Microalgal hydrogen production - A review

Algae: the game-changers in biohydrogen sector

Biohydrogen (H2) is an efficient form of renewable energy generated from various biological organisms. Specifically, primitive plants such as algae which are photosynthetic organisms can produce several commercial products, including biofuels due to their simple form, short life span, efficient photosynthetic capacity, and ability to grow in non-potable water sources. But these algae are often neglected and considered waste. Several studies have revealed the importance and role of algal species in generating biofuels, especially biohydrogen. Considerable research has been conducted in order to understand hydrogen production from algal sources. This review emphasizes the photolysis of water-based hydrogen production in algae apart from the metabolites fermentation process. The influence of physico-chemical factors, including oxygen scavengers, nanoparticles, and hydrogenases, was highlighted in this review to enhance H2 production from algal species. Also, several algal species used for hydrogen production are summarized in detail. Overall, this review intends to summarize the developments in hydrogen production from algal species keeping in view of excellent prospects. This knowledge certainly would provide a good opportunity for the industrial production of hydrogen using algal species, which is one of the most concerned areas in the energy sector.

Enhancing strategies of photosynthetic hydrogen production from microalgae: Differences in hydrogen production between prokaryotic and eukaryotic algae

Hydrogen production through the metabolic bypass of microalgae photosynthesis is an environmentally friendly method. This review examines the genetic differences in hydrogen production between prokaryotic and eukaryotic microalgae. Additionally, the pathways for enhancing microalgae-based photosynthetic hydrogen generation are summarized. The main strategies for enhancing microalgal hydrogen production involve inhibiting the oxygen-generating process of photosynthesis and promoting the oxygen tolerance of hydrogenase. Future research is needed to explore the regulation of physiological metabolism through quorum sensing in microalgae to enhance photosynthetic hydrogen production. Moreover, effective evaluation of carbon emissions and sequestration across the entire photosynthetic hydrogen production process is crucial for determining the sustainability of microalgae-based production approaches through comprehensive lifecycle assessment. This review elucidates the prospects and challenges associated with photosynthetic hydrogen production by microalgae.

Tetraselmis species for environmental sustainability: biology, water bioremediation, and biofuel production

With increasing demand of fossil fuels and water pollution and their environmental impacts, marine green microalgae have gained special attention in both scientific and industrial  fields. This is due to their fast growth in non-arable lands with high photosynthetic activity, their metabolic plasticity, as well as their high CO2 capture capacity. Tetraselmis species, green and eukaryotic microalgae, are not only considered as a valuable source of biomolecules including pigments, lipids, and starch but also widely used in biotechnological applications. Tetraselmis cultivation for high-value biomolecules and industrial use was demonstrated to be a non-cost-effective strategy because of its low demand in nutrients, such as phosphorus and nitrogen. Recently, phycoremediation of wastewater rich in nutrients, chemicals, and heavy metals has become an efficient and economic-alternative that allows the detoxification of waters and induces mechanisms in algal cells for biomolecules rich-energy synthesis to regulate their metabolic pathways. This review aims to shed light on Tetraselmis species for their different culture conditions and metabolites bioaccumulation, as well as their human health and environmental applications. Additionally, phycoremediation of contaminants associated to biofuel production in Tetraselmis cells and their different intracellular and extracellular mechanisms have also been investigated.

Biofuel production for circular bioeconomy: Present scenario and future scope

In recent years, biofuel production has attracted considerable attention, especially given the increasing worldwide demand for energy and emissions of greenhouse gases that threaten this planet. In this case, one possible solution is to convert biomass into green and sustainable biofuel, which can enhance the bioeconomy and contribute to sustainable economic development goals. Due to being in large quantities and containing high organic content, various biomass sources such as food waste, textile waste, microalgal waste, agricultural waste and sewage sludge have gained significant attention for biofuel production. Also, biofuel production technologies, including thermochemical processing, anaerobic digestion, fermentation and bioelectrochemical systems, have been extensively reported, which can achieve waste valorization through producing biofuels and re-utilizing wastes. Nevertheless, the commercial feasibility of biofuel production is still being determined, and it is unclear whether biofuel can compete equally with other existing fuels in the market. The concept of a circular economy in biofuel production can promote the environmentally friendly and sustainable valorization of biomass waste. This review comprehensively discusses the state-of-the-art production of biofuel from various biomass sources and the bioeconomy perspectives associated with it. Biofuel production is evaluated within the framework of the bioeconomy. Further perspectives on possible integration approaches to maximizing waste utilization for biofuel production are discussed, and what this could mean for the circular economy. More research related to pretreatment and machine learning of biofuel production should be conducted to optimize the biofuel production process, increase the biofuel yield and make the biofuel prices competitive.

Microalgae-bacteria nexus for environmental remediation and renewable energy resources: Advances, mechanisms and biotechnological applications

Microalgae and bacteria, known for their resilience, rapid growth, and proximate ecological partnerships, play fundamental roles in environmental and biotechnological advancements. This comprehensive review explores the synergistic interactions between microalgae and bacteria as an innovative approach to address some of the most pressing environmental issues and the demands of clean and renewable freshwater and energy sources. Studies indicated that microalgae-bacteria consortia can considerably enhance the output of biotechnological applications; for instance, various reports showed during wastewater treatment the COD removal efficiency increased by 40%-90.5 % due to microalgae-bacteria consortia, suggesting its great potential amenability in biotechnology. This review critically synthesizes research works on the microalgae and bacteria nexus applied in the advancements of renewable energy generation, with a special focus on biohydrogen, reclamation of wastewater and desalination processes. The mechanisms of underlying interactions, the environmental factors influencing consortia performance, and the challenges and benefits of employing these bio-complexes over traditional methods are also discussed in detail. This paper also evaluates the biotechnological applications of these microorganism consortia for the augmentation of biomass production and the synthesis of valuable biochemicals. Furthermore, the review sheds light on the integration of microalgae-bacteria systems in microbial fuel cells for concurrent energy production, waste treatment, and resource recovery. This review postulates microalgae-bacteria consortia as a sustainable and efficient solution for clean water and energy, providing insights into future research directions and the potential for industrial-scale applications.

New insights into Chlorella vulgaris applications

Environmental pollution is a big challenge that has been faced by humans in contemporary life. In this context, fossil fuel, cement production, and plastic waste pose a direct threat to the environment and biodiversity. One of the prominent solutions is the use of renewable sources, and different organisms to valorize wastes into green energy and bioplastics such as polylactic acid. Chlorella vulgaris, a microalgae, is a promising candidate to resolve these issues due to its ease of cultivation, fast growth, carbon dioxide uptake, and oxygen production during its growth on wastewater along with biofuels, and other productions. Thus, in this article, we focused on the potential of Chlorella vulgaris to be used in wastewater treatment, biohydrogen, biocement, biopolymer, food additives, and preservation, biodiesel which is seen to be the most promising for industrial scale, and related biorefineries with the most recent applications with a brief review of Chlorella and polylactic acid market size to realize the technical/nontechnical reasons behind the cost and obstacles that hinder the industrial production for the mentioned applications. We believe that our findings are important for those who are interested in scientific/financial research about microalgae.

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.

Exchange or Eliminate: The Secrets of Algal-Bacterial Relationships

Algae and bacteria have co-occurred and coevolved in common habitats for hundreds of millions of years, fostering specific associations and interactions such as mutualism or antagonism. These interactions are shaped through exchanges of primary and secondary metabolites provided by one of the partners. Metabolites, such as N-sources or vitamins, can be beneficial to the partner and they may be assimilated through chemotaxis towards the partner producing these metabolites. Other metabolites, especially many natural products synthesized by bacteria, can act as toxins and damage or kill the partner. For instance, the green microalga Chlamydomonas reinhardtii establishes a mutualistic partnership with a Methylobacterium, in stark contrast to its antagonistic relationship with the toxin producing Pseudomonas protegens. In other cases, as with a coccolithophore haptophyte alga and a Phaeobacter bacterium, the same alga and bacterium can even be subject to both processes, depending on the secreted bacterial and algal metabolites. Some bacteria also influence algal morphology by producing specific metabolites and micronutrients, as is observed in some macroalgae. This review focuses on algal-bacterial interactions with micro- and macroalgal models from marine, freshwater, and terrestrial environments and summarizes the advances in the field. It also highlights the effects of temperature on these interactions as it is presently known.

Harnessing diurnal dynamics: Deciphering the interplay of light cycles on algal-bacterial membrane bioreactors

Light profoundly modulates the algal-bacterial membrane bioreactor (algal-bacterial MBR) performance. Yet, its outdoor deployment grapples with the inherent diurnal cycle of sunlight, engendering suboptimal light conditions. The adaptability of such systems to these fluctuating light conditions and their implications for practical outdoor applications remained an under-explored frontier. In response, this study meticulously scrutinized two laboratory-scale algal-bacterial MBRs under varying light regimes: a 24-h continuous and a 12-h cyclic illumination. Over 70 days, continuous illumination was observed to yield superior biomass production and total nitrogen and total phosphorus removal efficiencies compared to its cyclic counterpart. Contrarily, when focusing on membrane fouling, the 12-h cyclic illumination exhibited lower membrane fouling. The spectral analyses coupled with adhesion ability evaluation, traced the enhanced membrane fouling under continuous illumination to the elevated organics and heightened adhesive properties of the flocs. Given the tangible benefits of reduced membrane fouling and the potential harnessing of solar radiation, the 12-h cyclic illumination emerges as an economically astute operational paradigm for algal-bacterial MBRs. The significance of this study is to promote the application of algal-bacterial MBR in sewage treatment and provide robust support for the development of green technology in the future.

Treatment of industrial wastewaters by algae-bacterial consortium with Bio-H2 production: Recent updates, challenges and future prospects

Currently, scarcity/security of clean water and energy resources are the most serious problems worldwide. Industries use large volume of ground water and a variety of chemicals to manufacture the products and discharge large volume of wastewater into environment, which causes severe impacts on environment and public health. Fossil fuels are considered as major energy resources for electricity and transportation sectors, which release large amount of CO2 and micro/macro pollutants, leading to cause the global warming and public health hazards. Therefore, algae-bacterial consortium (A-BC) may be eco-friendly, cost-effective and sustainable alternative way to treat the industrial wastewaters (IWWs) with Bio-H2 production. A-BC has potential to reduce the global warming and eutrophication. It also protects environment and public health as it converts toxic IWWs into non or less toxic (biomass). It also reduces 94%, 90% and 50% input costs of nutrients, freshwater and energy, respectively during IWWs treatment and Bio-H2 production. Most importantly, it produce sustainable alternative (Bio-H2) to replace use of fossil fuels and fill the world's energy demand in eco-friendly manner. Thus, this review paper provides a detailed knowledge on industrial wastewaters, their pollutants and toxic effects on water/soil/plant/humans and animals. It also provides an overview on A-BC, IWWs treatment, Bio-H2 production, fermentation process and its enhancement methods. Further, various molecular and analytical techniques are also discussed to characterize the A-BC structure, interactions, metabolites and Bio-H2 yield. The significance of A-BC, recent update, challenges and future prospects are also discussed.

Recent advances in sustainable hydrogen production from microalgae: Mechanisms, challenges, and future perspectives

The depletion of fossil fuel reserves has resulted from their application in the industrial and energy sectors. As a result, substantial efforts have been dedicated to fostering the shift from fossil fuels to renewable energy sources via technological advancements in industrial processes. Microalgae can be used to produce biofuels such as biodiesel, hydrogen, and bioethanol. Microalgae are particularly suitable for hydrogen production due to their rapid growth rate, ability to thrive in diverse habitats, ability to resolve conflicts between fuel and food production, and capacity to capture and utilize atmospheric carbon dioxide. Therefore, microalgae-based biohydrogen production has attracted significant attention as a clean and sustainable fuel to achieve carbon neutrality and sustainability in nature. To this end, the review paper emphasizes recent information related to microalgae-based biohydrogen production, mechanisms of sustainable hydrogen production, factors affecting biohydrogen production by microalgae, bioreactor design and hydrogen production, advanced strategies to improve efficiency of biohydrogen production by microalgae, along with bottlenecks and perspectives to overcome the challenges. This review aims to collate advances and new knowledge emerged in recent years for microalgae-based biohydrogen production and promote the adoption of biohydrogen as an alternative to conventional hydrocarbon biofuels, thereby expediting the carbon neutrality target that is most advantageous to the environment.

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.

Genetic engineering for biohydrogen production from microalgae

The development of biohydrogen as an alternative energy source has had great economic and environmental benefits. Hydrogen production from microalgae is considered a clean and sustainable energy production method that can both alleviate fuel shortages and recycle waste. Although algal hydrogen production has low energy consumption and requires only simple pretreatment, it has not been commercialized because of low product yields. To increase microalgal biohydrogen production several technologies have been developed, although they struggle with the oxygen sensitivity of the hydrogenases responsible for hydrogen production and the complexity of the metabolic network. In this review, several genetic and metabolic engineering studies on enhancing microalgal biohydrogen production are discussed, and the economic feasibility and future direction of microalgal biohydrogen commercialization are also proposed.

Epitomizing biohydrogen production from microbes: Critical challenges vs opportunities

Hydrogen is a clean and green biofuel choice for the future because it is carbon-free, non-toxic, and has high energy conversion efficiency. In exploiting hydrogen as the main energy, guidelines for implementing the hydrogen economy and roadmaps for the developments of hydrogen technology have been released by several countries. Besides, this review also unveils various hydrogen storage methods and applications of hydrogen in transportation industry. Biohydrogen productions from microbes, namely, fermentative bacteria, photosynthetic bacteria, cyanobacteria, and green microalgae, via biological metabolisms have received significant interests off late due to its sustainability and environmentally friendly potentials. Accordingly, the review is as well outlining the biohydrogen production processes by various microbes. Furthermore, several factors such as light intensity, pH, temperature and addition of supplementary nutrients to enhance the microbial biohydrogen production are highlighted at their respective optimum conditions. Despite the advantages, the amounts of biohydrogen being produced by microbes are still insufficient to be a competitive energy source in the market. In addition, several major obstacles have also directly hampered the commercialization effors of biohydrogen. Thus, this review uncovers the constraints of biohydrogen production from microbes such as microalgae and offers solutions associated with recent strategies to overcome the setbacks via genetic engineering, pretreatments of biomass, and introduction of nanoparticles as well as oxygen scavengers. The opportunities of exploiting microalgae as a suastainable source of biohydrogen production and the plausibility to produce biohydrogen from biowastes are accentuated. Lastly, this review addresses the future perspectives of biological methods to ensure the sustainability and economy viability of biohydrogen production.

Photosynthesis driven continuous hydrogen production by diazotrophic cyanobacteria in high cell density capillary photobiofilm reactors

Hydrogen (H₂) is a promising fuel in the context of climate neutral energy carriers and photosynthesis-driven H₂-production is an interesting option relying mainly on sunlight and water as resources. However, this approach depends on suitable biocatalysts and innovative photobioreactor designs to maximize cell performance and H₂ titers. Cyanobacteria were used as biocatalysts in capillary biofilm photobioreactors (CBRs). We show that biofilm formation/stability depend on light and CO₂ availabilityH₂ production rates correlate with these parameters but differ between Anabaena and Nostoc. We demonstrate that high light and corresponding O₂ levels influence biofilm stability in CBR. By adjusting these parameters, biofilm formation/stability could be enhanced, and H₂ formation was stable for weeks. Final biocatalyst titers reached up to 100 g l^-1 for N. punctiforme atcc 29133 NHM5 and Anabaena sp. pcc 7120 AMC 414. H₂ production rates were up to 300 µmol H₂ l^-1h^-1 and 3 µmol H₂ g_cdw^-1h^-1 in biofilms.

A high-value biohythane production: Feedstocks, reactor configurations, pathways, challenges, technoeconomics and applications

In recent years, the demand for high-quality biofuels from renewable sources has become an aspirational goal to offer a clean environment by alternating the depleting fossil fuels to meet future energy needs. In this aspect, biohythane production from wastes has received extensive research interest since it contains superior fuel characteristics than the promising conventional biofuel i.e. biogas. The main aim is to promote research and potentials of biohythane production by a systematic review of scientific literature on the biohythane production pathways, substrate/microbial consortium suitability, reactor design, and influential process/operational factors. Reactor configuration also decides the product yield in addition to other key factors like waste composition, temperature, pH, retention time and loading rates. Hence, a detailed emphasis on different reactor configurations with respect to the type of feedstock has also been given. The technical challenges are highlighted towards process optimization and system scale up. Meanwhile, solutions to improve product yield, technoeconomics, applications and key policy and governance factors to build a hydrogen based society have also been discussed.

Hydrogenase and Nitrogenase: Key Catalysts in Biohydrogen Production

Hydrogen with high energy content is considered to be a promising alternative clean energy source. Biohydrogen production through microbes provides a renewable and immense hydrogen supply by utilizing raw materials such as inexhaustible natural sunlight, water, and even organic waste, which is supposed to solve the two problems of "energy supply and environment protection" at the same time. Hydrogenases and nitrogenases are two classes of key enzymes involved in biohydrogen production and can be applied under different biological conditions. Both the research on enzymatic catalytic mechanisms and the innovations of enzymatic techniques are important and necessary for the application of biohydrogen production. In this review, we introduce the enzymatic structures related to biohydrogen production, summarize recent enzymatic and genetic engineering works to enhance hydrogen production, and describe the chemical efforts of novel synthetic artificial enzymes inspired by the two biocatalysts. Continual studies on the two types of enzymes in the future will further improve the efficiency of biohydrogen production and contribute to the economic feasibility of biohydrogen as an energy source.

Role of microalgae in achieving sustainable development goals and circular economy

In 2015, the United Nations General Assembly (UNGA) set out 17 Sustainable Development Goals (SDGs) to be achieved by 2030. These goals highlight key objectives that must be addressed. Each target focuses on a unique perspective crucial to meeting these goals. Social, political, and economic issues are addressed to comprehensively review the main issues combating climate change and creating sustainable and environmentally friendly industries, jobs, and communities. Several mechanisms that involve judicious use of biological entities are among instruments that are being explored to achieve the targets of SDGs. Microalgae have an increasing interest in various sectors, including; renewable energy, food, environmental management, water purification, and the production of chemicals such as biofertilizers, cosmetics, and healthcare products. The significance of microalgae also arises from their tendency to consume CO2, which is the main greenhouse gas and the major contributor to the climate change. This work discusses the roles of microalgae in achieving the various SDGs. Moreover, this work elaborates on the contribution of microalgae to the circular economy. It was found that the microalgae contribute to all the 17th SDGs, where they directly contribute to 9th of the SDGs and indirectly contribute to the rest. The major contribution of the Microalgae is clear in SDG-6 "Clean water and sanitation", SDG-7 "Affordable and clean energy", and SDG-13 "Climate action". Furthermore, it was found that Microalgae have a significant contribution to the circular economy.

Chlorella sp.-ameliorated undesirable microenvironment promotes diabetic wound healing

Chronic diabetic wound remains a critical challenge suffering from the complicated negative microenvironments, such as high-glucose, excessive reactive oxygen species (ROS), hypoxia and malnutrition. Unfortunately, few strategies have been developed to ameliorate the multiple microenvironments simultaneously. In this study, Chlorella sp. (Chlorella) hydrogels were prepared against diabetic wounds. In vitro experiments demonstrated that living Chlorella could produce dissolved oxygen by photosynthesis, actively consume glucose and deplete ROS with the inherent antioxidants, during the daytime. At night, Chlorella was inactivated in situ by chlorine dioxide with human-body harmless concentration to utilize its abundant contents. It was verified in vitro that the inactivated-Chlorella could supply nutrition, relieve inflammation and terminate the oxygen-consumption of Chlorella-respiration. The advantages of living Chlorella and its contents were integrated ingeniously. The abovementioned functions were proven to accelerate cell proliferation, migration and angiogenesis in vitro. Then, streptozotocin-induced diabetic mice were employed for further validation. The in vivo outcomes confirmed that Chlorella could ameliorate the undesirable microenvironments, including hypoxia, high-glucose, excessive-ROS and chronic inflammation, thereby synergistically promoting tissue regeneration. Given the results above, Chlorella is considered as a tailor-made therapeutic strategy for diabetic wound healing.

Sustainable production of biohydrogen from algae biomass: Critical review on pretreatment methods, mechanism and challenges

The production of chemicals and energy from sustainable biomass with an important objective decreasing carbon impressions has recently become one of the key areas of attention. Algae biomass have been recognized and researched as a potential renewable biomass of biohydrogen production attributed to their limited multiplying time, fast growing qualities and ability of lipid accumulation. This review additionally envelops various key perspectives such as composition and properties of algae biomass and pretreatment strategies such as physical, chemical and biological methods adopted for the algae biomass. This review is mainly focused on pretreatment strategies which have been developed to enhance biohydrogen production. The present review deals with methods and mechanism, enzymes involved and factors influencing on biohydrogen production which help to grasp various bottlenecks, challenges and constraints. Finally, the significant progressions and economical perspective on improving biohydrogen yield because of the expansion of co-substrates and the current trends are examined.

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.

Employing algal biomass for fabrication of biofuels subsequent to phytoremediation

An alga belongs to the multi-pertinent group which can add to a significant sector of environment. They show a prevailing gathering of microorganisms for bioremediation due to their significant capacity to inactivate toxic heavy metals. It can easily absorb or neutralize the toxicity of heavy metals from water and soil through phytoremediation. Biosorption is a promising innovation that focuses on novel, modest, and exceptionally successful materials to apply in phytoremediation technology. Furthermore, algal biomass can be used for biofuel generation after phytoremediation using thermochemical or biological transformation processes. The algal components get affected by heavy metals during phytoremediation, but with the help of different techniques, these are yield efficient. The extreme lipid and mineral substances of microalgae have been proven helpful for biofuel manufacturing and worth extra products. Biofuels produced are bio-oil, biodiesel, bioethanol, biogas, etc. The reuse capability of algae can be utilized toward ecological manageability and economic facility. In this review article, the reuse and recycling of algal biomass for biofuel production have been represented. This novel technique has numerous benefits and produces eco-friendly and economically beneficial products.

Light-Driven Synthetic Biology: Progress in Research and Industrialization of Cyanobacterial Cell Factory

Light-driven synthetic biology refers to an autotrophic microorganisms-based research platform that remodels microbial metabolism through synthetic biology and directly converts light energy into bio-based chemicals. This technology can help achieve the goal of carbon neutrality while promoting green production. Cyanobacteria are photosynthetic microorganisms that use light and CO₂ for growth and production. They thus possess unique advantages as "autotrophic cell factories". Various fuels and chemicals have been synthesized by cyanobacteria, indicating their important roles in research and industrial application. This review summarized the progresses and remaining challenges in light-driven cyanobacterial cell factory. The choice of chassis cells, strategies used in metabolic engineering, and the methods for high-value CO₂ utilization will be discussed.

Bioproducts from microalgae biomass: Technology, sustainability, challenges and opportunities

Microalgae are a potential feedstock for several bioproducts, mainly from its primary and secondary metabolites. Lipids can be converted in high-value polyunsaturated fatty acids (PUFA) such as omega-3, carbohydrates are potential biohydrogen (bioH₂) sources, proteins can be converted into biopolymers (such as bioplastics) and pigments can achieve high concentrations of valuable carotenoids. This work comprehends the current practices for the production of such products from microalgae biomass, with insights on technical performance, environmental and economical sustainability. For each bioproduct, discussion includes insights on bioprocesses, productivity, commercialization, environmental impacts and major challenges. Opportunities for future research, such as wastewater cultivation, arise as environmentally attractive alternatives for sustainable production with high potential for resource recovery and valorization. Still, microalgae biotechnology stands out as an attractive topic for it research and market potential.

A novel cascaded fractional fuzzy approach for control of fermentation process

In this work, a cascaded control strategy based on fractional-order fuzzy PD/PI (FOFPD/PI) is proposed for temperature control of the bioreactor. The FOFPI is used to control the ethanol concentration in the inner loop, while the FOFPD is used for temperature control of the bioreactor in the outer loop. The integer order fuzzy PD/PI (IOFPD/PI), 2DOF FOPID, 2DOF PID, and PID are also designed for comparison purposes. The design parameter of FOFPD/PI and IOFPD/PI are estimated using non-dominated sorting genetic algorithm II (NSGA-II). Results revealed that the proposed cascaded control scheme reduced the IAE by 33.5 %, 40.5%, 47%, and 64% compared to IOFPD/PI, 2DOF FOPID, 2DOF PID, and PID, respectively. Hence, it can be concluded that the proposed FOFPD/PI controller provides precise control of reactor temperature in different operating conditions compared to other controllers.

Exploring indigenous freshwater chlorophytes in integrated biophotovoltaic system for simultaneous wastewater treatment, heavy metal biosorption, CO2 biofixation and biodiesel generation

The study explored the combined photosynthetic activities of two green microalgal species, Tetradesmus obliquus and Tetradesmus reginae, on an integrated biophotovoltaic (BPV) platform for simultaneous wastewater treatment, toxic metal biosorption, carbon biofixation, bioelectricity generation and biodiesel production. The experimental setup comprised of a dual-chambered BPV with copper anode surrounded by T. obliquus in BG11 media, and copper cathode with T. reginae in municipal wastewater separated by Nafion 117 membrane. The study reported a maximum power density of 0.344 Wm^-2 at a cell potential of 0.415 V with external resistance of 1000 Ω and 0.3268 V maximum open-circuit voltage. The wastewater electrical conductivity and pH increased from 583 ± 22 to 2035 ± 29.31 mS/cm and 7.403 ± 0.174 to 8.263 ± 0.055 respectively, signifying increased photosynthetic and electrochemical activities. Residual nitrogen, phosphorus, chemical oxygen demand, arsenic, cadmium, chromium and lead removal efficiencies by T. reginae were 100%, 80.68%, 71.91%, 47.6%, 88.82%, 71.24% and 92.96%, respectively. T. reginae accumulated maximum biomass of 0.605 ± 0.033 g/L with a CO₂ biosequestration rate of 0.166 ± 0.010 gCO₂/L/day and 42.40 ± 1.166% lipid content. Methyl palmitate, methyl undecanoate and 13-octadecenoic acid with relative abundances of 37.24%, 24.80% and 12.02%, respectively were confirmed.

Integration of Waste to Bioenergy Conversion Systems: A Critical Review

Sustainable biofuel production is the most effective way to mitigate greenhouse gas emissions associated with fossil fuels while preserving food security and land use. In addition to producing bioenergy, waste biorefineries can be incorporated into the waste management system to solve the future challenges of waste disposal. Biomass waste, on the other hand, is regarded as a low-quality biorefinery feedstock with a wide range of compositions and seasonal variability. In light of these factors, biomass waste presents limitations on the conversion technologies available for value addition, and therefore more research is needed to enhance the profitability of waste biorefineries. Perhaps, to keep waste biorefineries economically and environmentally sustainable, bioprocesses need to be integrated to process a wide range of biomass resources and yield a diverse range of bioenergy products. To achieve optimal integration, the classification of biomass wastes to match the available bioprocesses is vital, as it minimizes unnecessary processes that may increase the production costs of the biorefinery. Based on biomass classification, this study discusses the suitability of the commonly used waste-to-energy conversion methods and the creation of integrated biorefineries. In this study, the integration of waste biorefineries is discussed through the integration of feedstocks, processes, platforms, and the symbiosis of wastes and byproducts. This review seeks to conceptualize a framework for identifying and integrating waste-to-energy technologies for the varioussets of biomass wastes.

Biohydrogen production from microalgae for environmental sustainability

Hydrogen as a clean energy that is conducive to energy and environmental sustainability, playing a significant role in the alleviation of global climate change and energy crisis. Biohydrogen generation from microalgae has been reported as a highly attractive approach that can produce a benign clean energy carrier to achieve carbon neutrality and bioenergy sustainability. Thus, this review explored the mechanism of biohydrogen production from microalgae containing direct biophotolysis, indirect biophotolysis, photo fermentation, and dark fermentation. In general, dark fermentation of microalgae for biohydrogen production is relatively better than photo fermentation, biophotolysis, and microbial electrolysis, because it is able to consecutively generate hydrogen and is not reliant on energy supplied by natural sunlight. Besides, this review summarized potential algal strains for hydrogen production focusing on green microalgae and cyanobacteria. Moreover, a thorough review process was conducted to present hydrogen-producing enzymes targeting biosynthesis and localization of enzymes in microalgae. Notably, the most powerful hydrogen-producing enzymes are [Fe-Fe]-hydrogenases, which have an activity nearly 10-100 times better than [Ni-Fe]-hydrogenases and 1000 times better than nitrogenases. In addition, this work highlighted the major factors affecting low energy conversion efficiency and oxygen sensitivity of hydrogen-producing enzymes. Noting that the most practical pathway of biohydrogen generation was sulfur-deprivation compared with phosphorus, nitrogen, and magnesium deficiency. Further discussions in this work summarized the recent advancement in biohydrogen production from microalgae such as genetic engineering, microalgae-bacteria consortium, electro-bio-hydrogenation, and nanomaterials for developing enzyme stability and hydrolytic efficiency. More importantly, this review provided a summary of current limitations and future perspectives on the sustainable production of biohydrogen from microalgae.

Structural and spectroscopic characterization of CO inhibition of [NiFe]-hydrogenase from Citrobacter sp. S-77

Hydrogenases catalyze the reversible oxidation of H2. Carbon monoxide (CO) is known to be a competitive inhibitor of O2-sensitive [NiFe]-hydrogenases. Although the activities of some O2-tolerant [NiFe]-hydrogenases are unaffected by CO, the partially O2-tolerant [NiFe]-hydrogenase from Citrobacter sp. S-77 (S77-HYB) is inhibited by CO. In this work, the CO-bound state of S77-HYB was characterized by activity assays, spectroscopic techniques and X-ray crystallography. Electron paramagnetic resonance spectroscopy showed a diamagnetic Ni2+ state, and Fourier-transform infrared spectroscopy revealed the stretching vibration of the exogenous CO ligand. The crystal structure determined at 1.77 Å resolution revealed that CO binds weakly to the nickel ion in the Ni-Fe active site of S77-HYB. These results suggest a positive correlation between O2 and CO tolerance in [NiFe]-hydrogenases.

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.

Advances in biopolymer composites and biomaterials for the removal of emerging contaminants

Domestic, agriculture, and industrial activities contaminate the waterbodies by releasing toxic substances and pathogens. Removal of pollutants from wastewater is critical to ensuring the quality of accessible water resources. Several wastewater treatments are often used. Researchers are increasingly focusing on adsorption, ion exchange, electrostatic interactions, biodegradation, flocculation, and membrane filtration for the efficient reduction of pollutants. Biopolymers are a combination of two or more products produced by the living organisms used to give the desired finished product with a unique attribute. Biomaterials are also similar to traditional polymers by having higher flexibility, biodegradability, low toxicity, and nontoxic secondary byproducts producing ability. Grafting, functionalization, and crosslinking will be used to enhance the characteristics of biopolymers. The present chapter will illustrate some of the important biopolymers and its compos that will impact wastewater treatment in the future. Most commonly used biopolymers including chitosan (CS), activated carbon (AC), carbon-nanotubes (CNTs), and graphene oxide (GO) are discussed. Finally, the opportunities and difficulties for applying adsorbents to water pollution treatment are discussed.

Recent Achievements in Microalgal Photobiological Hydrogen Production

It is well known that over the last 60 years the trend of long-lived greenhouse gas emissions have shown a strong acceleration. There is an increasing concern and a mounting opposition by public opinion to continue with the use of fossil energy. Western countries are presently involved in a so-called energy transition with the objective of abandoning fossil energy for renewable sources. In this connection, hydrogen can play a central role. One of the sustainable ways to produce hydrogen is the use of microalgae which possess two important natural catalysts: photosystem II and hydrogenase, used to split water and to combine protons and electrons to generate gaseous hydrogen, respectively. For about 20 years of study on photobiological hydrogen production, our scientific hopes were based on the application of the sulfur protocol, which indisputably represented a very important advancement in the field of hydrogen production biotechnology. However, as reported in this review, there is increasing evidence that this strategy is not economically viable. Therefore, a change of paradigm for the photobiological production of hydrogen based on microalgae seems mandatory. This review points out that an increasing number of microalgal strains other than Chlamydomonas reinhardtii are being tested and are able to produce sustainable amount of hydrogen without nutrient starvation and to fulfill this goal including the application of co-cultures.

Rewiring cyanobacterial photosynthesis by the implementation of an oxygen-tolerant hydrogenase

Molecular hydrogen (H₂) is considered as an ideal energy carrier to replace fossil fuels in future. Biotechnological H₂ production driven by oxygenic photosynthesis appears highly promising, as biocatalyst and H₂ syntheses rely mainly on light, water, and CO₂ and not on rare metals. This biological process requires coupling of the photosynthetic water oxidizing apparatus to a H₂-producing hydrogenase. However, this strategy is impeded by the simultaneous release of oxygen (O₂) which is a strong inhibitor of most hydrogenases. Here, we addressed this challenge, by the introduction of an O₂-tolerant hydrogenase into phototrophic bacteria, namely the cyanobacterial model strain Synechocystis sp. PCC 6803. To this end, the gene cluster encoding the soluble, O₂-tolerant, and NAD(H)-dependent hydrogenase from Ralstonia eutropha (ReSH) was functionally transferred to a Synechocystis strain featuring a knockout of the native O₂ sensitive hydrogenase. Intriguingly, photosynthetically active cells produced the O₂ tolerant ReSH, and activity was confirmed in vitro and in vivo. Further, ReSH enabled the constructed strain Syn_ReSH⁺ to utilize H₂ as sole electron source to fix CO₂. Syn_ReSH⁺ also was able to produce H₂ under dark fermentative conditions as well as in presence of light, under conditions fostering intracellular NADH excess. These findings highlight a high level of interconnection between ReSH and cyanobacterial redox metabolism. This study lays a foundation for further engineering, e.g., of electron transfer to ReSH via NADPH or ferredoxin, to finally enable photosynthesis-driven H₂ production.

Recent advances in Co-based co-catalysts for efficient photocatalytic hydrogen generation

Recent progress in photocatalytic hydrogen generation reaction highlights the critical role of co-catalysts in enhancing the solar-to-fuel conversion efficiency of diverse band-matched semiconductors. Because of the compositional flexibility, adjustable microstructure, tunable crystal phase and facet, cobalt-based co-catalysts have stimulated tremendous attention as they have high potential to promote hydrogen evolution reaction. However, a comprehensive review that specifically focuses on these promising materials has not been reported so far. Therefore, this present review emphasizes the recent progress in the pursuing of highly efficient Co-based co-catalysts for water splitting, and the advances in such materials are summarized through the analysis of structure-activity relationships. The fundamental principles of photocatalytic hydrogen production are profoundly outlined, followed by an elaborate discussion on the crucial parameters influencingthe reaction kinetics. Then, the co-catalytic reactivities of various Co-based materials involving Co, Co oxides, Co hydroxides, Co sulfides, Co phosphides and Co molecular complexes, etc, are thoroughly discussed when they are coupled with host semiconductors, with an insight towards the ultimateobjective of achieving a rationally designed photocatalyst for enhancing water splitting reaction dynamics. Finally, the current challenge and future perspective of Co-based co-catalysts as the promising noble-metal alternative materials for solar hydrogen generation are proposed and discussed.

Key Targets for Improving Algal Biofuel Production

A number of technological challenges need to be overcome if algae are to be utilized for commercial fuel production. Current economic assessment is largely based on laboratory scale up or commercial systems geared to the production of high value products, since no industrial scale plant exits that are dedicated to algal biofuel. For macroalgae (‘seaweeds’), the most promising processes are anaerobic digestion for biomethane production and fermentation for bioethanol, the latter with levels exceeding those from sugar cane. Currently, both processes could be enhanced by increasing the rate of degradation of the complex polysaccharide cell walls to generate fermentable sugars using specifically tailored hydrolytic enzymes. For microalgal biofuel production, open raceway ponds are more cost-effective than photobioreactors, with CO2 and harvesting/dewatering costs estimated to be ~50% and up to 15% of total costs, respectively. These costs need to be reduced by an order of magnitude if algal biodiesel is to compete with petroleum. Improved economics could be achieved by using a low-cost water supply supplemented with high glucose and nutrients from food grade industrial wastewater and using more efficient flocculation methods and CO2 from power plants. Solar radiation of not <3000 h·yr−1 favours production sites 30° north or south of the equator and should use marginal land with flat topography near oceans. Possible geographical sites are discussed. In terms of biomass conversion, advances in wet technologies such as hydrothermal liquefaction, anaerobic digestion, and transesterification for algal biodiesel are presented and how these can be integrated into a biorefinery are discussed.

Unravelling the attributes of novel cyanobacteria Jacksonvillea sp. ISTCYN1 by draft genome sequencing

Filamentous cyanobacteria, Jacksonvillea sp. ISTCYN1 was isolated from agriculture field and cultured in BG-11 medium. This study, report the genome sequence of cyanobacteria Jacksonvillea thatto the best of our knowledgeis the firstgenome sequenceof thisgenus. The 5.7 MB draft genome sequence of this cyanobacterium contains 5134 protein-coding genes. The phylogenetic tree was constructed based on genome and Desertifilum sp. IPPAS B-1220 validated the closest relationship with Jacksonvillea sp. ISTCYN1. The growth of strain ISTCYN1 has been reported in the presence of different types of plastic when used as a sole carbon source. SEM analysis revealed biofilm formation by cyanobacterial strain ISTCYN1 on the surface of high and low-density polyethylene and polypropylene. In the presence of these plastics, EPS production has also been reported by this strain. Whole genome sequence analysis reveals the presence of many genes involved in biofilm formation. The presence of key enzymes responsible for plastic degradation laccase, esterase, lipase, thioesterase, and peroxidase have been predicted in the genome analysis. Genome analysis also provides insight into the genes involved in biotin biosynthetic pathways. Furthermore, the presence of many selenoproteins reveals the selenium acquisition by this cyanobacterium.

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.

Bio-products from algae-based biorefinery on wastewater: A review

Increasing resource demand, predicted fossil resources shortage in the near future, and environmental concerns due to the production of greenhouse gas carbon dioxide have motivated the search for alternative 'circular' pathways. Among many options, microalgae have been recently 'revised' as one of the most promising due to their high growth rate (with low land use and without competing with food crops), high tolerance to nutrients and salts stresses and their variability in biochemical composition, in so allowing the supply of a plethora of possible bio-based products such as animal feeds, chemicals and biofuels. The recent raising popularity of Circular Bio-Economy (CBE) further prompted investment in microalgae, especially in combination with wastewater treatment, under the twofold aim of allowing the production of a wide range of bio-based products while bioremediating wastewater. With the aim of discussing the potential bio-products that may be gained from microalgae grown on urban wastewater, this paper presents an overview on microalgae production with particular emphasis on the main microalgae species suitable for growth on wastewater and the obtainable bio-based products from them. By selecting and reviewing 76 articles published in Scopus between 1992 and 2020, a number of interesting aspects, including the selection of algal species suitable for growing on urban wastewater, wastewater pretreatment and algal-bacterial cooperation, were carefully reviewed and discussed in this work. In this review, particular emphasis is placed on understanding of the main mechanisms driving formation of microalgal products (such as biofuels, biogas, etc.) and how they are affected by different environmental factors in selected species. Lastly, the quantitative information gathered from the articles were used to estimate the potential benefits gained from microalgae grown on urban wastewater in Campania Region, a region sometimes criticized for poor wastewater management.

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.

Advanced nonlinear control strategies for a fermentation bioreactor used for ethanol production

This study addresses the design of advanced control schemes implemented for a continuous fermentation process used to produce ethanol. Due to the inaccuracy of the models that express this complex process, a feasible controller is required to maximize the production of ethanol and to minimize its environmental impact, despite the existence of some significant uncertainties. Therefore, novel estimation and control schemes are designed and tested. These schemes are adaptive control laws including nonlinear estimation algorithms: a sliding mode observer to estimate the unknown influent concentration, but also state observers and parameter estimators used to estimate the unknown states and kinetics. Since the temperature is an important factor for an efficient operation of the process, an algorithm for temperature control in the bioreactor is also developed. To verify the control algorithms effectiveness, several tests performed via numerical simulations under realistic conditions are 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.