Recent insights into biohydrogen production by microalgae - From biophotolysis to dark fermentation

Kinetics of Hydrogen Generation from Oxidation of Hydrogenated Silicon Nanocrystals in Aqueous Solutions

Hydrogen generation rate is one of the most important parameters which must be considered for the development of engineering solutions in the field of hydrogen energy applications. In this paper, the kinetics of hydrogen generation from oxidation of hydrogenated porous silicon nanopowders in water are analyzed in detail. The splitting of the Si-H bonds of the nanopowders and water molecules during the oxidation reaction results in powerful hydrogen generation. The described technology is shown to be perfectly tunable and allows us to manage the kinetics by: (i) varying size distribution and porosity of silicon nanoparticles; (ii) chemical composition of oxidizing solutions; (iii) ambient temperature. In particular, hydrogen release below 0 °C is one of the significant advantages of such a technological way of performing hydrogen generation.

Advances in nanomaterials induced biohydrogen production using waste biomass

Recent advances on biohydrogen production using different types of waste biomass with the implementation of nanomaterials are summarized. Inspired by exceptional physicochemical and catalytic properties of nanomaterials, the present review focuses on several approaches including impact of nanomaterials on cellulosic biohydrogen production, possible pretreatment technology, as well as improved enzyme & sugar production in order to enhance the biohydrogen yield. Particularly, impacts of nanomaterial are elaborated in detail on different pathways of biohydrogen production (e.g. dark fermentation, photo-fermentation and hybrid-fermentation) using variety of waste biomass. Additionally, emphases are made on the feasibility of nanomaterials for making the biohydrogen production process more economical and sustainable and hence to develop advanced techniques for biohydrogen production using waste biomass.

Algae-Bacteria Consortia as a Strategy to Enhance H2 Production

Biological hydrogen production by microalgae is a potential sustainable, renewable and clean source of energy. However, many barriers limiting photohydrogen production in these microorganisms remain unsolved. In order to explore this potential and make biohydrogen industrially affordable, the unicellular microalga Chlamydomonas reinhardtii is used as a model system to solve barriers and identify new approaches that can improve hydrogen production. Recently, Chlamydomonas–bacteria consortia have opened a new window to improve biohydrogen production. In this study, we review the different consortia that have been successfully employed and analyze the factors that could be behind the improved H₂ production.

A Review of Biohydrogen Productions from Lignocellulosic Precursor via Dark Fermentation: Perspective on Hydrolysate Composition and Electron-Equivalent Balance

This paper reviews the current technological development of bio-hydrogen (BioH2) generation, focusing on using lignocellulosic feedstock via dark fermentation (DF). Using the collected reference reports as the training data set, supervised machine learning via the constructed artificial neuron networks (ANNs) imbedded with feed backward propagation and one cross-out validation approach was deployed to establish correlations between the carbon sources (glucose and xylose) together with the inhibitors (acetate and other inhibitors, such as furfural and aromatic compounds), hydrogen yield (HY), and hydrogen evolution rate (HER) from reported works. Through the statistical analysis, the concentrations variations of glucose (F-value = 0.0027) and acetate (F-value = 0.0028) were found to be statistically significant among the investigated parameters to HY and HER. Manipulating the ratio of glucose to acetate at an optimal range (approximate in 14:1) will effectively improve the BioH2 generation (HY and HER) regardless of microbial strains inoculated. Comparative studies were also carried out on the evolutions of electron equivalent balances using lignocellulosic biomass as substrates for BioH2 production across different reported works. The larger electron sinks in the acetate is found to be appreciably related to the higher HY and HER. To maintain a relative higher level of the BioH2 production, the biosynthesis needs to be kept over 30% in batch cultivation, while the biosynthesis can be kept at a low level (2%) in the continuous operation among the investigated reports. Among available solutions for the enhancement of BioH2 production, the selection of microbial strains with higher capacity in hydrogen productions is still one of the most phenomenal approaches in enhancing BioH2 production. Other process intensifications using continuous operation compounded with synergistic chemical additions could deliver additional enhancement for BioH2 productions during dark fermentation.

A perspective on novel cascading algal biomethane biorefinery systems

Synergistic opportunities to combine biomethane production via anaerobic digestion whilst cultivating microalgae have been previously suggested in literature. While biomethane is a promising and flexible renewable energy vector, microalgae are increasingly gaining importance as an alternate source of food and/or feed, chemicals and energy for advanced biofuels. However, simultaneously achieving, grid quality biomethane, effective microalgal digestate treatment, high microalgae growth rate, and the most sustainable use of the algal biomass is a major challenge. In this regard, the present paper proposes multiple configurations of an innovative Cascading Algal Biomethane-Biorefinery System (CABBS) using a novel two-step bubble column-photobioreactor photosynthetic biogas upgrading technology. To overcome the limitations in choice of microalgae for optimal system operation, a microalgae composition based biorefinery decision tree has also been conceptualised to maximise profitability. Techno-economic, environmental and practical aspects have been discussed to provide a comprehensive perspective of the proposed systems.

Enhancement of dark fermentative H2 production by gas separation membranes: A review

Biohydrogen production via dark fermentation is currently the most developed method considering its practical readiness for scale-up. However, technological issues to be resolved are still identifiable and should be of concern, particularly in terms of internal mass transfer. If sufficient liquid-to-gas H₂ mass transfer rates are not ensured, serious problems associated with the recovery of biohydrogen and consequent inhibition of the process can occur. Therefore, the continuous and effective removal of H₂ gas is required, which can be performed using gas separation membranes. In this review, we aim to analyze the literature experiences and knowledge regarding mass transfer enhancement approaches and show how membranes may contribute to this task by simultaneously processing the internal (headspace) gas, consisting mainly of H₂ and CO₂. Promising strategies related to biogas recirculation and integrated schemes using membranes will be presented and discussed to detect potential future research directions for improving biohydrogen technology.

Extraction of high-quality, high-molecular-weight DNA depends heavily on cell homogenization methods in green microalgae

PREMISE

New sequencing technologies have facilitated genomic studies in green microalgae; however, extracting high-quality DNA is often a bottleneck for long-read sequencing.

METHODS AND RESULTS

Here, we present a low-cost, highly transferrable method for the extraction of high-molecular-weight (HMW), high-purity DNA from microalgae. We first determined the effect of sample preparation on DNA quality using three homogenization methods: manual grinding using a mini-pestle, automatic grinding using a vortex adapter, and grinding in liquid nitrogen. We demonstrated the versatility of grinding in liquid nitrogen followed by a modified cetyltrimethylammonium bromide (CTAB) extraction across a suite of aquatic- and desert-evolved algal taxa. Finally, we tested the protocol's robustness by doubling the input material to increase yield, producing per sample up to 20 μg of high-purity DNA longer than 21.2 kbp.

CONCLUSIONS

All homogenization methods produced DNA within acceptable parameters for purity, but only liquid nitrogen grinding resulted in HMW DNA. The optimization of cell lysis while minimizing DNA shearing is therefore crucial for the isolation of DNA for long-read genomic sequencing because template DNA length strongly affects read output and length.

Industrial wastewater to biohydrogen: Possibilities towards successful biorefinery route

The aim of this review is to summarize the modern developments and enhancement strategies reported for improving the biorefinery route of industrial wastewater to biohydrogen. Recent developments towards biohydrogen production chiefly involves culture enrichment, pretreatment of biocatalysts, co culture fermentation, metabolic and genetic engineering, ecobiotechnological approaches and the coupling process of biohydrogen. In addition, an overview of dark fermentation, pathways involved, microbes involved in biohydrogen production, industrial wastewater as substrate have been focused. The utilization of organic residuals of dark fermentation for subsequent value added products are highlighted. More apparently, the two stage coupling process and its possibilities towards biorefinery has been reviewed comprehensively. Moreover, comparative energy and economic aspects of biohydrogen production from industrial wastewater and its prospects towards pilot scale applications are also spotlighted. Though all the enhancement strategies have both benefits and disadvantages, coupling process is considered as the most successful biorefinery route for biohydrogen production.

Estimation of equilibrium times and maximum capacity of adsorption of heavy metals by E. crassipes (review)

Cellulose emerges as an alternative for the treatment of water contaminated with heavy metals due to its abundant biomass and its proven potential in the adsorption of pollutants. The aquatic plant Eichhornia crassipes is an option as raw material in the contribution of cellulose due to its enormous presence in contaminated wetlands, rivers, and lakes. The efficiency in the removal of heavy metals is due to the cation exchange between the hydroxyl groups and carboxyl groups present in the biomass of E. crassipes with heavy metals. Through different chemical and physical transformations of the biomass of E. crassipesThe objective of this review article is to provide a discussion on the different mechanisms of adsorption of the biomass of E. crassipes to retain heavy metals and dyes. In addition to estimating equilibrium, times through kinetic models of adsorption and maximum capacities of this biomass through equilibrium models with isotherms, in order to design one biofilter for treatment systems on a larger scale represented the effluents of a real industry.

Estimation of equilibrium times and maximum capacity of adsorption of heavy metals by E. crassipes (review)

Cellulose emerges as an alternative for the treatment of water contaminated with heavy metals due to its abundant biomass and its proven potential in the adsorption of pollutants. The aquatic plant Eichhornia crassipes is an option as raw material in the contribution of cellulose due to its enormous presence in contaminated wetlands, rivers, and lakes. The efficiency in the removal of heavy metals is due to the cation exchange between the hydroxyl groups and carboxyl groups present in the biomass of E. crassipes with heavy metals. Through different chemical and physical transformations of the biomass of E. crassipesThe objective of this review article is to provide a discussion on the different mechanisms of adsorption of the biomass of E. crassipes to retain heavy metals and dyes. In addition to estimating equilibrium, times through kinetic models of adsorption and maximum capacities of this biomass through equilibrium models with isotherms, in order to design one biofilter for treatment systems on a larger scale represented the effluents of a real industry.

Recent advancement and strategy on bio-hydrogen production from photosynthetic microalgae

Recently, ensuring energy security is a key challenge to political and economic strength in the world. Bio-hydrogen production from microalgae is the promising alternative source for potential renewable and self-sustainability energy but still in the initial phase of development. Practically and sustainability of microalgae hydrogen production is still debatable. The genetic engineering and metabolic pathway engineering of hydrogenase and nitrogenase play a key role to enhance hydrogen production. Microalgae have photosynthetic efficiency and synthesize huge carbohydrate biomass, used as 4th generation feedstock to generate bio-hydrogen. Recent genetically modified strains of microalgae are the attractive source for enhancing bio-hydrogen production in the future. The potential of hydrogen production from microRNAs are gaining great interest of researcher. The main objective of this review is attentive discussed recent approaches on new molecular genetics engineering and metabolic pathway developments, modern photo-bioreactors efficiency, economic assessment, limitations and knowledge gap of bio-hydrogen production from microalgae.

Grid columnar flat panel photobioreactor with immobilized photosynthetic bacteria for continuous photofermentative hydrogen production

A biophotoreactor with a transparent glass flat panel with polymethyl methacrylate (PMMA) grid columnar for enhanced biofilm growth with Rhodopseudomonas palustris GCA009 was developed and tested at 590 nm incident light. Continuous photofermentative hydrogen production from glucose was tested using this novel reactor. At light intensity of 210 W/m², feed substrate concentration of 56.0 mmol/L, and crossflow velocity of 1.68 × 10^-6 m/s, a maximum hydrogen production rate of 32.6 mmol/L-d (3.56 mmol/m²-h), hydrogen yield of 1.15 mol H₂/mol glucose and light conversion efficiency of 5.34% can be achieved. Since the revised grid columnar effectively enlarged the surface area of reactor and enhanced cell attachment, the present reactor design led to higher hydrogen production rates than literature works.

Microalgal bioenergy production under zero-waste biorefinery approach: Recent advances and future perspectives

In view of the globalization and energy consumption, an economic and sustainable biorefinery model is essential to address the energy security and climate change. From this perspective, renewable biofuel production from microalgae along with a wide range of value-added co-products define its potential as a biorefinery feedstock. However, economic viability of microalgal biorefinery at its current state is not considered sustainable. Reduce, recycle, and reuse of waste derived from algal bioenergy conversion process will lead to an energy efficient and sustainable zero-waste microalgal biorefinery. This review focuses on three major aspects of zero-waste microalgal biorefinery approach; (1) recent advances on microalgal bioenergy conversion processes (chemical, biochemical and thermochemical); (2) mitigation and transformation of liquid and solid waste and (3) techno-economic analysis (TEA) and lifecycle assessment (LCA). In addition, the study also focuses on the challenges and future perspectives for an advanced microalgal biorefinery model.

Microbial Production of Hydrogen by Mixed Culture Technologies: A Review

With its high energy content and clean combustion, hydrogen is recognized as a renewable clean energy source with enormous potential. Biological hydrogen production is a promising alternative with significant advantages over conventional petroleum-derived chemical processes. Sustainable hydrogen production from renewable resources such as cassava, wastewater, and other agricultural waste is economically feasible for industrial applications. So far, the major bottlenecks in large-scale biological hydrogen production are the low production rate and yield. This review discusses the various factors that affect the metabolic pathways of dark hydrogen production, and highlights the state-of-the-art development of mixed culture technology. The aim of this review is to provide suggestions for the future directions of mixed culture technology, as well as by-product valorization in dark fermentation.

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.

Biological hydrogen production: molecular and electrolytic perspectives

Exploration of renewable energy sources is an imperative task in order to replace fossil fuels and to diminish atmospheric pollution. Hydrogen is considered one of the most promising fuels for the future and implores further investigation to find eco-friendly ways toward viable production. Expansive processes like electrolysis and fossil fuels are currently being used to produce hydrogen. Biological hydrogen production (BHP) displays recyclable and economical traits, and is thus imperative for hydrogen economy. Three basic modes of BHP were investigated, including bio photolysis, photo fermentation and dark fermentation. Photosynthetic microorganisms could readily serve as powerhouses to successively produce this type of energy. Cyanobacteria, blue green algae (bio photolysis) and some purple non-sulfur bacteria (Photo fermentation) utilize solar energy and produce hydrogen during their metabolic processes. Ionic species, including hydrogen (H⁺) and electrons (e^-) are combined into hydrogen gas (H₂), with the use of special enzymes called hydrogenases in the case of bio photolysis, and nitrogenases catalyze the formation of hydrogen in the case of photo fermentation. Nevertheless, oxygen sensitivity of these enzymes is a drawback for bio photolysis and photo fermentation, whereas, the amount of hydrogen per unit substrate produced appears insufficient for dark fermentation. This review focuses on innovative advances in the bioprocess research, genetic engineering and bioprocess technologies such as microbial fuel cell technology, in developing bio hydrogen production.

Improved itaconic acid production by Aspergillus niveus using blended algal biomass hydrolysate and glycerol as substrates

Superfluous algal biomass hydrolysate and purified glycerol obtained from biodiesel production were utilized for the production of itaconic acid by Aspergillus niveus. The lipid extracted Gracilaria edulis algal biomass residual was subjected to a pretreatment for the enhanced production of itaconic acid. Glycerol acquired from biodiesel production was pretreated and utilized as a substrate for itaconic acid production. The effect of individual and combined substrate concentration on itaconic acid production was investigated. Ultrasonication combined with the acid pretreated algal biomass produces higher itaconic acid due to the higher level of the total carbohydrate content (58.47 ± 2.57% w/v). After 168 h of incubation, A. niveus utilizes algal biomass hydrolysate and purified glycerol as substrate and produced 31.55 ± 1.25 g/L of itaconic acid and the dry cell weight is 18.24 ± 0.23 g/L respectively. Glycerol and algal biomass hydrolysate was a potential substrate for itaconic acid production by fungal species.

Latest development in microalgae-biofuel production with nano-additives

BACKGROUND

Microalgae have been experimented as a potential feedstock for biofuel generation in current era owing to its' rich energy content, inflated growth rate, inexpensive culture approaches, the notable capacity of CO₂ fixation, and O₂ addition to the environment. Currently, research is ongoing towards the advancement of microalgal-biofuel technologies. The nano-additive application has been appeared as a prominent innovation to meet this phenomenon.

MAIN TEXT

The main objective of this study was to delineate the synergistic impact of microalgal biofuel integrated with nano-additive applications. Numerous nano-additives such as nano-fibres, nano-particles, nano-tubes, nano-sheets, nano-droplets, and other nano-structures' applications have been reviewed in this study to facilitate microalgae growth to biofuel utilization. The present paper was intended to comprehensively review the nano-particles preparing techniques for microalgae cultivation and harvesting, biofuel extraction, and application of microalgae-biofuel nano-particles blends. Prospects of solid nano-additives and nano-fluid applications in the future on microalgae production, microalgae biomass conversion to biofuels as well as enhancement of biofuel combustion for revolutionary advancement in biofuel technology have been demonstrated elaborately by this review. This study also highlighted the potential biofuels from microalgae, numerous technologies, and conversion processes. Along with that, the study recounted suitability of potential microalgae candidates with an integrated design generating value-added co-products besides biofuel production.

CONCLUSIONS

Nano-additive applications at different stages from microalgae culture to end-product utilization presented strong possibility in mercantile approach as well as positive impact on the environment along with valuable co-products generation into the near future.

A Review of the Enhancement of Bio-Hydrogen Generation by Chemicals Addition

Bio-hydrogen production (BHP) produced from renewable bio-resources is an attractive route for green energy production, due to its compelling advantages of relative high efficiency, cost-effectiveness, and lower ecological impact. This study reviewed different BHP pathways, and the most important enzymes involved in these pathways, to identify technological gaps and effective approaches for process intensification in industrial applications. Among the various approaches reviewed in this study, a particular focus was set on the latest methods of chemicals/metal addition for improving hydrogen generation during dark fermentation (DF) processes; the up-to-date findings of different chemicals/metal addition methods have been quantitatively evaluated and thoroughly compared in this paper. A new efficiency evaluation criterion is also proposed, allowing different BHP processes to be compared with greater simplicity and validity.

Mexican Microalgae Biodiversity and State-Of-The-Art Extraction Strategies to Meet Sustainable Circular Economy Challenges: High-Value Compounds and Their Applied Perspectives

In recent years, the demand for naturally derived products has hiked with enormous pressure to propose or develop state-of-the-art strategies to meet sustainable circular economy challenges. Microalgae possess the flexibility to produce a variety of high-value products of industrial interests. From pigments such as phycobilins or lutein to phycotoxins and several polyunsaturated fatty acids (PUFAs), microalgae have the potential to become the primary producers for the pharmaceutical, food, and agronomical industries. Also, microalgae require minimal resources to grow due to their autotrophic nature or by consuming waste matter, while allowing for the extraction of several valuable side products such as hydrogen gas and biodiesel in a single process, following a biorefinery agenda. From a Mexican microalgae biodiversity perspective, more than 70 different local species have been characterized and isolated, whereas, only a minimal amount has been explored to produce commercially valuable products, thus ignoring their potential as a locally available resource. In this paper, we discuss the microalgae diversity present in Mexico with their current applications and potential, while expanding on their future applications in bioengineering along with other industrial sectors. In conclusion, the use of available microalgae to produce biochemically revenuable products currently represents an untapped potential that could lead to the solution of several problems through green technologies. As such, if the social, industrial and research communities collaborate to strive towards a greener economy by preserving the existing biodiversity and optimizing the use of the currently available resources, the enrichment of our society and the solution to several environmental problems could be attained.

Enhancing Hydrogen Production from Chlorella sp. Biomass by Pre-Hydrolysis with Simultaneous Saccharification and Fermentation (PSSF)

Simultaneous saccharification and fermentation (SSF) and pre-hydrolysis with SSF (PSSF) were used to produce hydrogen from the biomass of Chlorella sp. SSF was conducted using an enzyme mixture consisting of 80 filter paper unit (FPU) g-biomass−1 of cellulase, 92 U g-biomass−1 of amylase, and 120 U g-biomass−1 of glucoamylase at 35 °C for 108 h. This yielded 170 mL-H2 g-volatile-solids−1 (VS), with a productivity of 1.6 mL-H2 g-VS−1 h−1. Pre-hydrolyzing the biomass at 50 °C for 12 h resulted in the production of 1.8 g/L of reducing sugars, leading to a hydrogen yield (HY) of 172 mL-H2 g-VS−1. Using PSSF, the fermentation time was shortened by 36 h in which a productivity of 2.4 mL-H2 g-VS−1 h−1 was attained. To the best of our knowledge, the present study is the first report on the use of SSF and PSSF for hydrogen production from microalgal biomass, and the HY obtained in the study is by far the highest yield reported. Our results indicate that PSSF is a promising process for hydrogen production from microalgal biomass.

Gemini surfactant: A novel flotation collector for harvesting of microalgae by froth flotation

Froth flotation has been proved to be a promising approach for commercial scale harvesting of microalgae. However, all the surfactants used in the microalgae flotation harvesting process are conventional monomeric surfactants contain a single similar hydrophobic group in the molecule, which results in a low harvesting efficiency. In this work, a novel Gemini surfactant, N,N'-bis(cetyldimethyl)-1,4-butane diammonium dibromide (BCBD) was prepared, and originally recommended as a collector for froth flotation harvesting of Chlorella vulgaris from culture medium. The performance of BCBD was compared with the results acquired using its conventional monomeric surfactant cetyl trimethyl ammonium bromide (CTAB). The bench-scale flotation results showed that BCBD had excellent collecting power for Chlorella vulgaris. Achieving the obviously superior flotation harvesting performance (flotation recovery increased by 21.4% and enrichment ratio increased by 22.9), the dosage of Gemini type BCBD collector is five times less than that of monomeric CTAB collector.

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.

Enhanced biomass and lipid accumulation of mixotrophic microalgae by using low-strength ultrasonic stimulation

Ultrasonic treatment was applied to enhance the biomass and lipid accumulation of mixotrophic microalgae. The optimal microalgal ultrasonic power, ultrasonic frequency, ultrasonic interval and growth phase were 20 W, 20 Hz, 2 s and logarithmic phase, respectively. The maximum biomass concentration and lipid content reached 2.78 g L^-1 and 28.5%, which were 26.9% and 37% higher than those of the control group. Microscope analysis shows that ultrasonic exposure caused tiny cracks or holes on the surface of cell walls, but did not damage the integrity of algal cell structure. After ultrasonic stimulation, the permeability of membrane and the transport of nutrients were improved, and the utilization rate of substrate and pigment concentration increased 22.7% and 18.4%, respectively. However, excessive ultrasonic irradiation significantly inhibited the cell growth and lipid accumulation of microalgae. This study indicates the feasibility and efficiency of using low-strength ultrasound in promoting biomass and lipid production of microalgae.

Electron balancing under different sink conditions reveals positive effects on photon efficiency and metabolic activity of Synechocystis sp. PCC 6803

BACKGROUND

Cyanobacteria are ideal model organisms to exploit photosynthetically derived electrons or fixed carbon for the biotechnological synthesis of high value compounds and energy carriers. Much effort is spent on the rational design of heterologous pathways to produce value-added chemicals. Much less focus is drawn on the basic physiological responses and potentials of phototrophs to deal with natural or artificial electron and carbon sinks. However, an understanding of how electron sinks influence or regulate cellular physiology is essential for the efficient application of phototrophic organisms in an industrial setting, i.e., to achieve high productivities and product yields.

RESULTS

The physiological responses of the cyanobacterium Synechocystis sp. PCC 6803 to electron sink variation were investigated in a systematic and quantitative manner. A variation in electron demand was achieved by providing two N sources with different degrees of reduction. By additionally varying light and CO₂ availabilities, steady state conditions with strongly differing source-sink ratios were established. Balancing absorbed photons and electrons used for different metabolic processes revealed physiological responses to sink/source ratio variation. Surprisingly, an additional electron sink under light and thus energy limitation was found not to hamper growth, but was compensated by improved photosynthetic efficiency and activity. In the absence of carbon and light limitation, an increase in electron demand even stimulated carbon assimilation and growth.

CONCLUSION

The metabolism of Synechocystis sp. PCC 6803 is highly flexible regarding the compensation of additional electron demands. Under light limitation, photosynthesis obviously does not necessarily run at its maximal capacity, possibly for the sake of robustness. Increased electron demands can even boost photosynthetic activity and growth.

A review of the innovative gas separation membrane bioreactor with mechanisms for integrated production and purification of biohydrogen

This review article focuses on an assessment of the innovative Gas Separation Membrane Bioreactor (GS-MBR), which is an emerging technology because of its potential for in-situ biohydrogen production and separation. The GS-MBR, as a special membrane bioreactor, enriches CO₂ directly from the headspace of the anaerobic H₂ fermentation process. CO₂ can be fed as a substrate to auxiliary photo-bioreactors to grow microalgae as a promising raw material for biocatalyzed, dark fermentative H₂-evolution. Overall, these features make the GS-MBR worthy of study. To the best of the authors' knowledge, the GS-MBR has not been studied in detail to date; hence, a comprehensive review of this topic will be useful to the scientific community.

A facile synthesis of cysteine-based diketopiperazine from thiol-protected precursor

l-Cysteine is one of the most promising biomass-based building blocks with great potential applications. Herein, we report a versatile synthetic route to produce cysteine-based 2,5-diketopiperazine (DKP) with good yield from the thiol-ene click reaction of l-cysteine and commercially available acrylates, followed by dimerization of the amino acid intermediates. The achieved DKP diastereomers were successfully separated and fully characterized by spectroscopic methods. Moreover, the chiroptical property of DKP in the presence of various metal ions was investigated by circular dichroism spectroscopy. The potential application of the optically active cysteine-based DKP as a chiral probe for detection of silver ion in water has been demonstrated.

Sequential dark and photo fermentation hydrogen production from hydrolyzed corn stover: A pilot test using 11 m3 reactor

Pilot tests of sequential dark and photo fermentation H₂ production were for the first time conducted in a 11 m³ reactor (3 m³ for dark and 8 m³ for photo compartments). A combined solar and light-emitting diode illumination system and a thermal controlling system was installed and tested. With dark fermentation unit maintained at pH 4.5 and 35 °C and photo fermentation unit at pH 7.0 and 30 °C, the overall biogas production rate using hydrolyzed corn stover as substrate reached 87.8 ± 3.8 m³/d with 68% H₂ content, contributed by dark unit at 7.5 m³-H₂/m³-d and by photo unit at 4.7 m³/m³-d. Large variation was noted for H₂ production rate in different compartments of the tested units, revealing the adverse effects of poor mixing, washout, and other inhomogeneity associated with large reactor operations.

A novel approach of modeling continuous dark hydrogen fermentation

In this study a novel modeling approach for describing fermentative hydrogen production in a continuous stirred tank reactor (CSTR) was developed, using the Aquasim modeling platform. This model accounts for the key metabolic reactions taking place in a fermentative hydrogen producing reactor, using fixed stoichiometry but different reaction rates. Biomass yields are determined based on bioenergetics. The model is capable of describing very well the variation in the distribution of metabolic products for a wide range of hydraulic retention times (HRT). The modeling approach is demonstrated using the experimental data obtained from a CSTR, fed with food industry waste (FIW), operating at different HRTs. The kinetic parameters were estimated through fitting to the experimental results. Hydrogen and total biogas production rates were predicted very well by the model, validating the basic assumptions regarding the implicated stoichiometric biochemical reactions and their kinetic rates.

Microalgal hydrogen production - A review

Bio-hydrogen from microalgae including cyanobacteria has attracted commercial awareness due to its potential as an alternative, reliable and renewable energy source. Photosynthetic hydrogen production from microalgae can be interesting and promising options for clean energy. Advances in hydrogen-fuel-cell technology may attest an eco-friendly way of biofuel production, since, the use of H₂ to generate electricity releases only water as a by-product. Progress in genetic/metabolic engineering may significantly enhance the photobiological hydrogen production from microalgae. Manipulation of competing metabolic pathways by modulating the certain key enzymes such as hydrogenase and nitrogenase may enhance the evolution of H₂ from photoautotrophic cells. Moreover, biological H₂ production at low operating costs is requisite for economic viability. Several photobioreactors have been developed for large-scale biomass and hydrogen production. This review highlights the recent technological progress, enzymes involved and genetic as well as metabolic engineering approaches towards sustainable hydrogen production from microalgae.

Synechococcus nidulans from a thermoelectric coal power plant as a potential CO2 mitigation in culture medium containing flue gas wastes

This study evaluated the intermittent addition of coal flue gas wastes (CO₂, SO₂, NO and ash) into a Synechococcus nidulans LEB 115 cultivation in terms of growth parameters, CO₂ biofixation and biomass characterization. The microalga from a coal thermoelectric plant showed tolerance up to 200ppm SO₂ and NO, with a maximum specific growth rate of 0.18±0.03d^-1. The addition of thermal coal ash to the cultivation increased the Synechococcus nidulans LEB 115 maximum cell growth by approximately 1.3 times. The best CO₂ biofixation efficiency was obtained with 10% CO₂, 60ppm SO₂, 100ppm NO and 40ppm ash (55.0±3.1%). The biomass compositions in the assays were similar, with approximately 9.8% carbohydrates, 13.5% lipids and 62.7% proteins.