Improvement of hydrogen production from Chlorella sp. biomass by acid-thermal pretreatment

Pretreatments of lignocellulosic and algal biomasses for sustainable biohydrogen production: Recent progress, carbon neutrality, and circular economy

Lignocellulosic and algal biomasses are known to be vital feedstocks to establish a green hydrogen supply chain toward achieving a carbon-neutral society. However, one of the most pressing issues to be addressed is the low digestibility of these biomasses in biorefinery processes, such as dark fermentation, to produce green hydrogen. To date, various pretreatment approaches, such as physical, chemical, and biological methods, have been examined to enhance feedstock digestibility. However, neither systematic reviews of pretreatment to promote biohydrogen production in dark fermentation nor economic feasibility analyses have been conducted. Thus, this study offers a comprehensive review of current biomass pretreatment methods to promote biohydrogen production in dark fermentation. In addition, this review has provided comparative analyses of the technological and economic feasibility of existing pretreatment techniques and discussed the prospects of the pretreatments from the standpoint of carbon neutrality and circular economy.

Algal biomass to biohydrogen: Pretreatment, influencing factors, and conversion strategies

Hydrogen has gained attention as an alternative source of energy because of its non-polluting nature as on combustion it produces only water. Biological methods are eco-friendly and have benefits in waste management and hydrogen production simultaneously. The use of algal biomass as feedstock in dark fermentation is advantageous because of its low lignin content, high growth rate, and carbon-fixation ability. The major bottlenecks in biohydrogen production are its low productivity and high production costs. To overcome these issues, many advances in the area of biomass pretreatment to increase sugar release, understanding of algal biomass composition, and development of fermentation strategies for the complete recovery of nutrients are ongoing. Recently, mixed substrate fermentation, multistep fermentation, and the use of nanocatalysts to improve hydrogen production have increased. This review article evaluates the current progress in algal biomass pretreatment, key factors, and possible solutions for increasing hydrogen production.

Bioethanol production from defatted biomass of Nannochloropsis oculata microalgae grown under mixotrophic conditions

In order to improve the economic feasibility and environmental sustainability of microalgal bioethanol production, a nontoxic, copious agricultural waste, sugarcane bagasse aqueous extract (SBAE) was used for cultivating Nannochloropsis oculata microalga (NNO-1 UTEX Culture LB 2164) as potential sources of substitutes for traditional nutrition to reduce the costs in cultivation through acid digestion and enzymatic treatment before being fermented by Saccharomyces cerevisiae (NRRLY-2034). The primary target of this research was to find out the ethanol from hydrolysate of the defatted biomass of N. oculata grown mixotrophically on SBAE and CO₂ as carbon sources. For acid hydrolysis (AH), the highest carbohydrate yield 252.84 mg/g DW has been obtained with 5.0% (v/v) H₂SO₄ at 121 °C for 15 min for defatted biomass cultivated mixotrophically on sugarcane bagasse aqueous extract (SBAE) regarding 207.41 mg/g DW for defatted biomass cultivated autotrophically (control treatment). Whereas, the highest levels of reducing sugars has been obtained with 4.0% (v/v) H₂SO₄ 157.47±1.60 mg/g DW for defatted biomass cultivated mixotrophically compared with 135.30 mg/g DW for the defatted control treatment. The combination of acid hydrolysis 2.0% (v/v) H₂SO₄ followed by enzymatic treatment (AEH) increased the carbohydrate yields to 268.53 mg/g DW for defatted biomass cultivated mixotrophically on SBAE regarding 177.73 mg/g DW for the defatted control treatment. However, the highest levels of reducing sugars have been obtained with 3.0% (v/v) H₂SO₄ followed by enzyme treatment that gave 232.39±1.77 for defatted biomass cultivated mixotrophically on SBAE and 150.75 mg/g DW for the defatted control treatment. The sugar composition of the polysaccharides showed that glucose was the principal polysaccharide sugar (60.7-62.49%) of N. oculata defatted biomass. Fermentation of the hydrolysates by Saccharomyces cerevisiae for the acid pretreated defatted biomass samples gave ethanol yield of 0.86 g/L (0.062 g/g sugar consumed) for control and 1.17 g/L (0.069 g/g sugar consumed) for SBAE mixotrophic. Whereas, the maximum ethanol yield of 6.17±0.47 g/L (0.26±0.11 g/g sugar consumed) has been obtained with samples from defatted biomass grown mixotrophically (SBAE mixotrophic) pretreated with acid coupled enzyme hydrolysis.

Strategies and advances in the pretreatment of microalgal biomass

Modification of structural components, especially the cell wall, through adequate pretreatment strategies is critical to the bioconversion efficiency of algal biomass to biorefinery products. Over the years, several physical, physicochemical, chemical and green pretreatment methods have been developed to achieve maximum productivity of desirable by-products to sustain a circular bioeconomy. The effectiveness of the pretreatment methods is however, species specific due to diversity in the innate nature of the microalgal cell wall. This review provides a comprehensive overview of the most notable and promising pretreatment strategies for several microalgae species. Methods including the application of stress, ultrasound, electromagnetic fields, pressure, heat as well as chemical solvents (ionic liquids, supercritical fluids, deep eutectic solvents etc.) have been detailed and analyzed. Enzyme and hydrolytic microorganism based green pretreatment methods have also been reviewed. Metabolic engineering of microorganisms for product specificity and lower inhibitors can be a future breakthrough in microalgal pretreatment.

A critical look into different salt removal treatments for the production of high value pigments and fatty acids from marine microalgae Chlorella vulgaris (NIOT-74)

The prime challenge in seawater culture of microalgae for high value biomolecules production is presence of salt. Hence, twelve different salt removal treatments were evaluated for their impact on the lutein, total carotenoid, chlorophyll yields and fatty acid profile of marine microalgae Chlorella vulgaris (NIOT-74). The effectiveness of different treatments on salt removal was also visualized with the aid of Scanning Electron Microscope (SEM). Among the tested treatments, washing the algal biomass with 0.5 % HCl augmented the lutein (11.56 mg/g) and total carotenoid yield (60.88 mg/g) 1.82 and 1.86 fold respectively, in comparison to untreated control. Highest chlorophyll content (30.64 mg/g) was noticed in the distilled water wash treatment. Different salt removal treatments also impacted the fatty acid profile and degree of unsaturation of the fatty acids significantly. This study thus, signified the importance of salt removal treatments for the commercial production of biomolecules from marine microalgae cultured in natural seawater.

Valorization of microalgal biomass for biohydrogen generation: A review

Third generation biomass, i.e. microalgae, has emerged as a promising alternative to first and second generation biomass for biohydrogen production. However, its utilization is still low at present, due to several reasons including the strong and rigidity of the microalgal cell wall that limit the hydrolysis efficiency during dark fermentation (DF) and photofermentation (PF) processes. To improve the utilization efficiency of microalgal biomass, it is crucial that important aspects related to the production of the biomass and the following processes are elaborated. Thus, this article provides detailed overview of algal strains, cultivation, and harvesting. It also presents recent research and detailed information on microalgal biomass pretreatment, and biohydrogen production through DF, PF, and co-digestion of microalgal biomass with organic materials. Furthermore, factors affecting fermentation processes performance and the use of molecular techniques in biohydrogen production are presented. This review also discusses challenges and future prospects towards biohydrogen production from microalgal biomass.

Anaerobic digestion of microalgal biomass for bioenergy production, removal of nutrients and microcystin: current status

Using renewable microalgal biomass as active feedstocks for biofuels and bioproducts is explored to substitute petroleum-based fuels and chemicals. In the last few years, the importance of microalgae biomass has been realized as a renewable feedstock due to several positive attributes associated with it. Biorefinery via anaerobic digestion (AD) of microalgal biomass is a promising and sustainable method to produce value-added chemicals, edible products and biofuels. Microalgal biomass pretreatment is a significant process to enhance methane production by AD. Findings on the AD microbial community's variety and organization can give novel in turn on digester steadiness and presentation. This review presents a vital study of the existing facts on the AD microbial community and AD production. Co-digestion of microalgal biomass with different co-substrates was used in AD to enhance biogas production, and the process was economically viable with improved biodegradability. Microcystins, which are produced by toxic cyanobacterial blooms, create a severe hazard to environmental health. Anaerobic biodegradation is an effective method to degrade the microcystins and convert into nontoxic products. However, for the cost-effective conversion of biomass to energy and other beneficial byproducts, additional highly developed research is still required for large-scale AD of microalgal biomass.

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.

Pretreatment of microalgal biomass for efficient biohydrogen production - Recent insights and future perspectives

Biohydrogen is a plausible alternative fuel solution for the contemporary issues regarding global warming and the steadily increasing greenhouse gas emissions, because of its high energy content and carbon-free combustion properties. Hydrogen does not exist in its natural state and the current hydrogen production technologies (steam methane reforming, water splitting) are energy-intensive, accompanied by a huge carbon footprint. Dark fermentative hydrogen production by anaerobic hydrogen-producing bacteria is a green, sustainable and emission-free pathway for hydrogen production. Microalgal biomass is considered as the third generation biofuel feedstock and is receiving academic and industrial research attention for its carbon sequestration abilities. This review discusses in detail about the pretreatment methods that could be adapted for microalgal biomass for effective biohydrogen production. Microalgal cell wall structure and the associated polymeric carbohydrates that offer certain recalcitrance are critically analyzed and future research perspectives are presented.

Algal biorefinery models with self-sustainable closed loop approach: Trends and prospective for blue-bioeconomy

Microalgae due to its metabolic versatility have received a focal attention in the biorefinery and bioeconomy context. Microalgae products have broad and promising application potential in the domain of renewable fuels/energy, nutraceutical, pharmaceuticals and cosmetics. Biorefining of microalgal biomass in a circular loop with an aim to maximize resource recovery is being considered as one of the sustainable option that will have both economical and environmental viability. The expansive scope of microalgae cultivation with self-sustainability approach was discussed in this communication in the framework of blue-bioeconomy. Microalgae based primary products, cultivation strategies, valorization of microalgae biomass for secondary products and integrated biorefinery models for the production of multi-based products were discussed. The need and prospect of self-sustainable models in closed loop format was also elaborated.