizes the latest development of primary microalgal biotechnologies in the two areas that will bring researchers a comprehensive view towards industrialization with an economic perspective.     

Controlling light-use by Rhodobacter capsulatus continuous cultures in a flat-panel photobioreactor

The main bottleneck in scale-up of phototrophic fermentation is the low efficiency of light energy conversion to the desired product, which is caused by an excessive dissipation of light energy to heat. The photoheterotrophic formation of hydrogen from acetate and light energy by the microorganism Rhodobacter capsulatus NCIMB 11773 was chosen as a case study in this work. A light energy balance was set up, in which the total bacterial light energy absorption is split up and attributed to its destinations. These are biomass growth and maintenance, generation of hydrogen and photosynthetic heat dissipation. The constants defined in the light energy balance were determined experimentally using a flat-panel photobioreactor with a 3-cm optical path. An experimental method called D-stat was applied. Continuous cultures were kept in a so-called pseudo steady state, while the dilution rate was reduced slowly and smoothly. The biomass yield and maintenance coefficients of Rhodobacter capsulatus biomass on light energy were determined at 12.4 W/m(2) (400-950 nm) and amounted to 2.58 x 10(-8) +/- 0.04 x 10(-8) kg/J and 102 +/- 3.5 W/kg, respectively. The fraction of the absorbed light energy that was dissipated to heat at 473 W/m(2) depended on the biomass concentration in the reactor and varied between 0.80 and 0.88, as the biomass concentration was increased from 2.0 to 8.0 kg/m(3). The process conditions were estimated at which a 3.7% conversion efficiency of absorbed light energy to produced hydrogen energy should be attainable at 473 W/m(2).     

The energetics of asexual reproduction: Pedal laceration in the symbiotic sea anemone Aiptasia pulchella (Carlgren, 1943)

Asexual reproduction of the symbiotic sea anemone Aiptasia pulchella (Carlgren, 1943) was studied in the context of energy budgets determined under a variety of light and feeding conditions. Continuous darkness significantly increased rates of pedal laceration (the method of asexual reproduction), while different feeding rates had no significant effect. Digestive assimilation efficiency averaged 63% for whole brine shrimp, and conversion efficiencies (assimilated energy into biomass) averaged 31%. There were no significant differences between the conversion efficiencies of energy obtained from light or food. The index of reproductive effort (RE) for pedal laceration was very low, ranging from 0.004 to 0.044. This low RE associated with a substantial rate of asexual reproduction may, in part, explain the widespread success of certain sea anemones.     

Solar‐to‐Hydrogen Energy Conversion Based on Water Splitting

Artificial photosynthesis provides a blueprint to harvest solar energy to sustain the future energy demands. Solar‐driven water splitting, converting solar energy into hydrogen energy, is the prototype of photosynthesis. Various systems have been designed and evaluated to understand the reaction pathways and/or to meet the requirements of potential applications. In solar‐to‐hydrogen conversion, electrocatalytic hydrogen and oxygen evolution reactions are key research areas that are meaningful both theoretically and practically. To utilize hydrogen energy, fuel cell technology has been extensively investigated because of its high efficiency in releasing chemical energy. In this review, general concepts of the photosynthesis in green plants are discussed, different strategies for the light‐driven water splitting proposed in laboratories are introduced, the progress of electrocatalytic hydrogen and oxygen evolution reactions are reviewed, and finally, the reactions in hydrogen fuel cells are briefly discussed. Overall, the mass and energy circulation in the solar‐hydrogen‐electricity circle are delineated. The authors conclude that attention from scientists and engineers of relevant research areas is still highly needed to eliminate the wide disparity between the aspirations and realities of artificial photosynthesis.     

Hydrogen production by cyanobacteria in an automated outdoor photobioreactor under aerobic conditions

The possibility of hydrogen production by a hydrogenase impaired mutant strain of Anabaena variabilis in outdoor culture was studied. A computer-controlled rooftop (outdoor) tubular photobioreactor (4.35 L) was assembled. H(2) production rates by A. variabilis PK84 grown in CO(2) + air in the photobioreactor were measured together with other parameters such as temperature, irradiance, pH, dry biomass weight, and pO(2), and Chl a concentrations during summer months of 1998 and 1999. Efficiencies of light energy bioconversion to H(2) energy and energy accumulated in biomass were calculated. The influence of irradiance, temperature, and mode of cultivation on H(2) production and efficiency of light energy bioconversion were evaluated. The culture produced up to 1.1 L H(2) day(-1) PhBR(-1). The efficiency of light energy to H(2) energy bioconversion on some days was 0.094%. However, the conditions for maximum H(2) photoproduction and for maximum efficiency of light energy to H(2) energy bioconversion were not the same. A. variabilis PK84 could produce hydrogen for prolonged periods (up to 40 days) without injection of fresh inoculum. During this period photobioreactor produced 24.5 L of H(2). Possibilities for increasing the efficiency of light energy conversion are discussed.