Photobiological hydrogen production: Recent advances and state of the art

Ammonia-dependent reducing power redistribution for purple phototrophic bacteria culture-based biohydrogen production

The global energy crisis has intensified the search for sustainable and clean alternatives, with biohydrogen emerging as a promising solution to address environmental challenges. Leveraging photo fermentation (PF) process, purple phototrophic bacteria (PPB) can harness reducing power derived from organic substrates to facilitate hydrogen production. However, existing studies report much lower H2 yields than theoretical value when using acetate as carbon source and ammonia as nitrogen source, primarily attributed to the widely employed pulse-feeding mode which suffers from ammonia inhibition effect on nitrogenase. To address this issue, a continuous feeding mode was applied to avoid ammonia accumulation in this study. On the other hand, other pathways like carbon fixation and polyhydroxyalkanoate (PHA) formation could compete reducing power with H2 production. However, the reducing power allocation under continuous feeding mode is not yet clear. In this study, the reducing power allocation and hydrogen production performance were evaluated under various ammonia loading, using acetate as carbon source and infrared LED at around 50 W·m-2 as light source. The results show that (a) The absence of ammonia resulted in the best performance for hydrogen production, with 44 % of the reducing power distributed to H2 and the highest H2 volumetric productivity, while the allocation of reducing power to hydrogen production stopped when ammonia loading was above 7.6 mg NH4-N·L-1·d-1; (b) when PPB required to eliminate reducing power under ammonia limited conditions, PHA production was the preferred pathway followed by the hydrogen production pathway, but once PHA accumulation reached saturation, hydrogen generation pathway dominated; (c) under ammonia limited conditions, the TCA cycle was more activated rendering higher NADH (i.e. reducing power) production compared with that under ammonia sufficient conditions which was verified by metagenomics analysis, and all the hydrogen production, PHA accumulation and carbon fixation pathways were highly active to dissipate reducing power. This work provides the insight of reducing power distribution and PPB biohydrogen production variated by ammonia loading under continuous feeding mode.

Towards industrial biological hydrogen production: a review

Increased production of renewable energy sources is becoming increasingly needed. Amidst other strategies, one promising technology that could help achieve this goal is biological hydrogen production. This technology uses micro-organisms to convert organic matter into hydrogen gas, a clean and versatile fuel that can be used in a wide range of applications. While biohydrogen production is in its early stages, several challenges must be addressed for biological hydrogen production to become a viable commercial solution. From an experimental perspective, the need to improve the efficiency of hydrogen production, the optimization strategy of the microbial consortia, and the reduction in costs associated with the process is still required. From a scale-up perspective, novel strategies (such as modelling and experimental validation) need to be discussed to facilitate this hydrogen production process. Hence, this review considers hydrogen production, not within the framework of a particular production method or technique, but rather outlines the work (bioreactor modes and configurations, modelling, and techno-economic and life cycle assessment) that has been done in the field as a whole. This type of analysis allows for the abstraction of the biohydrogen production technology industrially, giving insights into novel applications, cross-pollination of separate lines of inquiry, and giving a reference point for researchers and industrial developers in the field of biohydrogen production.

Realization process of microalgal biorefinery: The optional approach toward carbon net-zero emission

Increasing carbon dioxide (CO2) emission has already become a dire threat to the human race and Earth's ecology. Microalgae are recommended to be engineered as CO2 fixers in biorefinery, which play crucial roles in responding climate change and accelerating the transition to a sustainable future. This review sorted through each segment of microalgal biorefinery to explore the potential for its practical implementation and commercialization, offering valuable insights into research trends and identifies challenges that needed to be addressed in the development process. Firstly, the known mechanisms of microalgal photosynthetic CO2 fixation and the approaches for strain improvement were summarized. The significance of process regulation for strengthening fixation efficiency and augmenting competitiveness was emphasized, with a specific focus on CO2 and light optimization strategies. Thereafter, the massive potential of microalgal refineries for various bioresource production was discussed in detail, and the integration with contaminant reclamation was mentioned for economic and ecological benefits. Subsequently, economic and environmental impacts of microalgal biorefinery were evaluated via life cycle assessment (LCA) and techno-economic analysis (TEA) to lit up commercial feasibility. Finally, the current obstacles and future perspectives were discussed objectively to offer an impartial reference for future researchers and investors.

A review on biological biohydrogen production: Outlook on genetic strain enhancements, reactor model and techno-economics analysis

Modernisation of industrial and transportation sector would have not imaginable without the help of fossil fuels, but constant usage has led to many environmental concerns. As a step forward, for safer next generation living we are forced to look into green fuels like bio‑hydrogen and higher alcohols. This review mainly focuses on bio‑hydrogen production via biological pathways, genetic improvements, knowledge gap, economics, and future directions. Dark and photo fermentation process with the factor influence the process (pH regulation, temperature, hydraulic retention time, organic loading rate, Maintenance, Nutrient) is studied. Integration of dark fermentation and microbial electrolysis cell is the most trending progression for sustainable bio‑hydrogen production. Genetic improvement of microbe for biohydrogen production via inactivation of hydrogenase (H2ase) and improve oxygen tolerant H2ase. In future, bioaugmentation, multidisciplinary integrated process and microbial electrolysis needs to be experimented in industrial level scale for successful commercialization. About 41.47 mmol H2/g DCW h at 40 g/L of optimum biohydrogen production was obtained through glycerol fermentation. From the studies, the cost of biohydrogen production was found to high with respect to the direct bio photolysis it cost around $7.24 kg-1; for indirect bio photolysis it cost around $7.54 kg-1 and for fermentation it cost around $7.61 kg-1.

Prediction of radiative properties of spherical microalgae considering internal heterogeneity and optical constants of various components

Most of the current predictions of the radiative properties of microalgae use the homogeneous sphere approximation based on the Mie scattering theory, and the refractive indices of the model were regarded as fixed values. Using the recently measured optical constants of various microalgae components, we propose a spherical heterogeneous model for spherical microalgae. The optical constants of the heterogeneous model were characterized by the measured optical constants of microalgae components for the first time. The radiative properties of the heterogeneous sphere were calculated using the T-matrix method and were well verified by measurements. It shows that the internal microstructure has a more significant effect on scattering cross-section and scattering phase function than absorption cross-section. Compared with the traditional homogeneous models selected with fixed values as refractive index, the calculation accuracy of scattering cross-section of the heterogeneous model improved by 15%-150%. The scattering phase function of the heterogeneous sphere approximation agreed better with measurements than the homogeneous models due to the more detailed description of the internal microstructure. It can be concluded that considering the internal microstructure of microalgae and characterizing the microstructure of the model by the optical constants of the microalgae components helps to reduce the error caused by the simplification of the actual cell.

Intensification of Hydrogen Production by a Co-culture of Syntrophomonas wolfei and Rhodopseudomonas palustris Employing High Concentrations of Butyrate as a Substrate

The purpose of this study is to present an effective form of developing a sequential dark (DF) and photo (PF) fermentation using volatile fatty acids (VFAs) and nitrogen compounds as bonding components between both metabolic networks of microbial growing in each fermentation. A simultaneous (co-)culture of Syntrophomonas wolfei (with its ability to consume butyrate and produce acetate) and Rhodopseudomonas palustris (that can use the produced acetate as a carbon source) performed a syntrophic metabolism. The former bacteria consumed the acetate/butyrate mixture reducing the butyrate concentration below 2.0 g/L, permitting Rhodopseudomonas palustris to produce hydrogen. Considering that the inoculum composition (Syntrophomonas wolfei/Rhodopseudomonas palustris) and the nitrogen source (yeast extract) define the microbial biomass specific productivity and the butyrate consumption, a response surface methodology defined the best inoculum design and yeast extract (YE) yielding to the highest biomass concentration of 1.1 g/L after 380.00 h. A second culture process (without a nitrogen source) showed the biomass produced in the previous culture process yields to produce a total cumulated hydrogen concentration of 3.4 mmol. This value was not obtained previously with the pure strain Rhodopseudomonas palustris if the culture medium contained butyrate concentration above 2.0 g/L, representing a contribution to the sequential fermentation scheme based on DF and PF.

Enhancement of static magnetic field on biological hydrogen production via photo-fermentation of giant reed

The effect of static magnetic field (SMF) on the system of photo-fermentation biological hydrogen production remains dimness. The goal of this study was to clarify the correlation between external SMF addition and hydrogen production via photo-fermentation from giant reed. SMF with 20 mT improved the cumulative H₂ yield by 26.1% and reduced the lag time of hydrogen production by 56.7% compared with that of without external magnetic field. Moreover, 20 mT of SMF not only enhanced the activity of nitrogenase by 94.52%, but also obtained the maximum energy conversion efficiency of 27.27%. The distribution of volatile fatty acids proved that the concentration of acetic acid and butyric acid were 137% and 81% higher than that of without SMF, respectively. The results would help to trigger the positive interaction between SMF and microorganism and to avoid the possible negative interaction.

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.

Advancement of renewable energy technologies via artificial and microalgae photosynthesis

There has been an urgent need to tackle global climate change and replace conventional fuels with alternatives from sustainable sources. This has led to the emergence of bioenergy sources like biofuels and biohydrogen extracted from microalgae biomass. Microalgae takes up carbon dioxide and absorbs sunlight, as part of its photosynthesis process, for growth and producing useful compounds for renewable energy. While, the developments in artificial photosynthesis to a chemical process that biomimics the natural photosynthesis process to fix CO₂ in the air. However, the artificial photosynthesis technology is still being investigated for its implementation in large scale production. Microalgae photosynthesis can provide the same advantages as artificial photosynthesis, along with the prospect of having final microalgae products suitable for various application. There are significant potential to adapt either microalgae photosynthesis or artificial photosynthesis to reduce the CO₂ in the climate and contribute to a cleaner and green cultivation method.

Research progress of additives in photobiological hydrogen production system to enhance biohydrogen

Photosynthetic biohydrogen has the advantages of extensive raw materials, clean and renewable, etc. But, its low substrate utilization rate limit its commercial application. It is reported that the use of additives in the process of biohydrogen by photofermentation is beneficial to increase biohydrogen. However, in practical application, the mechanism of additives in hydrogen production is not understood. This paper, the promotion effect of some additives on biohydrogen by photofermentation was reviewed. Whatever, the existing problems and development trends of various additives are also discussed. It is necessary to select appropriate additives according to the hydrogen-producing characteristics. The use of composite additives may further enhance biohydrogen, but the specific situation needs further exploration. The research results of this paper can help readers to further understand the role of additives in the crouse of photofermentative biohydrogen, provide reference for the research of photofermentative biohydrogen.

Prospects and environmental sustainability of phyconanotechnology: A review on algae-mediated metal nanoparticles synthesis and mechanism

In recent years, researchers have proven that the employment of natural green components in the biogenesis of nanoparticles from microalgae species is one of the ways to delight the global environment issues. The application of nanotechnology with the exploitation of phycochemical produced from algae species is known as 'phyconanotechnology'. The use of biological compounds by microalgae as reducing agents for the synthesis of inorganic nanoparticles has shown promising results such as cost-effective and environmentally friendly. Different classifications of algae such as brown algae, red algae, green algae, and cyanobacteria are studied for the synthesis of different types of metal nanoparticles. It is also an important motive to acknowledge the mechanisms of the microalgae-mediated biosynthesis of nanoparticles via an intracellular pathway or extracellular pathway. Besides, microalgae species as biogenic sources preclude the use of conventional methods reagents, such as sodium borohydride (NaBH₄) and N,N-dimethylformamide (DMF), which further consolidates their position as the best choice for sustainable (economically and environmentally) nanoparticle synthesis compared to the conventional nanoparticles synthesis pathway.

Simazine Enhances Dark Fermentative H2 Production by Unicellular Halotolerant Cyanobacterium Aphanothece halophytica

The halotolerant cyanobacterium Aphanothece halophytica is a potential H₂ producer that induces H₂ evolution under nitrogen deprivation. H₂ is mainly produced via the catabolism of stored glycogen under dark anaerobic condition. H₂ evolution is catalyzed by O₂-sensitive bidirectional hydrogenase. The aim of this study was to improve H₂ production by A. halophytica using various kinds of inhibitors. Among all types of inhibitors, simazine efficiently promoted the highest H₂ production under dark conditions. High simazine concentration and long-term incubation resulted in a decrease in cell and chlorophyll concentrations. The optimal simazine concentration for H₂ production by A. halophytica was 25 µM. Simazine inhibited photosynthetic O₂ evolution but promoted dark respiration, resulting in a decrease in O₂ level. Hence, the bidirectional hydrogenase activity and H₂ production was increased. A. halophytica showed the highest H₂ production rate at 58.88 ± 0.22 µmol H₂ g⁻¹ dry weight h⁻¹ and H₂ accumulation at 356.21 ± 6.04 μmol H₂ g⁻¹ dry weight after treatment with 25 µM simazine under dark anaerobic condition for 2 and 24 h, respectively. This study demonstrates the potential of simazine for the enhancement of dark fermentative H₂ production by A. halophytica.

Microalgae-bacterial granular consortium: Striding towards sustainable production of biohydrogen coupled with wastewater treatment

Anthropogenic activities have drastically affected the environment, leading to increased waste accumulation in atmospheric bodies, including water. Wastewater treatment is an energy-consuming process and typically requires thousands of kilowatt hours of energy. This enormous energy demand can be fulfilled by utilizing the microbial electrolysis route to breakdown organic pollutants in wastewater which produces clean water and biohydrogen as a by-product of the reaction. Microalgae are the promising microorganism for the biohydrogen production, and it has been investigated that the interaction between microalgae and bacteria can be used to boost the yield of biohydrogen. Consortium of algae and bacteria resulting around 50-60% more biohydrogen production compared to the biohydrogen production of algae and bacteria separately. This review summarises the recent development in different microalgae-bacteria granular consortium systems successfully employed for biohydrogen generation. We also discuss the limitations in biohydrogen production and factors affecting its production from wastewater.

Valorisation of algal biomass to value-added metabolites: emerging trends and opportunities

Algal biomass is a promising feedstock for sustainable production of a range of value-added compounds and products including food, feed, fuel. To further augment the commercial value of algal metabolites, efficient valorization methods and biorefining channels are essential. Algal extracts are ideal sources of biotechnologically viable compounds loaded with anti-microbial, anti-oxidative, anti-inflammatory, anti-cancerous and several therapeutic and restorative properties. Emerging technologies in biomass valorisation tend to reduce the significant cost burden in large scale operations precisely associated with the pre-treatment, downstream processing and waste management processes. In order to enhance the economic feasibility of algal products in the global market, comprehensive extraction of multi-algal product biorefinery is envisaged as an assuring strategy. Algal biorefinery has inspired the technologists with novel prospectives especially in waste recovery, carbon concentration/sequestration and complete utilisation of the value-added products in a sustainable closed-loop methodology. This review critically examines the latest trends in the algal biomass valorisation and the expansive feedstock potentials in a biorefinery perspective. The recent scope dynamics of algal biomass utilisation such as bio-surfactants, oleochemicals, bio-stimulants and carbon mitigation have also been discussed. The existing challenges in algal biomass valorisation, current knowledge gaps and bottlenecks towards commercialisation of algal technologies are discussed. This review is a comprehensive presentation of the road map of algal biomass valorisation techniques towards biorefinery technology. The global market view of the algal products, future research directions and emerging opportunities are reviewed.

Recent advances, current issues and future prospects of bioenergy production: A review

With the immense potential of bioenergy to drive carbon neutrality and achieve the climate targets of the Paris Agreement, this paper aims to present the recent advances in bioenergy production as well as their limitations. The novelty of this review is that it covers a comprehensive range of strategies in bioenergy production and it provides the future prospects for improvement. This paper reviewed more than 200 peer-reviewed scholarly papers mainly published between 2010 and 2021. Bioenergy is derived from biomass, which, through thermochemical and biochemical processes, is converted into various forms of biofuels. This paper reveals that bioenergy production is temperature-dependent and thermochemical processes currently have the advantage of higher efficiency over biochemical processes in terms of lower response time and higher conversion. However, biochemical processes produce more volatile organic compounds and have lower energy and temperature requirements. The combination of the two processes could fill the shortcomings of a single process. The choices of feedstock are diverse as well. In the future, it can be anticipated that continuous technological development to enhance the commercial viability of different processes, as well as approaches of ensuring their sustainability, will be among the main aspects to be studied in greater detail.

Significance of Hydrogen as Economic and Environmentally Friendly Fuel

The major demand of energy in today’s world is fulfilled by the fossil fuels which are not renewable in nature and can no longer be used once exhausted. In the beginning of the 21st century, the limitation of the fossil fuels, continually growing energy demand, and growing impact of green-house gas emissions on the environment were identified as the major challenges with current energy infrastructure all over the world. The energy obtained from fossil fuel is cheap due to its established infrastructure; however, these possess serious issues, as mentioned above, and cause bad environmental impact. Therefore, renewable energy resources are looked to as contenders which may fulfil most energy requirements. Among them, hydrogen is considered as the most environmentally friendly fuel. Hydrogen is clean, sustainable fuel and it has promise as a future energy carrier. It also has the ability to substitute the present energy infrastructure which is based on fossil fuel. This is seen and projected as a solution for the above-mentioned problems including rise in global temperature and environmental degradation. Environmental and economic aspects are the important factors to be considered to establish hydrogen infrastructure. This article describes the various aspects of hydrogen including production, storage, and applications with a focus on fuel cell based electric vehicles. Their environmental as well as economic aspects are also discussed herein.

Enhancement strategies for photo-fermentative biohydrogen production: A review

Biohydrogen production by photo fermentation is an attractive clean energy production approach with less environmental pollution and higher substrate conversion. In recent years, various measures have been used to improve biohydrogen production performance, but there is a lack of systematic and comprehensive summary and analysis. Hence, the recent literatures on enhancing biohydrogen production by photo fermentation were summarized, and the functional mechanisms of enhancement strategies were explained. In this work, these measures were divided into four categories according to their roles in photo fermentation, including substrate pretreatment, bacterial modification and immobilization, additive addition, reactor design optimization. It can be concluded that the optimal enhancement conditions of each strategy were affected by substrate type, strain and process parameters. According to the results of this work, it was expected to give readers a clear understanding and provide a scientific reference of the research of photosynthetic biohydrogen production.

Harnessing Solar Energy using Phototrophic Microorganisms: A Sustainable Pathway to Bioenergy, Biomaterials, and Environmental Solutions

Phototrophic microorganisms (microbial phototrophs) use light as an energy source to carry out various metabolic processes producing biomaterials and bioenergy and supporting their own growth. Among them, microalgae and cyanobacteria have been utilized extensively for bioenergy, biomaterials, and environmental applications. Their superior photosynthetic efficiency, lipid content, and shorter cultivation time compared to terrestrial biomass make them more suitable for efficient production of bioenergy and biomaterials. Other phototrophic microorganisms, especially anoxygenic phototrophs, demonstrated the ability to survive and flourish while producing renewable energy and high-value products under harsh environmental conditions. This review presents a comprehensive overview of microbial phototrophs on their (i) production of bioenergy and biomaterials, (ii) emerging and innovative applications for environmental conservation, mitigation, and remediation, and (iii) physical, genetic, and metabolic pathways to improve light harvesting and biomass/biofuel/biomaterial production. Both physical (e.g., incremental irradiation) and genetic approaches (e.g., truncated antenna) are implemented to increase the light-harvesting efficiency. Increases in biomass yield and metabolic products are possible through the manipulation of metabolic pathways and selection of a proper strain under optimal cultivation conditions and downstream processing, including harvesting, extraction, and purification. Finally, the current barriers in harnessing solar energy using phototrophic microorganisms are presented, and future research perspectives are discussed, such as integrating phototrophic microorganisms with emerging technologies.

Reliable determination of the growth and hydrogen production parameters of the photosynthetic bacterium Rhodobacter capsulatus in fed batch culture using a combination of the Gompertz function and the Luedeking-Piret model

In this study, experimental results of hydrogen producing process based on anaerobic photosynthesis using the purple non-sulfur bacterium Rhodobacter capsulatus are scrutinized. The bacterial culture was carried out in a photo-bioreactor operated in a quasi-continuous mode, using lactate as a carbon source. The method is based on the continuous stirred tank reactors (CSTR) technique to access kinetic parameters. The dynamic evolution of hydrogen production as a function of time was accurately simulated using Luedeking-Piret model and the growth of R. capsulatus was computed using Gompertz model. The combination of both models was successfully applied to determine the relevant parameters (λ, μ_max, α and β) for two R. capsulatus strains studied: the wild-type strain B10 and the H₂ over-producing mutant IR3. The mathematical description indicates that the photofermentation is more promising than dark fermentation for the conversion of organic substrates into biogas.

Photofermentative biohydrogen generation from organic acids by Rhodobacter sphaeroides O.U.001: Computational fluid dynamics modeling of hydrodynamics and temperature

Hydrogen gas is a clean-burning fuel suitable for powering public vehicles. Hydrogen fuel has the highest energy density (143 MJ kg^-1 ). This research paper emphasizes three-dimensional hydrodynamics and temperature distribution during photobiohydrogen generation by Rhodobacter sphaeroides strain O.U.001 in a triple-jacketed 1 L photobioreactor (PBR). The fermentation broth has turbulent flow conditions and light gradients among various layers, which affect the light conversion efficiency of the PBR. From the carbon source (malic acid), various organic acids are produced within fermentation (lactate, acetate, and formate). Modeling and simulation studies by computational fluid dynamics confirmed uniform fluid dynamics and heat transfer throughout the annular PBR. The modified Gompertz equation gave good simulated fitting with an experimental value for H₂ generation. R. sphaeroides O.U. 001 gave good simulated results for H₂ generation with mathematical modeling of substrate consumption kinetics and substrate utilization for biomass.

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.

Isolation of Industrial Important Bioactive Compounds from Microalgae

Microalgae are known as a rich source of bioactive compounds which exhibit different biological activities. Increased demand for sustainable biomass for production of important bioactive components with various potential especially therapeutic applications has resulted in noticeable interest in algae. Utilisation of microalgae in multiple scopes has been growing in various industries ranging from harnessing renewable energy to exploitation of high-value products. The focuses of this review are on production and the use of value-added components obtained from microalgae with current and potential application in the pharmaceutical, nutraceutical, cosmeceutical, energy and agri-food industries, as well as for bioremediation. Moreover, this work discusses the advantage, potential new beneficial strains, applications, limitations, research gaps and future prospect of microalgae in industry.

Metabolic pathways for microalgal biohydrogen production: Current progress and future prospectives

Microalgal biohydrogen (bioH₂) has attracted global interest owing to its potential carbon-free source of sustainable renewable energy. Most of previous reviews which focused on microalgal bioH₂, have shown unclear differentiation among the metabolic pathways. In this review, investigation of all different metabolic pathways for microalgal bioH₂ production along with discussion on the recent research work of last 5-years have been considered. The major factors (such as light, vital nutrients, microalgal cell density, and substrate bioavailability) are highlighted. Moreover, effect of various pretreatment approaches on the constituent's bioaccessibility is reported. Microbial electrolysis cells as a new strategy for bioH₂ production is stated. Comparison between the operation conditions of various bioreactors and economic feasibility is also emphasized. Genetic, metabolic engineering, and synthetic biology as recent technologies improved the microalgal bioH₂ production through inactivation of uptake hydrogenase (H₂ase), inhibition of the competing pathways in polysaccharide synthesis, and improving the O₂ tolerant H₂ase.

Green technology for sustainable biohydrogen production (waste to energy): A review

Perceiving and detecting a sustainable source of energy is very critical issue for current modern society. Hydrogen on combustion releases energy and water as a byproduct and has been considered as an environmental pollution free energy carrier. From the last decade, most of the researchers have recommended hydrogen as one of the cleanest fuels and its demand is rising ever since. Hydrogen having the highest energy density is more advantageous than any other fuel. Hydrogen obtained from the fossil fuels produces carbon dioxide as a byproduct and creates environment negative effect. Therefore, biohydrogen production from green algae and cyanobacteria is an attractive option that generates a benign renewable energy carrier. Microalgal feedstocks show a high potential for the generation of fuel such as biohydrogen, bioethanol and biodiesel. This article has reviewed the different methods of biohydrogen production while also trying to find out the most economical and ecofriendly method for its production. A thorough review process has been carried out to study the methods, enzymes involved, factors affecting the rate of hydrogen production, dual nature of algae, challenges and commercialization potential of algal biohydrogen.

Energy conservation in photosynthetic microorganisms

Photosynthesis is a biological process of energy conversion from solar radiation to useful organic compounds for the photosynthetic organisms themselves. It, thereby, also plays a role of food production for almost all animals on the Earth. The utilization of photosynthesis as an artificial carbon cycle is also attracting a lot of attention regarding its benefits for human life. Hydrogen and biofuels, obtained from photosynthetic microorganisms, such as microalgae and cyanobacteria, will be promising products as energy and material resources. Considering that the efficiency of bioenergy production is insufficient to replace fossil fuels at present, techniques for the industrial utilization of photosynthesis processes need to be developed intensively. Increase in the efficiency of photosynthesis, the yields of target substances, and the growth rates of algae and cyanobacteria must be subjects for efficient industrialization. Here, we overview the whole aspect of the energy production from photosynthesis to biomass production of various photosynthetic microorganisms.

Rectification of twitching bacteria through narrow channels: A numerical simulations study

Bacteria living on surfaces use different types of motility mechanisms to move on the surface in search of food or to form microcolonies. Twitching is one such form of motility employed by bacteria such as Neisseria gonorrhoeae, in which the polymeric extensions known as type IV pili mediate its movement. Pili extending from the cell body adhere to the surface and pull the bacteria by retraction. The bacterial movement is decided by the two-dimensional tug-of-war among the pili attached to the surface. Natural surfaces on which these microcrawlers dwell are generally spatially inhomogeneous and have varying surface properties. Their motility is known to be affected by the topography of the surfaces. Therefore, it is possible to control bacterial movement by designing structured surfaces which can be potentially utilized for controlling biofilm architecture. In this paper, we numerically investigate the twitching motility in a two-dimensional corrugated channel. The bacterial movement is simulated by two different models: (a) a detailed tug-of-war model which extensively describe the twitching motility of bacteria assisted by pili and (b) a coarse-grained run-and-tumble model which depicts the motion of wide-ranging self-propelled particles. The simulation of bacterial motion through asymmetric corrugated channels using the above models show rectification. The bacterial transport depends on the geometric parameters of the channel and inherent system parameters such as persistence length and self-propelled velocity. In particular, the variation of the particle current with the geometric parameters of the microchannels shows that one can optimize the particle current for specific values of these parameters.

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.

The influence of hydrofluoric acid etching processes on the photocatalytic hydrogen evolution reaction using mesoporous silicon nanoparticles

H2 has been identified as one of the potential energy vectors that can provide a sustainable energy supply when produced through solar-driven water-splitting reaction. Si is the second most abundant element in the Earth's crust and can absorb a significant fraction of the solar spectrum while presenting little toxicity risk, making it an attractive material for photocatalytic H2 production. Hydrogen-terminated mesoporous Si (mp-Si) nanoparticles can be utilized to effectively drive the hydrogen evolution reaction using UV-to-visible light. In this work, the response of the photocatalytic activity of mp-Si nanoparticles to a series of HF acid treatments was investigated. A two-step magnesiothermic reduction method was used to prepare crystalline mp-Si nanoparticles with a specific surface area of 573 m2 g-1. The HF etching process was optimized as a function of the amount of acid added and the reaction time. The reaction time did not influence the H2 evolution rate substantially. However, the amount of HF used did have a significant effect on the photocatalytic activity. In the presence of ≥1.0 mL HF acid per 0.010 g of Si, morphological damage was observed using electron microscopy. N2 adsorption measurements indicated that the pore size and surface area were also altered. Solution-phase 19F{1H} NMR studies indicated the formation of SiF5- and SiF62- when larger volumes of HF were used. Both factors, morphological damage and the presence of byproducts in the pores, likely result in a lowering of the photocatalytic H2 evolution rate. Under the optimized HF treatment conditions (0.5 mL of HF per 0.010 g of Si), a H2 evolution rate of 1398 ± 30 μmol g-1 h-1 was observed.

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.

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.

Anoxic growth optimization for metal respiration and photobiological hydrogen production by arsenic-resistant Rhodopseudomonas and Rhodobacter species

The current research focuses on anaerobic respiration of arsenic and other toxic metals by purple nonsulfur bacteria (PNSB). Among the optimization assays performed were carbon utilization, cross metal resistance, and metal respiration, along with a comparison of each assay in photoheterotrophic and chemoheterotrophic growth. The bacteria were identified by the classification of 16S ribosomal RNA gene sequences. Rhodobacter sp. PI3 proved to be more versatile in carbon source utilization (acetate, lactate, citrate, and oxalate), whereas Rhodopseudomonas palustris PI5 proved to be more versatile in metal resistance (arsenate, arsenite, cobalt, lead, selenium, and nickel). Both the strains were found to be positive for photofermentative hydrogen production along with arsenic respiration. This study reveals that anaerobic conditions are more appropriate for better efficiency of PNSB. Our study demonstrates that R. palustris PI5 and Rhodobacter sp. PI3 can be promising candidates for the biohydrogen production along with metal detoxification using heavy metal-polluted effluents as a substrate.

Marine Purple Photosynthetic Bacteria as Sustainable Microbial Production Hosts

Photosynthetic microorganisms can serve as the ideal hosts for the sustainable production of high-value compounds. Purple photosynthetic bacteria are typical anoxygenic photosynthetic microorganisms and are expected to be one of the suitable microorganisms for industrial production. Purple photosynthetic bacteria are reported to produce polyhydroxyalkanoate (PHA), extracellular nucleic acids and hydrogen gas. We characterized PHA production as a model compound in purple photosynthetic bacteria, especially focused on marine strains. PHA is a family of biopolyesters synthesized by a variety of microorganisms as carbon and energy storage materials. PHA have recently attracted attention as an alternative to conventional petroleum-based plastics. Production of extracellular nucleic acids have been studied in Rhodovulum sulfidophilum, a marine purple non-sulfur bacterium. Several types of artificial RNAs have been successfully produced in R. sulfidophilum. Purple photosynthetic bacteria produce hydrogen via nitrogenase, and genetic engineering strategies have been investigated to enhance the hydrogen production. This mini review describes the microbial production of these high-value compounds using purple photosynthetic bacteria as the host microorganism.

Omics Application of Bio-Hydrogen Production Through Green Alga Chlamydomonas reinhardtii

This article summarizes the current knowledge regarding omics approaches, which include genomics, transcriptomics, proteomics and metabolomics, in the context of bio-hydrogen production in Chlamydomonas reinhardtii. In this paper, critical genes (HydA1, Hyd A2, Sulp, Tla1, Sta7, PFL1) involved in H₂ metabolism were identified and analyzed for their function in H₂ accumulation. Furthermore, the advantages of gene microarrays and RNA-seq were compared, as well as their applications in transcriptomic analysis of H₂ production. Moreover, as a useful tool, proteomic analysis could identify different proteins that participate in H₂ metabolism. This review provides fundamental theory and an experimental basis for H₂ production, and further research effort is needed in this field.

Hydrogen gas alleviates toxic effects of cadmium in Brassica campestris seedlings through up-regulation of the antioxidant capacities: Possible involvement of nitric oxide

Hydrogen gas (H₂) has been shown as an important factor in plant tolerance to abiotic stresses, but the underlying mechanisms remain unclear. In the present study, the effects of H₂ and its interaction with nitric oxide (NO) on alleviating cadmium (Cd) stress in Brassica campestris seedlings were investigated. NO donor (SNP) or hydrogen-rich water (HRW) treatment showed a significant improvement in growth of Cd-stressed seedlings. Cd treatment upregulated both endogenous NO and H₂ (36% and 66%, respectively), and the increase of H₂ was prior to NO increase. When treated with NO scavenger (PTIO) or NO biosynthesis enzyme inhibitors (L-NAME and Gln), HRW-induced alleviation under Cd stress was prevented. Under Cd stress, HRW pretreatment significantly enhanced the NO accumulation, and together up-regulated the activity of NR (nitrate reductase) and expression of NR. HRW induced lower reactive oxygen species (ROS), higher AsA content, enhanced activity of POD (peroxidase) and SOD (superoxide dismutase) in seedling roots were inhibited by PTIO, L-NAME and Gln. Through proteomic analysis, the level of 29 proteins were changed in response to H₂ and NO-induced amelioration of Cd stress. Nearly half of them were involved in oxidation-reduction processes (about 20%) or antioxidant enzymes (approximately 20%). These results strongly indicate that in Cd-stressed seedlings, pretreatment with HRW induces the accumulation of H₂ (biosynthesized or permeated), which further stimulates the biosynthesis of NO through the NR pathway. Finally, H₂ and NO together enhance the antioxidant capabilities of seedlings in response to Cd toxicity.

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.

Transcriptome analysis of Rhodobacter capsulatus grown on different nitrogen sources

This study investigated the effect of different nitrogen sources, namely, ammonium chloride and glutamate, on photoheterotrophic metabolism of Rhodobacter capsulatus grown on acetate as the carbon source. Genes that were significantly differentially expressed according to Affymetrix microarray data were categorized into Clusters of Orthologous Groups functional categories and those in acetate assimilation, hydrogen production, and photosynthetic electron transport pathways were analyzed in detail. Genes related to hydrogen production metabolism were significantly downregulated in cultures grown on ammonium chloride when compared to those grown on glutamate. In contrast, photosynthetic electron transport and acetate assimilation pathway genes were upregulated. In detail, aceA encoding isocitrate lyase, a unique enzyme of the glyoxylate cycle and ccrA encoding the rate limiting crotonyl-CoA carboxylase/reductase enzyme of ethylmalonyl-coA pathway were significantly upregulated. Our findings indicate for the first time that R. capsulatus can operate both glyoxylate and ethylmalonyl-coA cycles for acetate assimilation.

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.

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

One of the best options to alleviate the problems associated with global warming and climate change is to reduce burning of fossil fuels and search for new alternative energy resources. In case of biodiesel and bioethanol production, the choice of feedstock and the process design influences the GHG emissions and appropriate methods need to be adapted. Hydrogen is a zero-carbon and energy dense alternative energy carrier with clean burning properties and biohydrogen production by microalgae can reduce production associated GHG emissions to a great extent. Biohydrogen can be produced through dark fermentation using sugars, starch, or cellulosic materials. Microalgae-based biohydrogen production is recently regarded as a promising pathway for biohydrogen production via photolysis or being a substrate for anaerobic fermentation. This review lists the methods of hydrogen production by microalgae. The enzymes involved and the factors affecting the biohydrogen production process are discussed. The bottlenecks in microalgae-based biohydrogen production are critically reviewed and future research areas in hydrogen production are presented.