Algal biomass valorisation to high-value chemicals and bioproducts: Recent advances, opportunities and challenges

Recent developments and challenges in algal protein and peptide extraction strategies, functional and technological properties, bioaccessibility, and commercial applications

The burgeoning demand for protein, exacerbated by population growth and recent disruptions in the food supply chain, has prompted a rapid exploration of sustainable protein alternatives. Among these alternatives, algae stand out for their environmental benefits, rapid growth, and rich protein content. However, the widespread adoption of algae-derived proteins faces significant challenges. These include issues related to harvesting, safety, scalability, high cost, standardization, commercialization, and regulatory hurdles. Particularly daunting is the efficient extraction of algal proteins, as their resilient cell walls contain approximately 70% of the protein content, with conventional methods accessing only a fraction of this. Overcoming this challenge necessitates the development of cost-effective, scalable, and environmentally friendly cell disruption techniques capable of breaking down these rigid cell walls, often laden with viscous polysaccharides. Various approaches, including physical, chemical, and enzymatic methods, offer potential solutions, albeit with varying efficacy depending on the specific algal strain and energy transfer efficiency. Moreover, there remains a pressing need for further research to elucidate the functional, technological, and bioaccessible properties of algal proteins and peptides, along with exploring their diverse commercial applications. Despite these obstacles, algae hold considerable promise as a sustainable protein source, offering a pathway to meet the escalating nutritional demands of a growing global population. This review highlights the nutritional, technological, and functional aspects of algal proteins and peptides while underscoring the challenges hindering their widespread adoption. It emphasizes the critical importance of establishing a sustainable trajectory for food production, with algae playing a pivotal role in this endeavor.

Towards Lipid from Microalgae: Products, Biosynthesis, and Genetic Engineering

Microalgae can convert carbon dioxide into organic matter through photosynthesis. Thus, they are considered as an environment-friendly and efficient cell chassis for biologically active metabolites. Microalgal lipids are a class of organic compounds that can be used as raw materials for food, feed, cosmetics, healthcare products, bioenergy, etc., with tremendous potential for commercialization. In this review, we summarized the commercial lipid products from eukaryotic microalgae, and updated the mechanisms of lipid synthesis in microalgae. Moreover, we reviewed the enhancement of lipids, triglycerides, polyunsaturated fatty acids, pigments, and terpenes in microalgae via environmental induction and/or metabolic engineering in the past five years. Collectively, we provided a comprehensive overview of the products, biosynthesis, induced strategies and genetic engineering in microalgal lipids. Meanwhile, the outlook has been presented for the development of microalgal lipids industries, emphasizing the significance of the accurate analysis of lipid bioactivity, as well as the high-throughput screening of microalgae with specific lipids.

Direct and robust citramalate production from brown macroalgae using fast-growing Vibrio sp. dhg

Brown macroalgae is a promising feedstock for biorefinery owing to its high biomass productivity and contents of carbohydrates such as alginate and mannitol. However, the limited availability of microbial platforms efficiently catabolizing the brown macroalgae sugars has restricted its utilization. In this study, the direct production of citramalate, an important industrial compound, was demonstrated from brown macroalgae by utilizing Vibrio sp. dhg, which has a remarkably efficient catabolism of alginate and mannitol. Specifically, citramalate synthase from Methanocaldococcus jannaschii was synthetically expressed, and competing pathways were removed to maximally redirect the carbon flux toward citramalate production. Notably, a resulting strain, VXHC, produced citramalate up to 9.8 g/L from a 20 g/L mixture of alginate and mannitol regardless of their ratios. Citramalate was robustly produced even when diverse brown macroalgae were provided directly. Collectively, this study showcased the high potential of brown macroalgae biorefinery using Vibrio sp. dhg.

Using algae in Li-ion batteries: A sustainable pathway toward greener energy storage

This paper reviews and analyzes the innovations and advances in using algae and their derivatives in different parts of Li-ion batteries. Applications in Li-ion battery anodes, electrolytes, binders, and separators were discussed. Algae provides a sustainable feedstock for different materials that can be used in Li-ion batteries, such as carbonaceous material, biosilica, biopolymers, and other materials that have unique micro- and nano-structures that act as biotemplates for composites structure design. Natural materials and biotemplates provided by algae have various advantages, such as electrochemical and thermal stability, porosity that allows higher storage capacity, nontoxicity, and other properties discussed in the paper. Results reveal that despite algae and its derivatives being a promising renewable feedstock for different applications in Li-ion batteries, more research is yet to be performed to evaluate its feasibility of being used in the industry.

Advances in Algomics technology: Application in wastewater treatment and biofuel production

Advanced sustainable bioremediation is gaining importance with rising global pollution. This review examines microalgae's potential for sustainable bioremediation and process enhancement using multi-omics approaches. Recently, microalgae-bacterial consortia have emerged for synergistic nutrient removal, allowing complex metabolite exchanges. Advanced bioremediation requires effective consortium design or pure culture based on the treatment stage and specific roles. The strain potential must be screened using modern omics approaches aligning wastewater composition. The review highlights crucial research gaps in microalgal bioremediation. It discusses multi-omics advantages for understanding microalgal fitness concerning wastewater composition and facilitating the design of microalgal consortia based on bioremediation skills. Metagenomics enables strain identification, thereby monitoring microbial dynamics during the treatment process. Transcriptomics and metabolomics encourage the algal cell response toward nutrients and pollutants in wastewater. Multi-omics role is also summarized for product enhancement to make algal treatment sustainable and fit for sustainable development goals and growing circular bioeconomy scenario.

Oscillatoria sancta Cultivation Using Fruit and Vegetable Waste Formulated Media and Its Potential as a Functional Food: Assessment of Cultivation Optimization

The most cost-effective technique to cultivate microalgae is with low-cost resources, like fruit and vegetable peels. This study examined the viability of culturing microalgae (Oscillatoria sancta PCC 7515) isolated from a waterlogged region of Punjab, India, in a low-cost medium (fruit and vegetable waste peels) for pharmaceutical use. 16S rRNA sequencing discovered O. sancta PCC 7515. Fruit and vegetable peels were mineralized and chemically analyzed. At a 5% Bacillus flexus concentration, fruit and vegetable peels were liquefied at room temperature for 24 h. Response Surface Methodology (RSM) was used to assess and improve important cultural variables. The RSM predicted the best results at 10 pH, 30 days of incubation, 5% inoculum concentration, and 5% fruit and vegetable waste liquid leachate. The optimum conditions yielded more biomass than the basal conditions (0.8001 g/L). O. sancta PCC 7515 produced more lipids, proteins, Chl a, and Chl b in a formulated alternate medium than standard media. This study shows that O. sancta PCC 7515 may thrive on fruit and vegetable peel media. Fruit and vegetable waste (FVW) media assure low-cost microalgae-based functional foods.

An integrated approach for the extraction of lipids from marine macroalgae consortium using RSM optimization and thermo-kinetic analysis

This work presents an integrated approach for the extraction of lipids from marine macroalgae using RSM optimization and thermo-kinetic analysis. The lipids were extracted from marine macroalgal biomass using a Soxhlet extractor. The Soxhlet extraction parameters, including temperature (60-80 °C), solvent-to-algae ratio (3:1-7:1), algal particle size (0.05-0.25 mm), and extraction time (60-180 min), were optimized using RSM to achieve the maximum possible lipid extraction yield from marine macroalgae. The highest lipid extraction yield of 12.76% was obtained using the optimized conditions, which included an extraction temperature of 72 °C, a solvent-to-algae ratio of 5:1, an algal particle size of 0.16 mm, and an extraction time of 134 min. The kinetic analysis revealed an activation energy of 52.79 kJ mol-1 for the Soxhlet extraction process. The thermodynamic analysis of the Soxhlet extraction process demonstrated the following results: ΔH = 49.98 kJ mol-1, ΔS = -128.24 J K-1 mol-1, and ΔG = 93.98 kJ mol-1. The GC-MS analysis confirmed that the extracted algal lipids exhibited a composition of 14.20% palmitic acid, 4.89% stearic acid, and 76.97% oleic acid. The physiochemical analysis ensured that the extracted algal lipids possess excellent qualities, making them desirable for sustainable biofuel production.

Hydrothermal conversion of biomass to fuels, chemicals and materials: A review holistically connecting product properties and marketable applications

Biomass is a renewable and carbon-neutral resource with good features for producing biofuels, biochemicals, and biomaterials. Among the different technologies developed to date to convert biomass into such commodities, hydrothermal conversion (HC) is a very appealing and sustainable option, affording marketable gaseous (primarily containing H₂, CO, CH₄, and CO₂), liquid (biofuels, aqueous phase carbohydrates, and inorganics), and solid products (energy-dense biofuels (up to 30 MJ/kg) with excellent functionality and strength). Given these prospects, this publication first-time puts together essential information on the HC of lignocellulosic and algal biomasses covering all the steps involved. Particularly, this work reports and comments on the most important properties (e.g., physiochemical and fuel properties) of all these products from a holistic and practical perspective. It also gathers vital information addressing selecting and using different downstream/upgrading processes to convert HC reaction products into marketable biofuels (HHV up to 46 MJ/kg), biochemicals (yield >90 %), and biomaterials (great functionality and surface area up to 3600 m²/g). As a result of this practical vision, this work not only comments on and summarizes the most important properties of these products but also analyzes and discusses present and future applications, establishing an invaluable link between product properties and market needs to push HC technologies transition from the laboratory to the industry. Such a practical and pioneering approach paves the way for the future development, commercialization and industrialization of HC technologies to develop holistic and zero-waste biorefinery processes.

Hydrothermal Processing of Agar Waste to Levulinic acid and Fermentation of Hydrolysate to Bioethanol

Increasing global energy consumption and depleting fossil-fuel supplies prompted the search for green-alternatives. This study focuses on conversion of waste agar using different acids/alkalis (0.5% and 1%) as catalysts under varied temperature and time towards galactose (Gal), 5-hydroxymethylfurfural (HMF) and levulinic acid (LA) production in a sequential reaction. The optimized process for agar depolymerisation was achieved using 1 % acid (H₂SO₄/HCl) catalysed conditions with a maximum of 11 g/L Gal yield (121 °C; 15 min). Increase in temperature (150 °C) and time (180 min) with 1% HCl/H₂SO₄ catalyst resulted in improved LA production along with Gal and HMF. The hydrolysis process was optimised for the selective production of LA (10 g/L) at 175 °C; 180 min. Further, galactose-rich hydrolysates were assessed for bioethanol fermentation using Saccharomyces cerevisiae and resulted 3 g/L ethanol. Thus, the study comprehensively demonstrates waste agar utilization to yield biochemicals/fuels in a circular bio-based economy approach.

Application of regression and artificial neural network analysis of Red-Green-Blue image components in prediction of chlorophyll content in microalgae

This study presented a novel methodology to predict microalgae chlorophyll content from colour models using linear regression and artificial neural network. The analysis was performed using SPSS software. Type of extractant solvents and image indexes were used as the input data for the artificial neural network calculation. The findings revealed that the regression model was highly significant, with high R² of 0.58 and RSME of 3.16, making it a useful tool for predicting the chlorophyll concentration. Simultaneously, artificial neural network model with R² of 0.66 and low RMSE of 2.36 proved to be more accurate than regression model. The model which fitted to the experimental data indicated that acetone was a suitable extraction solvent. In comparison to the cyan-magenta-yellow-black model in image analysis, the red-greenblue model offered a better correlation. In short, the estimation of chlorophyll concentration using prediction models are rapid, more efficient, and less expensive.

Emerging trends in the pretreatment of microalgal biomass and recovery of value-added products: A review

Microalgae are a promising source of raw material (i.e., proteins, carbohydrates, lipids, pigments, and micronutrients) for various value-added products and act as a carbon sink for atmospheric CO₂. The rigidity of the microalgal cell wall makes it difficult to extract different cellular components for its applications, including biofuel production, food and feed supplements, and pharmaceuticals. To improve the recovery of products from microalgae, pretreatment strategies such as biological, physical, chemical, and combined methods have been explored to improve whole-cell disruption and product recovery efficiency. However, the diversity and uniqueness of the microalgal cell wall make the pretreatment process more species-specific and limit its large-scale application. Therefore, advancing the currently available technologies is required from an economic, technological, and environmental perspective. Thus, this paper provides a state-of-art review of the current trends, challenges, and prospects of sustainable microalgal pretreatment technologies from a microalgae-based biorefinery concept.

Current trends in the pretreatment of microalgal biomass for efficient and enhanced bioenergy production

Biofuels from microalgal biomass is among some of the promising sustainable energy technologies that can significantly replace the dependence on fossil fuels worldwide due to potentiality to lower CO₂ emissions. Nevertheless, the extraction of biomolecules for biofuel generation is inhibited by the rigidity of the cellular structure of microalgal biomass. Various pretreatment strategies have been evaluated for their efficacy in microalgal cell wall disruption to enhance microalgal bioenergy production. However, the efficiency of the pretreatment methods depend on the particular species being treated due to the inherent variability of the composition of the cell wall. This paper reviews pretreatment strategies (mainly novel physical, chemical and physicochemical) employed in bioenergy generation from microalgal biomass, address existing constraints and provides prospects for economic and industrial-scale production. The authors have also discussed the different pretreatment methods used for biodiesel, bioethanol, and biohydrogen production.

Recent advances in lignocellulosic and algal biomass pretreatment and its biorefinery approaches for biochemicals and bioenergy conversion

As the global demand for sustainable energy increases, lignocellulosic (such as agricultural residues, forest biomass, municipal waste, and dedicated energy crops) and algal (including macroalgae and microalgae) biomass have attracted considerable attention, because of their high availability of carbohydrates. This is a potential feedstock to produce biochemical and bioenergy. Pretreatment of biomass can disrupt their complex structure, increasing conversion efficiency and product yield. Therefore, this review comprehensively discusses recent advances in different pretreatments (physical, chemical, physicochemical, and biological pretreatments) for lignocellulosic and algal biomass and their biorefining methods. Life cycle assessment (LCA) which enables the quantification of the environmental impact assessment of a biorefinery also be introduced. Biorefinery processes such as raw material acquisition, extraction, production, waste accumulation, and waste conversion are all monitored under this concept. Nevertheless, there still exist some techno-economic barriers during biorefinery and extensive research is still needed to develop cost-effective processes.

Potential use of algae for the bioremediation of different types of wastewater and contaminants: Production of bioproducts and biofuel for green circular economy

Remediation by algae is a very effective strategy for avoiding the use of costly, environmentally harmful chemicals in wastewater treatment. Recently, industries based on biomass, especially the bioenergy sector, are getting increasing attention due to their environmental acceptability. However, their practical application is still limited due to the growing cost of raw materials such as algal biomass, harvesting and processing limitations. Potential use of algal biomass includes nutrients recovery, heavy metals removal, COD, BOD, coliforms, and other disease-causing pathogens reduction and production of bioenergy and valuable products. However, the production of algal biomass using the variable composition of different wastewater streams as a source of growing medium and the application of treated water for subsequent use in agriculture for irrigation has remained a challenging task. The present review highlights and discusses the potential role of algae in removing beneficial nutrients from different wastewater streams with complex chemical compositions as a biorefinery concept and subsequent use of produced algal biomass for bioenergy and bioactive compounds. Moreover, challenges in producing algal biomass using various wastewater streams and ways to alleviate the stress caused by the toxic and high concentrations of nutrients in the wastewater stream have been discussed in detail. The technology will be economically feasible and publicly accepted by reducing the cost of algal biomass production and reducing the loaded or attached concentration of micropollutants and pathogenic microorganisms. Algal strain improvement, consortium development, biofilm formation, building an advanced cultivation reactor system, biorefinery concept development, and life-cycle assessment are all possible options for attaining a sustainable solution for sustainable biofuel production. Furthermore, producing valuable compounds, including pharmaceutical, nutraceutical and pigment contents generated from algal biomass during biofuel production, could also help reduce the cost of wastewater management by microalgae.

Cell disruption and lipid extraction from Chlorella species for biorefinery applications: Recent advances

Chlorella is a promising microalga for CO₂-neutral biorefinery that co-produces drop-in biofuels and multiple biochemicals. Cell disruption and selective lipid extraction steps are major technical bottlenecks in biorefinement because of the inherent robustness and complexity of algal cell walls. This review focuses on the state-of-the-art achievements in cell disruption and lipid extraction methods for Chlorella species within the last five years. Various chemical, physical, and biological approaches have been detailed theoretically, compared, and discussed in terms of the degree of cell wall disruption, lipid extractability, chemical toxicity, cost-effectiveness, energy use, scalability, customer preferences, environment friendliness, and synergistic combinations of different methods. Future challenges and prospects of environmental-friendly and efficient extraction technologies are also outlined for practical applications in sustainable Chlorella biorefineries. Given the diverse industrial applications of Chlorella, this review may provide useful information for downstream processing of the advanced biorefineries of other algae genera.

Microalgal remediation and valorisation of polluted wastewaters for zero-carbon circular bioeconomy

Overexploitation of natural resources to meet human needs has considerably impacted CO₂ emissions, contributing to global warming and severe climatic change. This review furnishes an understanding of the sources, brutality, and effects of CO₂ emissions and compelling requirements for metamorphosis from a linear to a circular bioeconomy. A detailed emphasis on microalgae, its types, properties, and cultivation are explained with significance in attaining a zero-carbon circular bioeconomy. Microalgal treatment of a variety of wastewaters with the conversion of generated biomass into value-added products such as bio-energy and pharmaceuticals, along with agricultural products is elaborated. Challenges encountered in large-scale implementation of microalgal technologies for low-carbon circular bioeconomy are discussed along with solutions and future perceptions. Emphasis on the suitability of microalgae in wastewater treatment and its conversion into alternate low-carbon footprint bio-energies and value-added products enforcing a zero-carbon circular bioeconomy is the major focus of this review.

Algal biorefinery towards decarbonization: Economic and environmental consideration

Algae biomass contains various biological elements, including lipids, proteins, and carbohydrates, making it a viable feedstock for manufacturing biofuels. However, the biggest obstacle to commercializing algal biofuels is their high production costs, primarily related to an algae culture. The extraction of additional high value added bioproducts from algal biomass is thus required to increase the economic viability of producing algae biofuel. This study aims to discuss the economic benefits of a zero-carbon economy and an environmentally sustainable algae resource in decarbonizing the environment through the manufacture of algal-based biofuels from algae biomass for a range of potential uses. In addition, research on the algae biorefineries, with an emphasis on case studies for various cultivation methods, as well as the commercialization of biofuel and bioenergy. Overall, the algal biorefinery offers fresh potential for synthesizing various products.

Advances in the utilisation of carbon-neutral technologies for a sustainable tomorrow: A critical review and the path forward

Global industrialisation and overexploitation of fossil fuels significantly impact greenhouse gas emissions, resulting in global warming and other environmental problems. Hence, investigations on capturing, storing, and utilising atmospheric CO2 create novel technologies. Few microorganisms, microalgae, and macroalgae utilise atmospheric CO2 for their growth and reduce atmospheric CO2 levels. Activated carbon and biochar from biomasses also capture CO2. Nanomaterials such as metallic oxides, metal-organic frameworks, and MXenes illustrate outstanding adsorption characteristics, and convert CO2 to carbon-neutral fuels, creating a balance between CO2 production and elimination, thus zeroing the carbon footprint. The need for a paradigm shift from fossil fuels and promising technologies on renewable energies, carbon capture mechanisms, and carbon sequestration techniques that help reduce CO2 emissions for a better tomorrow are reviewed to achieve the world's sustainable development goals. The challenges and possible solutions with future perspectives are also discussed.

Recent developments in biorefining of macroalgae metabolites and their industrial applications - A circular economy approach

The macroalgal industry is expanding, and the quest for novel ingredients to improve and develop innovative products is crucial. Consumers are increasingly looking for natural-derived ingredients in cosmetic products that have been proven to be effective and safe. Macroalgae-derived compounds have growing popularity in skincare products as they are natural, abundant, biocompatible, and renewable. Due to their high biomass yields, rapid growth rates, and cultivation process, they are gaining widespread recognition as potentially sustainable resources better suited for biorefinery processes. This review demonstrates macroalgae metabolites and their industrial applications in moisturizers, anti-aging, skin whitening, hair, and oral care products. These chemicals can be obtained in combination with energy products to increase the value of macroalgae from an industrial perspective with a zero-waste approach by linking multiple refineries. The key challenges, bottlenecks, and future perspectives in the operation and outlook of macroalgal biorefineries were also discussed.

Improving the content of high value compounds in Nordic Desmodesmus microalgal strains

Nordic Desmodesmus microalgal strains (2-6) and (RUC-2) were exposed to abiotic stress (light and salt) to enhance lipids and carotenoids. The biomass output of both strains increased by more than 50% during light stress of 800 μmol m^-2 s^-1 compared to control light. The biomass of Desmodesmus sp. (2-6) contained most lipids (15% of dry weight) and total carotenoids (16.6 mg g^-1) when grown at moderate light stress (400 μmol m^-2 s^-1), which further could be enhanced up to 2.5-fold by salinity stress. Desmodesmus sp. (RUC-2) exhibited maximal lipid (26.5%) and carotenoid (43.8 mg L^-1) content at light intensities of 400 and 100 μmol m^-2 s^-1, respectively. Salinity stress stimulated lipid accumulation by 39%. Nordic Desmodesmus strains therefore are not only able to tolerate stress conditions, but their biomass considerably improves under stress. These strains have high potential to be used in algal bio-factories on low-cost medium like Baltic seawater.

Advances in algal biomass pretreatment and its valorisation into biochemical and bioenergy by the microbial processes

Urbanization and pollution are the major issues of the current time own to the exhaustive consumption of fossil fuels which have a detrimental effect on the nation's economies and air quality due to greenhouse gas (GHG) emissions and shortage of energy reserves. Algae, an autotrophic organism provides a green substitute for energy as well as commercial products. Algal extracts become an efficient source for bioactive compounds having anti-microbial, anti-oxidative, anti-inflammatory, and anti-cancerous potential. Besides the conventional approach, residual biomass from any algal-based process might act as a renewable substrate for fermentation. Likewise, lignocellulosic biomass, algal biomass can also be processed for sugar recovery by different pre-treatment strategies like acid and alkali hydrolysis, microwave, ionic liquid, and ammonia fiber explosion, etc. Residual algal biomass hydrolysate can be used as a feedstock to produce bioenergy (biohydrogen, biogas, methane) and biochemicals (organic acids, polyhydroxyalkanoates) via microbial fermentation.