Algae biorefinery: Review on a broad spectrum of downstream processes and products

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.

Chlamydomonas reinhardtii and Microbacterium forte sp. nov., a mutualistic association that favors sustainable hydrogen production

A naturally occurring multispecies bacterial community composed of Bacillus cereus and two novel bacteria (Microbacterium forte sp. nov. and Stenotrophomonas goyi sp. nov.) has been identified from a contaminated culture of the microalga Chlamydomonas reinhardtii. When incubated in mannitol- and yeast extract-containing medium, this bacterial community can promote and sustain algal hydrogen production up to 313 mL H2·L-1 for 17 days and 163.5 mL H2·L-1 for 25 days in high-cell (76.7 μg·mL-1 of initial chlorophyll) and low-cell density (10 μg·mL-1 of initial chlorophyll) algal cultures, respectively. In low-cell density algal cultures, hydrogen production was compatible with algal growth (reaching up to 60 μg·mL-1 of chlorophyll). Among the bacterial community, M. forte sp. nov. was the sole responsible for the improvement in hydrogen production. However, algal growth was not observed in the Chlamydomonas-M. forte sp. nov. consortium during hydrogen-producing conditions (hypoxia), suggesting that the presence of B. cereus and S. goyi sp. nov. could be crucial to support the algal growth during hypoxia. Still, under non‑hydrogen producing conditions (aerobiosis) the Chlamydomonas-M. forte sp. nov. consortium allowed algal growth (up to 40 μg·mL-1 of chlorophyll) and long-term algal viability (>45 days). The genome sequence and growth tests of M. forte sp. nov. have revealed that this bacterium is auxotroph for biotin and thiamine and unable to use sulfate as sulfur source; it requires S-reduced forms such as cysteine and methionine. Cocultures of Chlamydomonas reinhardtii and M. forte sp. nov. established a mutualistic association: the alga complemented the nutrient deficiencies of the bacterium, while the bacterium released ammonium (0.19 mM·day-1) and acetic acid (0.15 mM·day-1) for the alga. This work offers a promising avenue for photohydrogen production concomitant with algal biomass generation using nutrients not suitable for mixotrophic algal growth.

Potential applications of microalgae-bacteria consortia in wastewater treatment and biorefinery

The use of microalgae-bacteria consortia (MBC) for wastewater treatment has garnered attention as their interactions impart greater environmental adaptability and stability compared with that obtained by only microalgae or bacteria use, thereby improving the efficiency of pollutant removal and bio-product productivity. Additionally, the value-added bio-products produced via biorefineries can improve economic competitiveness and environmental sustainability. Therefore, this review focuses on the interaction between microalgae and bacteria that leads to nutrient exchange, gene transfer and signal transduction to comprehensively understand the interaction mechanisms underlying their strong adaptability. In addition, it includes recent research in which MBC has been efficiently used to treat various wastewater. Moreover, the review summarizes the use of MBC-produced biomass in a biorefining context to produce biofuel, biomaterial, high-value bio-products and bio-fertilizer. Overall, more effort is needed to identify the symbiotic mechanism in MBC to provide a foundation for circular bio-economy and environmentally friendly development programmes.

Strategies and challenges to enhance commercial viability of algal biorefineries for biofuel production

The rise in energy consumption would quadruple in the coming century and the, existing energy resources might be insufficient to meet the demand of the growing population. An alternative and sustainable energy resource is therefore needed to address the fossil fuel deficiency. The utility of microalgae strains in the aspect of biorefinery has been in research for quite some time. Algal biorefinery is an alternate way of renewable energy however even after decades of research it still suffers from commercialization bottlenecks. The current manuscript reviews the scenarios where the innovation needs an ignition for its commercialization. This review discusses the prospects of up-scale cultivation, and harvesting algal biomass for biorefineries. It narrates algal biorefinery hurdles that can be solved using integrated technology approach, life cycle assessment and applications of nanotechnology. The review also sheds light upon the ties of algal biorefineries with its economic viability.

Innovation in lignocellulosics dewatering and drying for energy sustainability and enhanced utilization of forestry, agriculture, and marine resources - A review

Efficient utilization of forestry, agriculture, and marine resources in various manufacturing sectors requires optimizing fiber transformation, dewatering, and drying energy consumption. These processes play a crucial role in reducing the carbon footprint and boosting sustainability within the circular bioeconomy framework. Despite efforts made in the paper industry to enhance productivity while conserving resources and energy through lower grammage and higher machine speeds, reducing thermal energy consumption during papermaking remains a significant challenge. A key approach to address this challenge lies in increasing dewatering of the fiber web before entering the dryer section of the paper machine. Similarly, the production of high-value-added products derived from alternative lignocellulosic feedstocks, such as nanocellulose and microalgae, requires advanced dewatering techniques for techno-economic viability. This critical and systematic review aims to comprehensively explore the intricate interactions between water and lignocellulosic surfaces, as well as the leading technologies used to enhance dewatering and drying. Recent developments in technologies to reduce water content during papermaking, and advanced dewatering techniques for nanocellulosic and microalgal feedstocks are addressed. Existing research highlights several fundamental and technical challenges spanning from the nano- to macroscopic scales that must be addressed to make lignocellulosics a suitable feedstock option for industry. By identifying alternative strategies to improve water removal, this review intends to accelerate the widespread adoption of lignocellulosics as feasible manufacturing feedstocks. Moreover, this review aims to provide a fundamental understanding of the interactions, associations, and bonding mechanisms between water and cellulose fibers, nanocellulosic materials, and microalgal feedstocks. The findings of this review shed light on critical research directions necessary for advancing the efficient utilization of lignocellulosic resources and accelerating the transition towards sustainable manufacturing practices.

Artificial intelligence and machine learning tools for high-performance microalgal wastewater treatment and algal biorefinery: A critical review

The increased water scarcity, depletion of freshwater resources, and rising environmental awareness are stressing for the development of sustainable wastewater treatment processes. Microalgae-based wastewater treatment has resulted in a paradigm shift in our approach toward nutrient removal and simultaneous resource recovery from wastewater. Wastewater treatment and the generation of biofuels and bioproducts from microalgae can be coupled to promote the circular economy synergistically. A microalgal biorefinery transforms microalgal biomass into biofuels, bioactive chemicals, and biomaterials. The large-scale cultivation of microalgae is essential for the commercialization and industrialization of microalgae biorefinery. However, the inherent complexity of microalgal cultivation parameters regarding physiological and illumination parameters renders it challenging to facilitate a smooth and cost-effective operation. Artificial intelligence (AI)/machine learning algorithms (MLA) offer innovative strategies for assessing, predicting, and regulating uncertainties in algal wastewater treatment and biorefinery. The current study presents a critical review of the most promising AI/MLAs that demonstrate a potential to be applied in microalgal technologies. The most commonly used MLAs include artificial neural networks, support vector machine, genetic algorithms, decision tree, and random forest algorithms. Recent developments in AI have made it possible to combine cutting-edge techniques from AI research fields with microalgae for accurate analysis of large datasets. MLAs have been extensively studied for their potential in microalgae detection and classification. However, the ML application in microalgal industries, such as optimizing microalgae cultivation for increased biomass productivity, is still in its infancy. Incorporating smart AI/ML-enabled Internet of Things (IoT) based technologies can help the microalgal industries to operate effectively with minimum resources. Future research directions are also highlighted, and some of the challenges and perspectives of AI/ML are outlined. As the world is entering the digitalized industrial era, this review provides an insightful discussion about intelligent microalgal wastewater treatment and biorefinery for researchers in the field of microalgae.

Synthesis and Characterization of a New Alginate/Carrageenan Crosslinked Biopolymer and Study of the Antibacterial, Antioxidant, and Anticancer Performance of Its Mn(II), Fe(III), Ni(II), and Cu(II) Polymeric Complexes

Natural polysaccharides are essential to a wide range of fields, including medicine, food, and cosmetics, for their various physiochemical and biological properties. However, they still have adverse effects limiting their further applications. Consequently, possible structural modifications should be carried out on the polysaccharides for their valorization. Recently, polysaccharides complexed with metal ions have been reported to enhance their bioactivities. In this paper, we synthesized a new crosslinked biopolymer based on sodium alginate (AG) and carrageenan (CAR) polysaccharides. The biopolymer was then exploited to form complexes with different metal salts including MnCl₂·4H₂O, FeCl₃·6H₂O, NiCl₂·6H₂O, and CuCl₂·2H₂O. The four polymeric complexes were characterized by Fourier-transform infrared spectroscopy (FT-IR), elemental analysis, ultraviolet-visible spectroscopy (UV-Vis), magnetic susceptibility, molar conductivity methods, and thermogravimetric analysis. The X-ray crystal structure of the Mn(II) complex is tetrahedral and belongs to the monoclinic crystal system with the space group P121/n1. The Fe(III) complex is octahedral and crystal data fit with the cubic crystal system with the space group Pm-3m. The Ni(II) complex is tetrahedral and crystal data correspond to the cubic crystal arrangement with the space group Pm-3m. The data estimated for the Cu(II) polymeric complex revealed that it is tetrahedral and belongs to the cubic system with the space group Fm-3m. The antibacterial study showed significant activity of all the complexes against both Gram-positive bacteria (Staphylococcus aureus and Micrococcus luteus) and Gram-negative (Escherichia coli and Salmonella typhimurium) pathogenic strains. Similarly, the various complexes revealed an antifungal activity against Candida albicans. The Cu(II) polymeric complex recorded a higher antimicrobial activity with an inhibitory zone reaching 4.5 cm against Staphylococcus aureus bacteria and the best antifungal effect of 4 cm. Furthermore, higher antioxidant values of the four complexes were obtained with DPPH scavenging activity varying from 73 to 94%. The two more biologically effective complexes were then selected for the viability cell assessments and in vitro anticancer assays. The polymeric complexes revealed excellent cytocompatibility with normal human breast epithelial cells (MCF10A) and a high anticancer potential with human breast cancer cells (MCF-7) which increase significantly in a dose-dependent manner.

Recent Progress on Tailoring the Biomass-Derived Cellulose Hybrid Composite Photocatalysts

Biomass-derived cellulose hybrid composite materials are promising for application in the field of photocatalysis due to their excellent properties. The excellent properties between biomass-derived cellulose and photocatalyst materials was induced by biocompatibility and high hydrophilicity of the cellulose components. Biomass-derived cellulose exhibited huge amount of electron-rich hydroxyl group which could promote superior interaction with the photocatalyst. Hence, the original sources and types of cellulose, synthesizing methods, and fabrication cellulose composites together with applications are reviewed in this paper. Different types of biomasses such as biochar, activated carbon (AC), cellulose, chitosan, and chitin were discussed. Cellulose is categorized as plant cellulose, bacterial cellulose, algae cellulose, and tunicate cellulose. The extraction and purification steps of cellulose were explained in detail. Next, the common photocatalyst nanomaterials including titanium dioxide (TiO₂), zinc oxide (ZnO), graphitic carbon nitride (g-C₃N₄), and graphene, were introduced based on their distinct structures, advantages, and limitations in water treatment applications. The synthesizing method of TiO₂-based photocatalyst includes hydrothermal synthesis, sol-gel synthesis, and chemical vapor deposition synthesis. Different synthesizing methods contribute toward different TiO₂ forms in terms of structural phases and surface morphology. The fabrication and performance of cellulose composite catalysts give readers a better understanding of the incorporation of cellulose in the development of sustainable and robust photocatalysts. The modifications including metal doping, non-metal doping, and metal-organic frameworks (MOFs) showed improvements on the degradation performance of cellulose composite catalysts. The information and evidence on the fabrication techniques of biomass-derived cellulose hybrid photocatalyst and its recent application in the field of water treatment were reviewed thoroughly in this review paper.

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.

Life cycle assessment of microalgal biorefinery: A state-of-the-art review

Microalgal biorefineries represent an opportunity to economically and environmentally justify the production of bioproducts. The generation of bioproducts within a biorefinery system must quantitatively demonstrate its viability in displacing traditional fossil-based refineries. To this end, several works have conducted life cycle analyses on microalgal biorefineries and have shown technological bottlenecks due to energy-intensive processes. This state-of-the-art review covers different studies that examined microalgal biorefineries through life cycle assessments and has identified strategic technologies for the sustainable production of microalgal biofuels through biorefineries. Different metrics were introduced to supplement life cycle assessment studies for the sustainable production of microalgal biofuel. Challenges in the comparison of various life cycle assessment studies were identified, and the future design choices for microalgal biorefineries were established.

Furfural degradation and its effect on Rhodosporidium toruloides-1588 during microbial growth and lipid accumulation

The presence of furfural in the hydrolysates obtained from lignocellulosic biomass sources represents an enormous challenge during their fermentation because furfural is a toxic compound for different microorganisms. Rhodosporidium toruloides-1588 can grow and accumulate lipids using wood hydrolysate as a substrate containing up to 1 g/L of furfural. In this study, the capacity of R. toruloides-1588 to grow and accumulate lipids using furfural without glucose in the media has been observed. R. toruloides-1588 degraded up to 3 g/L of furfural into furfuryl alcohol (1.8 g/L) and 2-furoic acid (0.9 g/L). Furthermore, R. toruloides-1588 accumulated 52% and 30% of its dry weight into lipids using YM media and YM media without glucose, respectively. Fatty acids such as palmitic, stearic and oleic were the most abundant. Finally, R. toruloides-1588 could potentially utilize furfural as a carbon source.

A Systematic Review on Waste as Sustainable Feedstock for Bioactive Molecules—Extraction as Isolation Technology

In today’s linear economy, waste streams, environmental pollution, and social–economic differences are increasing with population growth. The need to develop towards a circular economy is obvious, especially since waste streams are composed of valuable compounds. Waste is a heterogeneous and complex matrix, the selective isolation of, for example, polyphenolic compounds, is challenging due to its energy efficiency and at least partially its selectivity. Extraction is handled as an emerging technology in biorefinery approaches. Conventional solid liquid extraction with organic solvents is hazardous and environmentally unfriendly. New extraction methods and green solvents open a wider scope of applications. This research focuses on the question of whether these methods and solvents are suitable to replace their organic counterparts and on the definition of parameters to optimize the processes. This review deals with the process development of agro-food industrial waste streams for biorefineries. It gives a short overview of the classification of waste streams and focuses on the extraction methods and important process parameters for the isolation of secondary metabolites.

Solar spectral management for natural photosynthesis: from photonics designs to potential applications

Photosynthesis is the most important biological process on Earth that converts solar energy to chemical energy (biomass) using sunlight as the sole energy source. The yield of photosynthesis is highly sensitive to the intensity and spectral components of light received by the photosynthetic organisms. Therefore, photon engineering has the potential to increase photosynthesis. Spectral conversion materials have been proposed for solar spectral management and widely investigated for photosynthesis by modifying the quality of light reaching the organisms since the 1990s. Such spectral conversion materials manage the photon spectrum of light by a photoconversion process, and a primary challenge faced by these materials is increasing their efficiencies. This review focuses on emerging spectral conversion materials for augmenting the photosynthesis of plants and microalgae, with a special emphasis on their fundamental design and potential applications in both greenhouse settings and microalgae cultivation systems. Finally, a discussion about the future perspectives in this field is made to overcome the remaining challenges.

Gasification Characteristics and Kinetics of Lipid-Extracted Nannochloropsis gaditana

A thermal behavior study of lipid-extracted Nannochloropsis gaditana (LEA) was performed in a thermogravimetric analyzer. The study was performed by heating the sample under different heating rates (5, 10, and 15 °C/min) from room temperature to 1000 °C using N2 gas as the medium. This is crucial for thermal stability studies in a kinetic control regime. The following three stages of chemical decompositions were found: (1) moisture removal (2) devolatilization (3) fixed carbon decomposition; maximum decomposition was observed at the second stage. Activation energies of the LEA were studied using the Flynn–Wall–Ozawa model and Kissinger–Akahira–Sunose model. Main sample decomposition was observed from 100–700 °C during volatile matter evaporation. The thermal behavior study findings were used for the gasification of the sample with air to study the effect of varying reaction parameters on the compositions of the synthesis gas yield. Maximum H2 yield was found at 700 °C and 0.7 g, which were 51.2 mol% and 50.6 mol%, respectively. From the study, it was found that LEA is suitable to be used as feedstock in gasification for synthesis gas production.

Microalgae-Based Biorefineries: Challenges and Future Trends to Produce Carbohydrate Enriched Biomass, High-Added Value Products and Bioactive Compounds

Simple Summary

Microalgae-based biorefineries allow the simultaneous production of microalgae biomass enriched in a particular macromolecule and high-added and low-value products if a proper selection of the microalgae species and the cultivation conditions are adequate for the purpose. This review discusses the challenges and future trends related to microalgae-based biorefineries stressing the multi-product approach and the use of raw wastewater or pretreated wastewater to improve the cost-benefit ratio of biomass and products. Emphasis is given to the production of biomass enriched in carbohydrates. Microalgae-bioactive compounds as potential therapeutical and health promoters are also discussed. Future and novel trends following the circular economy strategy are also discussed.

Abstract

Microalgae have demonstrated a large potential in biotechnology as a source of various macromolecules (proteins, carbohydrates, and lipids) and high-added value products (pigments, poly-unsaturated fatty acids, peptides, exo-polysaccharides, etc.). The production of biomass at a large scale becomes more economically feasible when it is part of a biorefinery designed within the circular economy concept. Thus, the aim of this critical review is to highlight and discuss challenges and future trends related to the multi-product microalgae-based biorefineries, including both phototrophic and mixotrophic cultures treating wastewater and the recovery of biomass as a source of valuable macromolecules and high-added and low-value products (biofertilizers and biostimulants). The therapeutic properties of some microalgae-bioactive compounds are also discussed. Novel trends such as the screening of species for antimicrobial compounds, the production of bioplastics using wastewater, the circular economy strategy, and the need for more Life Cycle Assessment studies (LCA) are suggested as some of the future research lines.

Recent biotechnological developments in reshaping the microalgal genome: A signal for green recovery in biorefinery practices

The use of renewable energy sources as a substitute for nonrenewable fossil fuels is urgently required. Algae biorefinery platform provides an excellent alternate to overcome future energy problems. However, to let this viable biomass be competent with existing feedstocks, it is necessary to exploit genetic manipulation and improvement in upstream and downstream platforms for optimal bio-product recovery. Furthermore, the techno-economic strategies further maximize metabolites production for biofuel, biohydrogen, and other industrial applications. The experimental methodologies in algal photobioreactor promote high biomass production, enriched in lipid and starch content in limited environmental conditions. This review presents an optimization framework combining genetic manipulation methods to simulate microalgal growth dynamics, understand the complexity of algal biorefinery to scale up, and identify green strategies for techno-economic feasibility of algae for biomass conversion. Overall, the algal biorefinery opens up new possibilities for the valorization of algae biomass and the synthesis of various novel products.

Latest Expansions in Lipid Enhancement of Microalgae for Biodiesel Production: An Update

Research progress on sustainable and renewable biofuel has gained motion over the years, not just due to the rapid reduction of dwindling fossil fuel supplies but also due to environmental and potential energy security issues as well. Intense interest in microalgae (photosynthetic microbes) as a promising feedstock for third-generation biofuels has grown over recent years. Fuels derived from algae are now considered sustainable biofuels that are promising, renewable, and clean. Therefore, selecting the robust species of microalgae with substantial features for quality biodiesel production is the first step in the way of biofuel production. A contemporary investigation is more focused on several strategies and techniques to achieve higher biomass and triglycerides in microalgae. The improvement in lipid enhancement in microalgae species by genetic manipulation approaches, such as metabolic or genetic alteration, and the use of nanotechnology are the most recent ways of improving the production of biomass and lipids. Hence, the current review collects up-to-date approaches for microalgae lipid increase and biodiesel generation. The strategies for high biomass and high lipid yield are discussed. Additionally, various pretreatment procedures that may aid in lipid harvesting efficiency and improve lipid recovery rate are described.

Microalgal metabolic engineering strategies for the production of fuels and chemicals

Microalgae are promising sustainable resources because of their ability to convert CO₂ into biofuels and chemicals directly. However, the industrial production and economic feasibility of microalgal bioproducts are still limited. As such, metabolic engineering approaches have been undertaken to enhance the productivities of microalgal bioproducts. In the last decade, impressive advances in microalgae metabolic engineering have been made by developing genetic engineering tools and multi-omics analysis. This review presents comprehensive microalgal metabolic pathways and metabolic engineering strategies for producing lipids, long chain-polyunsaturated fatty acids, terpenoids, and carotenoids. Additionally, promising metabolic engineering approaches specific to target products are summarized. Finally, this review discusses current challenges and provides future perspectives for the effective production of chemicals and fuels via microalgal metabolic engineering.

Economic and environmental sustainability analysis of seaweed farming: Monetizing carbon offsets of a brown algae cultivation system in Ireland

This paper examines the economic and environmental costs of seaweed cultivation (Alaria esculenta) in Ireland and evaluates the potential revenue made on the voluntary carbon offset market (VCOM). The life cycle assessment (LCA) results revealed the cultivation equipment with the polypropylene used for the cultivation lines contributes the highest share of impacts due to their replacement rate. This study suggests long-term employment of farm infrastructure and increased seaweed yield could enhance the environmental sustainability of the system. Moreover, life cycle costing (LCC) indicates the seaweed farm in Ireland is economically feasible over a 20-year lifespan. However, the revenue generated on the VCOM from the seaweed carbon assimilation was minimal, contributing to only 5% of the revenue. This study concludes that further development of the seaweed market with stabilized biomass prices and producing a range of viable products from seaweed biomass will be a major factor in the economic sustainability.

Sustainable valorization of algae biomass via thermochemical processing route: An overview

Biofuels have become an attractive energy source because of the growing energy demand and environmental issues faced by fossil fuel consumption. Algal biomass, particularly microalgae, has excellent potential as feedstock to be converted to bio-oil, biochar, and combustible syngas via thermochemical conversion processes. Third-generation biofuels from microalgal feedstock are the promising option, followed by the first-generation and second-generation biofuels. This paper provides a review of the applications of thermochemical conversion techniques for biofuel production from algal biomass, comprising pyrolysis, gasification, liquefaction, and combustion processes. The progress in the thermochemical conversion of algal biomass is summarized, emphasizing the application of pyrolysis for its benefits over other processes. The review also encompasses the challenges and perspectives associated with the valorization of microalgae to biofuels ascertaining the potential opportunities and possibilities of extending the research into this area.

Simultaneous nutrient recovery and algal biomass production from anaerobically digested sludge centrate using a membrane photobioreactor

This study aims to evaluate the performance of C. vulgaris microalgae to simultaneously recover nutrients from sludge centrate and produce biomass in a membrane photobioreactor (MPR). Microalgae growth and nutrient removal were evaluated at two different nutrient loading rates (sludge centrate). The results show that C. vulgaris microalgae could thrive in sludge centrate. Nutrient loading has an indiscernible impact on biomass growth and a notable impact on nutrient removal efficiency. Nutrient removal increased as the nutrient loading rate decreased and hydraulic retention time increased. There was no membrane fouling observed in the MPR and the membrane water flux was fully restored by backwashing using only water. However, the membrane permeability varies with the hydraulic retention time (HRT) and biomass concentration in the reactor. Longer HRT offers higher permeability. Therefore, it is recommended to operate the MPR system in lower HRT to improve the membrane resistance and energy consumption.

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.

Mixotrophic cultivation of isolated Messastrum gracile SVMIICT7: Photosynthetic response and product profiling

The isolated Messastrum gracile SVMIICT7 was mixotrophically cultivated in flat panel photobioreactor (FP-PBR) towards understanding the photosynthetic transient and product profile. Biomass productivity attained a maximum of 45 mg L^-1d^-1, with COD, nitrate and phosphate removal of 83.3%, 84.05%, and 74.98% respectively. Messastrum sp. showed good assimilation of proteins (124 mg g^-1) (w/w), carbohydrates (119 mg g^-1) (w/w) and lipids (26%) (w/w). The myristoleic acid (C14:1-39.1%) and heptadecanoic acid (C17:0-29.1%) are abundant fatty acids with therapeutic, food and feed applications. The cellular ultrastructure studies revealed facile arrangement of chloroplast and starch covered pyrenoids supporting increased carbohydrate accumulation. Photosystem II (PSII) [Y(II), ETR(II), Y(NPQ), and Y(NO)] and photosystem I (PSI) [Y(I), ETR(I), Y(NA), and Y(ND)] transients showed improved photosynthetic efficiency directing microalgae growth and biomass productivity. Higher Fv/Fm values indicates relatively good water splitting and carbon fixation at PSII and PSI facilitating improved photosynthetic electron transport and synthesis of value-added products thereby enabling bioeconomy.

Microwave hydrothermal processing of Undaria pinnatifida for bioactive peptides

Microwave hydrothermal processing was employed to obtain valuable gelling or bioactive fractions from U. pinnatifida, assessing the processing conditions following a biorefinery concept. It was identified a relevant impact on the antioxidant properties, sulfate, protein and oligosaccharides content, with the highest values above 200 °C, although the maximum in fucose was obtained at 160 °C. The lowest temperature involved the highest minerals and sulfate content of the solid phases. Rheology indicated that hydrothermal treatment at 160 °C is adequate to extract alginates with structural and viscoelastic properties similar to those commercially available. The incorporation of the hydrothermal residual solids in the proposed alginate matrices favored the development of systems with potential non-food applications applications. Selected extracts, after an intensification stage using ultrasound, featured interesting biological activities for two human cancer cell lines (A2780; HeLa 229) with percentage of cellular inhibition > 83 and 57%, without positive effects on A549 and HCT-116.

Seasonal characterization and quantification of biomolecules from sargassum collected from Mexican Caribbean coast - A preliminary study as a step forward to blue economy

Since 2014, Mexican Caribbean coasts have experienced an atypical massive arrival of pelagic Sargassum accumulated on the shores triggers economic losses, public health problems, and ecosystem damaging near the coastline. Mechanical harvesting has been implemented ending in landfills. Since Sargassum algae represent abundant biomass in tropical regions of the world, it has shown potential as a feedstock to supply bioprocesses focused on obtaining high-value compounds and bioproducts. Nevertheless, there is a lack of data on the biochemical composition of Sargassum biomass from Mexican Caribbean coasts to propose valorization pathways. This study conducted a biochemical and elemental characterization of Sargassum biomass and compared, through statistical analysis, the effect of the season (dry and wet), place of collection (from the beach and shallow water), and method of extraction (Microwave-Assisted Extraction and Enzyme Assisted Extraction) on biomass composition. The biomass composition, expressed in dry weight basis, revealed 5-7% moisture content, 24-31 % ash, 2.6-3.8 % lipids, 1.8-7.0 %, total carbohydrates, 3-11 % total proteins, 1.5-2.31 mgGAg-1 total phenolic compounds (TPC), 2.7-2.9 kcal g-1 calorific power, and metals such as As (30-146.3 ppm), Fe (16.5-45 ppm), P (197-472 ppm). The most influential factor on the compositional content of Sargassum biomass was the season of the year, followed by the extraction method and the place of collection of Sargassum. These results will elucidate information on the biotechnological potential of Sargassum biomass from the Mexican Caribbean, contributing to sustainability challenges of the region, minimizing waste, and making the most of resources.

Microwave hydrothermal processing of the invasive macroalgae Sargassum muticum within a green biorefinery scheme

This study deals with the multiproduct valorization of the invasive macroalgae Sargassum muticum within a green biorefinery concept using microwave hydrothermal treatment. Temperatures of 160 and 180 °C for 0-60 min (severities 1.62-3.54) were evaluated, allowing a recovery of a liquid phase rich in fucoidan-derived compounds (up to 4.81 g/L), oligomers and phenolics with antioxidant capacity (up to 2.85 g TE/L by ABTS assay), and a high-enzymatically susceptible solid (glucan to glucose conversion 76-100% in 9 h) suitable for bioethanol production (20.5 g/L in 18 h, corresponding to 96% ethanol yield). Moreover, energy consumption of the pretreatments' temperature-time binomial was evaluated showing significant differences, demonstrating the advantages of microwave as alternative heating pretreatment.

Microalgae for high-value products: A way towards green nutraceutical and pharmaceutical compounds

Microalgae is a renewable bioresource with the potential to replace the conventional fossil-based industrial production of organic chemicals and pharmaceuticals. Moreover, the microalgal biomass contains carotenoids, vitamins, and other biomolecules that are widely used as food supplements. However, the microalgal biomass production, their composition variations, energy-intensive harvesting methods, optimized bio-refinery routes, and lack of techno-economic analysis are the major bottleneck for the life-sized commercialization of this nascent bio-industry. This review discusses the microalgae-derived key bioactive compounds and their applications in different sectors for human health. Furthermore, this review proposes advanced strategies to enhance the productivity of bioactive compounds and highlight the key challenges associated with a safety issue for use of microalgae biomass. It also provides a detailed global scenario and market demand of microalgal bioproducts. In conclusion, this review will provide the concept of microalgal biorefinery to produce bioactive compounds at industrial scale platform for their application in the nutraceutical and pharmaceutical sector considering their current and future market trends.

Recent advances on food waste pretreatment technology via microalgae for source of polyhydroxyalkanoates

Polyhydroxyalkanoates (PHAs) are biodegradable and biocompatible polyester which are biosynthesized from the intracellular cells of microalgae through the cultivation of organic food waste medium. Before cultivation process, food waste must undergo several pre-treatment techniques such as chemical, biological, physical or mechanical in order to solubilize complex food waste matter into simpler micro- and macronutrients in which allow bio-valorisation of microalgae and food waste compound during the cultivation process. This work reviews four microalgae genera namely Chlamydomonas, Chlorella, Spirulina, and Botryococcus, are selected as suitable species due to rapid growth rate, minimal nutrient requirement, greater adaptability and flexibility prior to lower the overall production cost and maximized the production of PHAs. This study also focuses on the different mode of cultivation for the accumulation of PHAs followed by cell wall destabilization, extraction, and purification. Nonetheless, this review provides future insights into enhancing the productivity of bioplastic derived from microalgae towards low-cost, large-scale, and higher productivity of PHAs.

Emerging technologies for conversion of sustainable algal biomass into value-added products: A state-of-the-art review

Concerns regarding high energy demand and gradual depletion of fossil fuels have attracted the desire of seeking renewable and sustainable alternatives. Similar to but better than the first- and second-generation biomass, algae derived third-generation biorefinery aims to generate value-added products by microbial cell factories and has a great potential due to its abundant, carbohydrate-rich and lignin-lacking properties. However, it is crucial to establish an efficient process with higher competitiveness over the current petroleum industry to effectively utilize algal resources. In this review, we summarize the recent technological advances in maximizing the bioavailability of different algal resources. Following an overview of approaches to enhancing the hydrolytic efficiency, we review prominent opportunities involved in microbial conversion into various value-added products including alcohols, organic acids, biogas and other potential industrial products, and also provide key challenges and trends for future insights into developing biorefineries of marine biomass.

Empowering blue economy: From underrated ecosystem to sustainable industry

With increasing demand for resources to achieve global food-water-energy nexus and rapid decline in land-based sources, oceans represent both solution and boost to sustainable environment and economy. In addition to fundamental part of earth's ecosystem for uncatalogued diversity of life, oceans are undervalued economy powerhouse with gross marine product value. With sustainable management of existing assets including shipping, transportation, manufacturing, fisheries, tourism and exploration of new business like marine biotechnology and renewable energy, the ocean or blue economy has potential to fulfill sustainable development goals (SDG). In spite of recognition of blue economy as a new economic frontier, investments by existing industries and emergence of new ones are limited and less known, hence require more in depth attention and scientific understanding. In the present study, authors present a systematic comparative assessment of blue economy sectors with distinct challenges and strategies to be further explored and implemented for industrial deployment. The conceptualization of integrated routes of bio(economy) by the current study can act as gateway for key stakeholders, i.e. governance, bluepreneurs (scientists and industries) to prioritize technologies for sustainable applications of marine resources.

Advanced near-zero waste treatment of food processing wastewater with water, carbon, and nutrient recovery

A near-zero waste treatment system for food processing wastewater was developed and studied. The wastewater was treated using an anaerobic membrane bioreactor (AnMBR), polished using an outdoor photobioreactor for microalgae cultivation (three species were studied), and excess sludge was treated using hydrothermal carbonization. The study was conducted under arid climate conditions for one year (four seasons). The AnMBR reduced the total organic carbon by 97%, which was mostly recovered as methane (~57%) and hydrochar (~4%). Microalgal biomass productivity in the AnMBR effluent ranged from 0.25 to 0.8 g·L^-1·day^-1. Nitrogen (N) and phosphorous (P) uptake varied seasonally, from 18 to 45 mg·L^-1·day^-1 and up to 5 mg·L^-1·day^-1, respectively. N and P mass balance analysis demonstrated that the process was highly efficient in the recovery of nitrogen (~77%), and phosphorus (~91%). The performance of the microalgal culture changed among seasons because of climatic variation, as a result of variation in the wastewater chemistry, and possibly due to differences among the microalgal species. Effluent standards for irrigation use were met throughout the year and were achieved within two days in summer and 4.5 days in winter. Overall, the study demonstrated a near-zero waste discharge system capable of producing high-quality effluent, achieving nutrient and carbon recovery into microalgae biomass, and energy production as biogas and hydrochar.

Algae as an attractive source for cosmetics to counter environmental stress

In recent times, a considerable amount of evidence has come to light regarding the effect that air pollution has on skin conditions. The human skin is the chief protection we have against environmental harm, whether biological, chemical, or physical. The stress from these environmental factors, along with internal influences, can be a cause of skin aging and enlarged pores, thinner skin, skin laxity, wrinkles, fine lines, dryness, and a more fragile dermal layer. This knowledge has led to greater demand for skin cosmetics and a requirement for natural raw ingredients with a high degree of safety and efficiency in combating skin complications. Recent developments in green technology have made the employment of naturally occurring bioactive compounds more popular, and novel extraction methods have ensured that the use of these compounds has greater compatibility with sustainable development principles. Thus, there is a demand for investigations into efficient non-harmful naturally occurring raw ingredients; compounds derived from algae could be beneficial in this area. Algae, both macroalgae and microalgae, consists of waterborne photosynthetic organisms that are potentially valuable as they have a range of bioactive compounds in their composition. Several beneficial metabolites can be obtained from algae, such as antioxidants, carotenoids, mycosporine-like amino acids (MAA), pigments, polysaccharides, and scytonemin. Various algae strains are now widely employed in skincare products for various purposes, such as a moisturizer, anti-wrinkle agent, texture-enhancing agents, or sunscreen. This research considers the environmental stresses on human skin and how they may be mitigated using cosmetics created using algae; special attention will be paid to external factors, both generally and specifically (amongst them light exposure and pollutants).

A critical review on production of biopolymers from algae biomass and their applications

Algae is abundantly present in our ecosystems and can be easily extracted and used for production of biopolymers. Algae does not produce any anthropogenic, harmful effects, has a good growth rate, and cultivable in wastewater. This literature elucidates the potential of algae biomass by comparing various seaweed and microalgae strains. The routes for biopolymer production were portrayed and their novel methods of isolation such as microwave assisted, ultrasound assisted, and subcritical water assisted extraction are discussed in detail. These novel methods are observed to be highly efficient compared to conventional solvent extraction, with the microwave assisted and ultrasound assisted processes yielding 33% and 5% more biopolymer respectively than the conventional method. Biopolymers are used in variety of applications such as environmental remediation, adsorbent and antioxidant. Biopolymer is shown to be highly effective in the removal of potentially toxic elements and is seen to extract more than 40 mg PTE/g biopolymer.

Phase equilibria modeling of biorefinery-related systems: a systematic review

The search for sustainable ideas has gained prominence in recent decades at all levels of society since it has become imperative an economic, social, and environmental development in an integrated manner. In this context, biorefineries are currently present as the technology that best covers all these parameters, as they add the benefits of waste reuse, energy cogeneration, and fossil fuel substitution. Thus, the study of the various applicable biological matrices and exploring the technical capabilities of these processes become highly attractive. Thermodynamic modeling acts in this scenario as a fundamental tool for phase behavior predictions in process modeling, design, and optimization. Thus, this work aimed to systematize, using the PRISMA statement for systematic reviews, the information published between 2010 and 2020 on phase equilibria modeling in systems related to biorefineries to organize what is already known about the subject. As a result, 236 papers were categorized in terms of the year, country, type of phase equilibria, and thermodynamic model used. Also, the phase behavior predictions of different thermodynamic models under the same process conditions were qualitatively compared, establishing PC-SAFT as the model that best represents the great diversity of interest systems for biorefineries in a wide range of conditions.

Production of Phenyllactic Acid from Porphyra Residues by Lactic Acid Bacterial Fermentation

The concept of algae biorefinery is attracting attention because of the abundant valuable compounds within algal biomass. Phenyllactic acid (PhLA), a broad-spectrum antimicrobial organic acid that can be produced by lactic acid bacteria (LAB), is considered a potential biopreservative. In this study, a cascading biorefinery approach was developed to harvest PhLA from Porphyra residues by LAB fermentation. LAB strains were cultivated in de Man, Rogosa and Sharpe (MRS) broth to screen their ability to produce PhLA. As the strains of Lactobacillus plantarum KP3 and L. plantarum KP4 produced higher concentrations of PhLA at 0.09 mg/mL, these two strains were employed for fermentation. After phycobiliprotein extraction, the Porphyra residues, ultrafiltration eluate, phenylalanine (Phe) and yeast extract with a volume of 20 mL were inoculated with LAB strain KP3 and fermented at 37 °C for 120 h. The PhLA content of the fermented broth was 1.86 mg. To optimize the biorefinery process, the ultrafiltration eluate was replaced by commercial cellulase. Up to 4.58 mg of PhLA, which was 2.5 times greater than that produced from KP3 cultured in MRS broth, could be harvested. Taken together, the findings provide an optimized process for LAB fermentation, which is shown to be a feasible algae biorefinery approach to obtaining PhLA from Porphyra residues.

Multiproduct Microalgae Biorefineries Mediated by Ionic Liquids

Ionic liquids (ILs) are salts with low melting points that can be used as solvents for mild extraction and selective fractionation of biomolecules (e.g., proteins, carbohydrates, lipids, and pigments), enabling the valorisation of microalgal biomass in a multiproduct biorefinery concept, while maintaining the biomolecules' structural integrity and activity. Aqueous biphasic systems and emulsions stabilised by core-shell particles have been used to fractionate disrupted microalgal biomass into hydrophobic (lipids and pigments) and hydrophilic (proteins and carbohydrates) components. From nondisrupted biomass, the hydrophobic components can be directly extracted using ILs from intact cells, while the most fragile hydrophilic components can be obtained upon further mechanical cell disruption. These multiproduct biorefinery concepts will be discussed in an outlook on future separations using IL-based systems.

Substrate properties as controlling parameters in attached algal cultivation

There is growing interest in attached algae cultivation systems because they could provide a more cost- and energy-efficient alternative to planktonic (suspended algae) cultivation systems for many applications. However, attached growth systems have been far less studied than planktonic systems and have largely emphasized algae strains of most interest for biofuels. New algal biorefinery pathways have assessed the commercial potentials of algal biomass beyond biofuel production and placed more emphasis on value-added products from that biomass. Therefore, algal strain selection criteria and biomass cultivation methods need to be updated to include additional strains for improved efficiency. One possible way of improving attached cultivation systems is through engineering substrate surface characteristics to boost algal adhesion and enable strain selective algal colonization and growth. This review explores the effect of substrate chemical and topographical characteristics on the cultivation of attached algae. It also highlights the importance of considering algal community structure and attachment mechanisms in investigating attached algae systems using the example of filamentous algae found in algal turf scrubber (ATS™) systems. KEY POINTS : • Attached algal cultivation is a promising alternative to planktonic cultivation. • Performance increase results from tuning surface qualities of attachment substrates. • Attachment adaptation of periphytic algae has innate potential for cultivation.

Microbial co-culturing strategies for the production high value compounds, a reliable framework towards sustainable biorefinery implementation - an overview

The microbial co-cultures or consortia are a natural set of microorganisms formed from different species or the same species but different strains, in which members can interact with each other. The co-culture systems have wide variety of technological applications such as the production of foods, treatment of wastewater, removal of toxic substances, environmental recovery, and all these without the need to work in sterile conditions. Therefore, the need of understanding communication mechanisms between cell-to-cell within co-culture will allow to construct and to program their biological behavior from the use of complex substrates to produce biocompounds. The technology of co-culture systems enables the development of biorefinery platforms to obtain biofuels, and high value compounds through biomass transformation by sustainable process. This review focuses on understanding the roles of consortia microbial to design and built co-culture systems to produce high value compounds in terms a sustainable biorefinery.

Cationic cassava starch and its composite as flocculants for microalgal biomass separation

Commercial- and laboratory modified- cationic cassava starches and their composites with magnetic particles were examined for characteristics and separation efficiency. Scanning electron micrographs showed that cationic starch with an increasing degree of substitution (DS) value (0.0180 to 0.91) showed greater clumped polyhedral granules and became markedly enlarged with disintegrated boundaries. Zeta potential analysis revealed that the increase in the DS value in cationic starches resulted in an increase in positive charge. The maximum harvesting efficiency of 92.86 ± 0.46% was achieved when commercial cationic starch with DS 0.040 at 1.0 g L^-1 was added to the Chlorella sp. solution. The maximum recovery capacity (10.20 ± 0.16 g DCW g starch^-1) was recorded by using commercial cationic starch with DS 0.040 at a lower dosage of 0.1 g L^-1. Their composites showed lower separation efficiency than the commercial cationic starches. The results suggest that the commercial cationic cassava starch with 0.040 DS shows great potential as a flocculant for algal separation. This first report of using commercial cationic cassava starch as a flocculant provides a low cost and convenient process to separate algal cells from the culture medium. Moreover, uncontaminated magnetic particle biomass allows for wide range of algal utilization in food and pharmaceutical biotechnologies.