Comparison of tubular and panel type photobioreactors for biohydrogen production utilizing Chlamydomonas reinhardtii considering mixing time and light intensity

Strategies to enhance biohydrogen production from microalgae: A comprehensive review

Microalgae represent a promising renewable feedstock for the sustainable production of biohydrogen. Their high growth rates and ability to fix carbon utilizing just sunlight, water, and nutrients make them well-suited for this application. Recent advancements have focused on improving microalgal hydrogen yields and cultivation methods. This review aims to summarize recent developments in microalgal cultivation techniques and genetic engineering strategies for enhanced biohydrogen production. Specific areas of focus include novel microalgal species selection, immobilization methods, integrated hybrid systems, and metabolic engineering. Studies related to microalgal strain selection, cultivation methods, metabolic engineering, and genetic manipulations were compiled and analyzed. Promising microalgal species with high hydrogen production capabilities such as Synechocystis sp., Anabaena variabilis, and Chlamydomonas reinhardtii have been identified. Immobilization techniques like encapsulation in alginate and integration with dark fermentation have led to improved hydrogen yields. Metabolic engineering through modulation of hydrogenase activity and photosynthetic pathways shows potential for enhanced biohydrogen productivity. Considerable progress has been made in developing microalgal systems for biohydrogen. However, challenges around process optimization and scale-up remain. Future work involving metabolic modeling, photobioreactor design, and genetic engineering of electron transfer pathways could help realize the full potential of this renewable technology.

Design and applications of photobioreactors- a review

There has been increasing attention in recent years on the use of photobioreactors for various biotechnological applications, especially for the cultivation of microalgae. Photobioreactors-based production of photosynthetic microorganisms furnish several advantages as minimising toxicity and providing improved conditions. However, the designing and scaling-up of photobioreactors (PBRs) remain a challenge. Due to huge capital investment and operating cost, there is a deficiency of suitable PBRs for development of photosynthetic microorganisms on large-scale. It is, therefore, highly desirable to understand the current state-of-the-art PBRs, their advantages and limitations so as to classify different PBRs as per their most suited applications. This review provides a holistic overview of the discreet features of diverse PBR designs and their purpose in microalgae growth and biohydrogen production and also summarizes the recent development in use of hybrid PBRs to increase their working efficiency and overall economics of their operation for the production of value-added products.

Establishment of a simple method to evaluate mixing times in a plastic bag photobioreactor using image processing based on freeware tools

Objective

Accurate determination of the mixing time in bioreactors is essential for the optimization of the productivity of bioprocesses. The aim of this work was to develop a simple optical method to determine the mixing time in a photobioreactor. The image processing method should be based on freeware tools, should not require programming skills, and thus could be used in education within high schools and in early stages of undergraduate programs.

Results

An optical method has been established to analyze images from recorded videos of mixing experiments. The steps are: 1. Extraction of a sequence of images from the video file; 2. Cropping of the pictures; 3. Background removal; and 4. Image analysis and mixing time evaluation based on quantification of pixel-to-pixel heterogeneity within a given area of interest. The novel method was generally able to track the dependency between aeration rate and mixing time within the investigated photobioreactor. In direct comparison, a pearson correlation coefficient of rho = 0.99 was obtained. Gas flow rates between 10 L h⁻¹, and 300 L h⁻¹ resulted from mixing times of between 48 and 14 s, respectively. This technique is applicable without programming skills and can be used in education with inexperienced user groups.

Emerging technologies for sustainable production of biohydrogen production from microalgae: A state-of-the-art review of upstream and downstream processes

Biohydrogen (BioH₂) is considered as one of the most environmentally friendly fuels and a strong candidate to meet the future demand for a sustainable source of energy. Presently, the production of BioH₂ from photosynthetic organisms has raised a lot of hopes in the fuel industry. Moreover, microalgal-based BioH₂ synthesis not only helps to combat current global warming by capturing greenhouse gases but also plays a key role in wastewater treatment. Hence, this manuscript provides a state-of-the-art review of the upstream and downstream BioH₂ production processes. Different metabolic routes such as direct and indirect photolysis, dark fermentation, photofermentation, and microbial electrolysis are covered in detail. Upstream processes (e.g. growth techniques, growth media) also have a great impact on BioH₂ productivity and economics, which is also explored. Technical and scientific obstacles of microalgae BioH₂ systems are finally addressed, allowing the technology to become more innovative and commercial.

Microalgal Hydrogen Production in Relation to Other Biomass-Based Technologies—A Review

Hydrogen is an environmentally friendly biofuel which, if widely used, could reduce atmospheric carbon dioxide emissions. The main barrier to the widespread use of hydrogen for power generation is the lack of technologically feasible and—more importantly—cost-effective methods of production and storage. So far, hydrogen has been produced using thermochemical methods (such as gasification, pyrolysis or water electrolysis) and biological methods (most of which involve anaerobic digestion and photofermentation), with conventional fuels, waste or dedicated crop biomass used as a feedstock. Microalgae possess very high photosynthetic efficiency, can rapidly build biomass, and possess other beneficial properties, which is why they are considered to be one of the strongest contenders among biohydrogen production technologies. This review gives an account of present knowledge on microalgal hydrogen production and compares it with the other available biofuel production technologies.

Emerging technologies for biofuel production: A critical review on recent progress, challenges and perspectives

Due to increasing anthropogenic activities, especially industry and transport, the fossil fuel demand and consumption have increased proportionally, causing serious environmental issues. This attracted researchers and scientists to develop new alternative energy sources. Therefore, this review covers the biofuel production potential and challenges related to various feedstocks and advances in process technologies. It has been concluded that the biofuels such as biodiesel, ethanol, bio-oil, syngas, Fischer-Tropsch H_2, and methane produced from crop plant residues, micro- and macroalgae and other biomass wastes using thermo-bio-chemical processes are an eco-friendly route for an energy source. Biofuels production and their uses in industries and transportation considerably minimize fossil fuel dependence. Literature analysis showed that biofuels generated from energy crops and microalgae could be the most efficient and attractive process. Recent progress in the field of biofuels using genetic engineering has larger perspectives in commercial-scale production. However, its large-scale production is still challenging; hence, to resolve this problem, it is essential to convert biomass in biofuels by developing novel technology to increase biofuel production to fulfil the current and future energy demand.

H2 production by photofermentation in an innovative plate-type photobioreactor with meandering channels

Hydrogen production by Rhodobacter capsulatus is an anaerobic, photobiological process requiring specific mixing conditions. In this study, an innovative design of a photobioreactor is proposed. The design is based on a plate-type photobioreactor with an interconnected meandering channel to allow culture mixing and H₂ degassing. The culture flow was characterized as a quasi-plug-flow with radial mixing caused by a turbulent-like regime achieved at a low Reynolds number. The dissipated volumetric power was decreased 10-fold while maintaining PBR performances (production and yields) when compared with a magnetically stirred tank reactor. To increase hydrogen production flow rate, several bacterial concentrations were tested by increasing the glutamate concentration using fed-batch cultures. The maximum hydrogen production flow rate (157.7 ± 9.3 ml H₂ /L/h) achieved is one of the highest values so far reported for H₂ production by R. capsulatus. These first results are encouraging for future scale-up of the plate-type reactor.

Wastewater based microalgal biorefinery for bioenergy production: Progress and challenges

Treatment of industrial and domestic wastewater is very important to protect downstream users from health risks and meet the freshwater demand of the ever-increasing world population. Different types of wastewater (textile, dairy, pharmaceutical, swine, municipal, etc.) vary in composition and require different treatment strategies. Wastewater management and treatment is an expensive process; hence, it is important to integrate relevant technology into this process to make it more feasible and cost-effective. Wastewater treatment using microalgae-based technology could be a global solution for resource recovery from wastewater and to provide affordable feedstock for bioenergy (biodiesel, biohydrogen, bio-alcohol, methane, and bioelectricity) production. Various microalgal cultivation systems (open or closed photobioreactors), turf scrubber, and hybrid systems have been developed. Although many algal biomass harvesting methods (physical, chemical, biological, and electromagnetic) have been reported, it is still an expensive process. In this review article, resource recovery from wastewater using algal cultivation, biomass harvesting, and various technologies applied in converting algal biomass into bioenergy, along with the various challenges that are encountered are discussed in brief.

Modelling and multi-objective optimization for simulation of hydrogen production using a photosynthetic consortium

This study was aimed at finding the optimal conditions for hydrogen production based on statistical experiments and using a simulation approach. A Plackett–Burman design and steepest ascent were used to screen the key factors to obtain the best hydrogen concentration. According to the regression analysis, cysteine, acetate, and aeration had the best effect. The optimal conditions, using the method of steepest ascent, were aeration (0.125 L/min), acetate (200 mg/L), cysteine (498 mg/L). Once this was determined, an experiment with more than two factors was considered. The combinations: acetate + cysteine without aeration and cysteine without aeration increased hydrogen concentration. These last two criteria were used to validate the dynamic model based on unstructured kinetics. Biomass, nitrogen, acetate, and hydrogen concentrations were monitored. The proposed model was used to perform the multi-objective optimization for various desired combinations. The simultaneous optimization for a minimum ratio of cysteine-acetate improved the concentration of hydrogen to 20 mg/L. Biomass optimized the concentration of hydrogen to 11.5 mg/L. The simultaneous optimization of reaction time (RT) and cysteine improved hydrogen concentration to 28.19 mg/L. The experimental hydrogen production was 11.4 mg/L at 24 h under discontinuous operation. Finally, the proposed model and the optimization methodology calculated a higher hydrogen concentration than the experimental data.

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.

The effect of medium and light wavelength towards Stichococcus bacillaris fatty acid production and composition

Introduction of novel species will highlight technical feasibility of microalgae-based biofuels for commercial applications. This paper reports the effect of culture medium and light wavelength on biomass and fatty acid production of S. bacillaris which holds some advantages as short life cycle, easy cultivation, high lipid content, diversity of fatty acids and stability under harsh environmental conditions. The results displayed that, soil extract (SE) greatly enhance growth rate of cultures. Maximum biomass and lipid productivity were achieved in TAP medium as 81 mg/L·day, 19.44 mg/L·day; respectively. Light wavelength didn't significantly change growth kinetics but played a critical role on chlorophyll-a accumulation. C14:0, C16:0 and C18:0 fatty acids were abundant which are suitable for biodiesel conversion. Interestingly, blue and red light increased longer chain fatty acids content. These results indicated that; S. bacillaris holds potential for further development of biodiesel production and feasibility of algal biodiesel for fundamental and applied sciences.

Photobiological hydrogen production via immobilization: understanding the nature of the immobilization and investigation on various conventional photobioreactors

Hydrogen photoproduction from microalgae has been an emerging topic for biofuel development. However, low yield for large-scale cultivations seems to be the main challenge. Immobilization seems to be an alternative method for sustainable hydrogen generation. In this study we examined the bead stability, bead diameter and immobilization method in accordance with photobioreactors (PBR). 2.1 mm diameter beads were selected for PBR experiments. CSTR, tubular and panel type PBRs give important results to develop suitable immobilization matrixes and techniques for mass production in scalable PBR systems. In conclusion, we suggest to develop techniques specific for the design and operation characteristic of the PBR for a yield efficient hydrogen generation.

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

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

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

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.

Quantifying the evolution of flow boiling bubbles by statistical testing and image analysis: toward a general model

A new index, the estimate of the error variance, which can be used to quantify the evolution of the flow patterns when multiphase components or tracers are difficultly distinguishable, was proposed. The homogeneity degree of the luminance space distribution behind the viewing windows in the direct contact boiling heat transfer process was explored. With image analysis and a linear statistical model, the F-test of the statistical analysis was used to test whether the light was uniform, and a non-linear method was used to determine the direction and position of a fixed source light. The experimental results showed that the inflection point of the new index was approximately equal to the mixing time. The new index has been popularized and applied to a multiphase macro mixing process by top blowing in a stirred tank. Moreover, a general quantifying model was introduced for demonstrating the relationship between the flow patterns of the bubble swarms and heat transfer. The results can be applied to investigate other mixing processes that are very difficult to recognize the target.

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

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

Properties of microalgal enzymatic protein hydrolysates: Biochemical composition, protein distribution and FTIR characteristics

Chlorella vulgaris SAG 211-12, a green microalga, as model organism was cultivated photomixotrophically using various organic carbon and nitrogen sources at Erlenmeyer scale. The modified medium selected for the experiments was standard BG11 supplemented with 5 g l^-1 glucose and 1 g l^-1 proteose peptone (PP). To evaluate the effects of light/dark cycles, 12:12; 18:6 and 24:0 light/dark cycle conditions were examined on hourly basis. 24:0 continuous illumination condition was chosen to continue 2 l continuous stirred tank photobioreactor (CSTR) experiments under 1 vvm aeration, 120 rpm mixing time, 23 ± 2 °C, and 70 μE m^-2 s^-1 illumination conditions. The results showed significant effect of the culture conditions on the cellular composition. To enhance digestibility of the intact cell; dry biomass was digested with pancreatin enzyme solution and in vitro protein digestibility (IVPD) of crude biomass (UTS), cell debris (CVA) and protein hydrolysates (CVH) was measured. IVPD values of UTS, CVA and CVH were found to be 33-41%, 46-58%, 67-89%; respectively with no significant changes regarding culture conditions (p > 0,05). Results also showed the positive effect of the enzyme treatment for digestion which is a key advantage for nutritional characteristic of the algal biomass.

Monitoring of Microalgal Processes

Process monitoring, which can be defined as the measurement of process variables with the smallest possible delay, is combined with process models to form the basis for successful process control. Minimizing the measurement delay leads inevitably to employing online, in situ sensors where possible, preferably using noninvasive measurement methods with stable, low-cost sensors. Microalgal processes have similarities to traditional bioprocesses but also have unique monitoring requirements. In general, variables to be monitored in microalgal processes can be categorized as physical, chemical, and biological, and they are measured in gaseous, liquid, and solid (biological) phases. Physical and chemical process variables can be usually monitored online using standard industrial sensors. The monitoring of biological process variables, however, relies mostly on sensors developed and validated using laboratory-scale systems or uses offline methods because of difficulties in developing suitable online sensors. Here, we review current technologies for online, in situ monitoring of all types of process parameters of microalgal cultivations, with a focus on monitoring of biological parameters. We discuss newly introduced methods for measuring biological parameters that could be possibly adapted for routine online use, should be preferably noninvasive, and are based on approaches that have been proven in other bioprocesses. New sensor types for measuring physicochemical parameters using optical methods or ion-specific field effect transistor (ISFET) sensors are also discussed. Reviewed methods with online implementation or online potential include measurement of irradiance, biomass concentration by optical density and image analysis, cell count, chlorophyll fluorescence, growth rate, lipid concentration by infrared spectrophotometry, dielectric scattering, and nuclear magnetic resonance. Future perspectives are discussed, especially in the field of image analysis using in situ microscopy, infrared spectrophotometry, and software sensor systems.