Insights into upstream processing of microalgae: A review

Enhanced accumulation of important bioproducts in Chlorella vulgaris through AGPase gene silencing coupled with polyethylene glycol treatment

Microalgae with rapid growth rate, ability to be cultivated in non-agricultural land, and producing numerous bioactive compounds have attracted attention for biofuel production and extraction of valuable co-products. This work focuses on the development of an industrially viable Chlorella vulgaris strain more suitable for biorefinery by downregulating the ADP-glucose pyrophosphorylase (AGPase) enzyme involved in starch biosynthesis. Transgenic lines of C. vulgaris generated through RNA interference (RNAi) demonstrated diminished starch content with an average of 6.7 % (dry cell weight, DCW) from 10 % (DCW) in the untransformed control, after 5 days of shake flask culture in tris acetate phosphate medium with 16 h:8 h light:dark cycle. Under the same growth condition, the total carbohydrate content decreased by an average of 23.5 %, while the lipid content and carotenoid level improved by an average of 19.3 % and 23 %, respectively, in the RNAi lines compared to the control. RNAi lines showed a higher yield of other important co-products, like protein and exopolysaccharide by an average of 30.6 % and 19.6 %, respectively, compared to the control. Interestingly, when polyethylene glycol (PEG) 6000 (0.5 % w/v) was supplemented in the culture medium of C. vulgaris grown till mid-log phase, RNAi lines along with the untransformed control exhibited enhanced level of valuable metabolites after 7 days of PEG treatment. The average carotenoid content of 25.5 μg/mg was recorded in PEG-treated RNAi lines compared to 15.34 μg/mg in PEG-untreated untransformed control alga. Under the similar growth condition, the average lipid content increased to 22.5 % (DCW) in PEG-treated RNAi lines compared to 16.28 % (DCW) in PEG-untreated untransformed control. Overall, the study encompasses use of genetic engineering tool in combination with the application of biochemical modulator PEG to divert the carbon flux from starch biosynthesis towards improved production of important metabolites in microalga.

Astaxanthin biosynthesis for functional food development and space missions

Astaxanthin (AXT), a natural carotenoid, has strong antioxidant and anti-ageing effects and can reduce ultraviolet light-induced damage to cells and DNA, stimulate the immune system, and improve cardiovascular disease prognosis. Despite its wide applications in the: nutraceutical, cosmetic, aquaculture, and pharmaceutical industries, AXT industrial production and application are hindered by natural source scarcity, low production efficiency, and high requirements. This review compares the qualitative differences of AXT derived from different natural sources, evaluates the upstream procedures for AXT expression in different chassis organisms, and investigates synthetic biology- and cell factory-based strategies for the industrial production of natural AXT. Synthetic biology is a promising novel strategy for reprogramming plants or microorganisms to produce AXT. Additionally, genetic engineering using cell factories extends beyond terrestrial applications, as it may contribute to the long-term sustainability of human health during space exploration and migration endeavors. This review provides a theoretical basis for the efficient and accurate genetic engineering of AXT from the microalga Haematococcuspluvialis, providing a valuable reference for future research on the biomanufacturing of AXT and other biological metabolites.

The microalgal sector in Europe: Towards a sustainable bioeconomy

Microalgae are a diverse group of photosynthetic microorganisms that can be exploited to produce sustainable food and feed products, alleviate environmental pollution, or sequester CO2 to mitigate climate change, among other uses. To optimize resource use and integrate industrial waste streams, it is essential to consider factors such as the biology and cultivation parameters of the microalgal strains, as well as the cultivation system and processing technologies employed. This paper reviews the main commercial applications of microalgae (including cyanobacteria) and examines the biological and biotechnological aspects critical to the sustainable processing of microalgal biomass and its derived compounds. We also provide an up-to-date overview of the microalgal sector in Europe considering the strain, cultivation system and commercial application. We have identified 146 different microalgal-derived products from 66 European microalgae producers, and 49 additional companies that provide services and technologies, such as optimization and scalability of the microalgal production. The most widely cultivated microalga is 'spirulina' (Limnospira spp.), followed by Chlorella spp. and Nannochloropsis spp., mainly for human consumption and cosmetics. The preferred cultivation system in Europe is the photobioreactor. Finally, we discuss the logistic and regulatory challenges of producing microalgae at industrial scale, particularly in the European Union, and explore the potential of new genomic techniques and bioprocessing to foster a sustainable bioeconomy in the microalgal sector.

Influence of Microalgal Cell Division Tendency on OD to DCW Conversion Factor and Chlorophyll Contents

As the importance of achieving a carbon-neutral society grow, research on the production of microalgal bioproducts have gained significant attention. Among the production processes, cultivation has emerged as a critical step as it determines overall productivity and feasibility. However, changes in cell properties during cultivation pose challenges, and there is currently no direct method to simultaneously assess cell growth, cellular division properties and cell enlargement. In this study, two green algae (Chlorella sp. ABC001 and Chlorella sp. HS2) were cultivated under photo and mixotrophic conditions to evaluate their growth, cell size profile, and changes in optical density. The mixotrophic condition resulted in 2.1 and 1.6-fold higher biomass yields than the photoautotrophic condition for ABC001 and HS2, respectively at day 4. Over the 4-day cultivation, cell sizes ranged from 2.51 to 5.57 μm for ABC001 and from 2.56 to 4.41 μm for HS2. By analyzing changes in the conversion factor between dry cell weight (DCW) and optical density (OD), it was observed that variations in the slope correlated with changes in cell size. Additionally, chlorophyll content fluctuated during cultivation, reaching maximum levels (14.34 and 16.58 μg/mg biomass, respectively) under phototrophic condition on day 2. This study highlights the relationship between cell division tendencies, DCW, cell size, and OD, demonstrating their critical role in determining cellular component content. For future optimization of algal product production processes, further research on these cellular differentiation mechanisms will be essential.

Biodegradable polymer films incorporating phycocyanin and polyhydroxybutyrate: advancing sustainable packaging technologies

The biocompatibility and degradability of poly hydroxybutyrate (PHB) make it an important material for food packaging. Poly hydroxybutyrate (PHB) is less flexible than typical polymers and more brittle due to its strong crystalline structure and low melting temperature. Furthermore, while poly hydroxybutyrate (PHB) does not have antibiotic capabilities, the inclusion of additional materials such as phycocyanin can boost its antibiotic and antifungal effects. This research focuses on increasing the yield of phycocyanin from the microalgae Spirulina and selecting an optimal green extraction method, and on the other hand, on the synthesis of biodegradable polymers derived from poly hydroxybutyrate in combination with phycocyanin for food packaging applications. This study shows that higher wavelengths of light, an optimal temperature of about 33 degrees Celsius, pH adjustments using changing the composition of the culture medium by increasing metal compounds and using non-invasive, green and inexpensive extraction methods significantly increase biomass production and phycocyanin yield, which in previous research only one to two of the above parameters were considered for protein extraction, which this study has addressed more comprehensively. Also, the synthesis of biodegradable polymers with special properties for packaging applications, including green polymer poly hydroxybutyrate, phycocyanin and beeswax, which has not been done in previous research on modifying the properties of poly hydroxybutyrate using these two research materials, so that with 60% phycocyanin in combination with poly hydroxybutyrate, they show promising properties for food packaging applications. By examining the physical and chemical properties, thermal strength and degradability of the polymer compared to pure poly hydroxybutyrate, it shows further improvement in addition to antibiotic and antifungal properties.

Enhancing Biomass and Lipid Production in Messastrum gracile Using Inorganic Carbon Substrates and Alternative Solvents for Lipid Extraction

Microalgae show promise as a biomass and bioproduct for applications in various industries. The cultivation of microalgae plays a crucial role in optimizing biomass yield and bioproduct accumulation. The provision of inorganic carbon substrates substantially enhances microalgal growth and lipid biosynthesis, resulting in marked increases in the production of biofuels and other bioproducts. This study examined biomass and lipid accumulation in Messastrum gracile IFRPD 1061 under inorganic stress conditions, previously unreported. M. gracile IFRPD 1061 was subjected to varying conditions of inorganic carbon substrates, 1-3 g·L-1 sodium carbonate and bicarbonate concentration, to enhance biomass and lipid accumulation. Optimal productivity levels were observed with sodium bicarbonate addition of 3 g·L-1 and 1 g·L-1 for biomass and lipids, resulting in productivities of 392.64 and 53.57 mg·L-1·d-1, respectively. Results underlined the effectiveness of sodium carbonate and bicarbonate as inorganic carbon sources for stimulating microalgal growth and enhancing the production of high-value products. The extraction of lipids from freeze-dried biomass of M. gracile IFRPD 1061 demonstrated optimal yield using methanol/hexane solvents compared with the control experiments. Lipid extraction yields using methanol/hexane were 42.18% and 46.81% from oven-dried and freeze-dried biomass, respectively. Lipids extracted from oven-dried M. gracile IFRPD 1061 using methanol/hexane/chloroform solvents indicated the potential of methanol/hexane as a solvent for lipid extraction from dry microalgal biomass using an ultrasonic-assisted technique. This study contributes valuable insights into maximizing biofuel and bioproduct production from microalgae, highlighting A. gracilis as a promising candidate for industrial applications.

Harnessing algal biomass for sustainable energy: cultivation, strain improvement, and biofuel production

The world faces pressing environmental challenges, including greenhouse gas emissions, global warming, climate change, and rising sea levels. Alongside, these issues, the depletion of fossil fuels has intensified the search for alternative energy sources. Algal biomass presents a promising long-term solution to these global problems. The quest for sustainable energy has driven significant research into algal biofuels as a viable alternative to fossil fuels. Algae offers several advantages as a feedstock for biofuel production, including high biomass yield, rapid growth rates, cost-effective cultivation, carbon dioxide fixation capabilities, and the potential to grow on non-arable land using non-potable water. This manuscript provides an overview of algal biomass cultivation using renewable feedstocks, identifies potential algal strains for biofuel production, and explores bioengineering advancements in algae. Additionally, strain improvement strategies to enhance biofuel yields are discussed. The review also addresses large-scale algal biomass cultivation for biofuel production, assesses its commercial viability, examines challenges faced by the biofuel industry, and outlines prospects for biofuel production using highly potent algal strains. By overcoming and addressing these challenges, algal biofuels have the potential to become a cornerstone of sustainable energy solutions.

Enhanced growth and metabolite production from a novel strain of Porphyridium sp

Microalgae are capable of generating numerous metabolites that possess notable biological activities and hold substantial promise for various industrial applications. Nevertheless, the taxonomic diversity of these photosynthetic microorganisms has not received thorough investigation. Using the 18S rRNA encoding gene, a recently discovered strain originating from the Tunisian coast (the governorate of Mahdia) was identified as a member of the Porphyridium genus. The growth response as well as the metabolite accumulation of Porphyridium sp. to different culture media (Pm, F/2, and Hemerick) was investigated over a period of 52 days. The highest biomass production was recorded with Pm medium (2 × 107 cell/mL). The apparent growth rates (µ) and the doubling time (Dt) were about 0.081 day-1 and 12.34 days, respectively. The highest chlorophyll a (0.678 ± 0.005 pg/cell), total carotenoids (0.18 ± 0.003 pg/cell), phycoerythrin (3.88 ± 0.003 pg/cell), and proteins (14.58 ± 0.35 pg/cell) contents were observed with F/2 medium. Cultivating Porphyridium sp. in both F/2 and Hemerick media yielded similar levels of starch accumulation. The Hemerick medium has proven to be the most suitable for the production of lipids (2.23% DW) and exopolysaccharides (5.41 ± 0.56 pg/cell).

A review on algal biomass dewatering and recovery of microalgal-based valuable products with different membrane technologies

Efficient microalgae harvesting and dewatering are critical processes for a range of applications, including the production of raw materials, nutritional supplements, pharmaceuticals, sustainable biofuels, and wastewater treatment. The optimization of these processes poses significant challenges due to the need for high efficiency and sustainability while managing costs and energy consumption. This review comprehensively addresses these challenges by focusing on the development and application of various membrane filtration technologies specifically designed for the effective harvesting and dewatering of algal biomass. Membrane filtration has emerged as a predominant method due to its ability to handle large volumes of microalgae with relatively low energy requirements. This review systematically examines the different membrane-based technologies and their effectiveness in recovering valuable components from algal biomass, such as lipids, proteins, and carbohydrates. The discussion begins with an overview of the physical characteristics of microalgae and their cultivation conditions, which are critical for understanding how these factors influence the performance of membrane filtration processes. Key aspects such as the features of algal cells, the presence of algal organic matter, and transparent exopolymer particles are explored in detail. The review also delves into various strategies for improving membrane antifouling properties, which are essential for maintaining the efficiency and longevity of the filtration systems. In addition, the advantages and disadvantages of different membrane techniques are reviewed, highlighting their respective performance in separating microalgae and dewatering. Finally, the review offers insights into future research directions and technological advancements that could further enhance the efficiency and sustainability of microalgae processing. This comprehensive evaluation aims to provide a thorough understanding of current membrane technologies, their applications, and the ongoing developments necessary to overcome existing limitations and improve overall process performance.

Effects of Culture Medium Enrichment with Zinc on Astaxanthin Accumulation in a New Strain of the Microalga Dysmorphococcus globosus

High Zn2+ concentrations in microalgal cells that produce astaxanthin as a feed additive can reduce the symptoms of malnutrition in aquatic animals. Therefore, in this study, we analysed the effect of Zn2+ in the culture medium on the growth of a newly isolated microalgal strain Dysmorphococcus globosus ZY24. Zn2+ and white light stress altered the pigment content in microalgal cells. In addition, high Zn2+ concentrations in the culture medium altered cell morphology and chlorophyll fluorescence and also increased intracellular Zn2+ accumulation. Further, an optimal Zn2+ concentration in the culture medium promoted the synthesis of astaxanthin and other pigments. When the concentration of Zn2+ was 45.5 mg L-1, Dysmorphococcus globosus ZY24 produced 0.31 mg g-1 astaxanthin, whereas the total zinc content of the microalgae was 4337 mg kg-1. This study confirmed that microalgae have a high capacity for Zn2+ enrichment, providing a theoretical basis for studying Zn2+ enrichment in microalgae. Furthermore, Zn2+ supplementation to stimulate astaxanthin production in microalgae is a practical method to enhance their nutritional value.

Antibiotic synergistic effect surge bioenergy potential and pathogen resistance of Chlorella variabilis biofilm

Tetracycline (TC) and ciprofloxacin (CF) induce a synergistic effect that alters the biochemical composition, leading to a decrease in the growth and photosynthetic efficiency of microalgae. But the current study provides a novel insight into stress-inducing techniques that trigger a change in macromolecules, leading to an increase in the bioenergy potential and pathogen resistance of Chlorella variabilis biofilm. The study revealed that in a closed system, a light intensity of 167 μmol/m2/s causes 93.5% degradation of TC and 16% degradation of CF after 7 days of exposure, hence availing the products for utilization by C. variabilis biofilm. The resistance to pathogens invasion was linked to 85% and 40% increase in the expression level of photosystem II oxygen-evolving enhancer protein 3 (PsbQ), and mitogen activated kinase (MAK) respectively. The results also indicate that a surge in light intensity triggers 49% increase in the expression level of lysophosphatidylcholine (LPC) (18:2), which is an important lipidomics that can easily undergo transesterification into bioenergy. The thermogravimetric result indicates that the biomass sample of C. variabilis biofilm cultivated under light intensity of 167 μmol/m2/s produces a higher residual mass of 45.5% and 57.5 under air and inert conditions, respectively. The Fourier transform infrared (FTIR) indicates a slight shift in the major functional groups, while the energy-dispersive X-ray spectroscopy (SEM-EDS) and X-ray fluorescence (XRF) indicate clear differences in the morphology and elemental composition of the biofilm biomass in support of the increase bioenergy potential of C. variabilis biofilm. The current study provides a vital understanding of a innovative method of cultivation of C. variabilis biofilm, which is resistant to pathogens and controls the balance between fatty acid and TAG synthesis leading to surge in bioenergy potential and environmental sustainability.

Improving Undernutrition with Microalgae

Undernutrition is an important global health problem, especially in children and older adults. Both reversal of maternal and child undernutrition and heathy ageing have become United Nations-supported global initiatives, leading to increased attention to nutritional interventions targeting undernutrition. One feasible option is microalgae, the precursor of all terrestrial plants. Most commercially farmed microalgae are photosynthetic single-celled organisms producing organic carbon compounds and oxygen. This review will discuss commercial opportunities to grow microalgae. Microalgae produce lipids (including omega-3 fatty acids), proteins, carbohydrates, pigments and micronutrients and so can provide a suitable and underutilised alternative for addressing undernutrition. The health benefits of nutrients derived from microalgae have been identified, and thus they are suitable candidates for addressing nutritional issues globally. This review will discuss the potential benefits of microalgae-derived nutrients and opportunities for microalgae to be converted into food products. The advantages of microalgae cultivation include that it does not need arable land or pesticides. Additionally, most species of microalgae are still unexplored, presenting options for further development. Further, the usefulness of microalgae for other purposes such as bioremediation and biofuels will increase the knowledge of these microorganisms, allowing the development of more efficient production of these microalgae as nutritional interventions.

Two-step polyhydroxybutyrate production from hydrogenic effluent by freshwater microalgae Coelastrella sp. KKU-P1 and Acutodesmus sp. KKU-P2 under mixotrophic cultivation

This study aimed to produce PHB using hydrogenic effluent discharged from the biohydrogen production process with freshwater microalgae including Coelastrella sp. KKU-P1, and Acutodesmus sp. KKU-P2. Batch experiments explored the influence of initial pH and hydrogenic effluent concentration, revealing optimal conditions at 10 % (v/v) effluent concentration and a pH of 6.5 for both KKU-P1 and KKU-P2. Subsequently, medium formulation and photoperiods were optimized to maximize biomass and PHB accumulation. The results showed that the optimal condition for PHB accumulation with KKU-P1 and KKU-P2 was nitrogen phosphorus (NP)-limited Bold's Basal Medium (BBM) under dark conditions. A two-step PHB accumulation in the upscale bioreactor was investigated under optimal conditions. The results showed that KKU-P1 achieved maximum PHB, protein, carbohydrate, and lipid contents of 4.57 %, 29.37 %, 24.76 %, and 13.21 %, respectively, whereas KKU-P2 achieved 6.35 %, 31.53 %, 16.16 %, and 4.77 %, respectively. Based on these findings, it appears that a mixotrophic approach under nutrient-limiting conditions is effective for PHB production in both KKU-P1 and KKU-P2 strains.

Applications of the Microalgae Chlamydomonas and Its Bacterial Consortia in Detoxification and Bioproduction

The wide metabolic diversity of microalgae, their fast growth rates, and low-cost production make these organisms highly promising resources for a variety of biotechnological applications, addressing critical needs in industry, agriculture, and medicine. The use of microalgae in consortia with bacteria is proving valuable in several areas of biotechnology, including the treatment of various types of wastewater, the production of biofertilizers, and the extraction of various products from their biomass. The monoculture of the microalga Chlamydomonas has been a prominent research model for many years and has been extensively used in the study of photosynthesis, sulphur and phosphorus metabolism, nitrogen metabolism, respiration, and flagellar synthesis, among others. Recent research has increasingly recognised the potential of Chlamydomonas-bacteria consortia as a biotechnological tool for various applications. The detoxification of wastewater using Chlamydomonas and its bacterial consortia offers significant potential for sustainable reduction of contaminants, while facilitating resource recovery and the valorisation of microalgal biomass. The use of Chlamydomonas and its bacterial consortia as biofertilizers can offer several benefits, such as increasing crop yields, protecting crops, maintaining soil fertility and stability, contributing to CO2 mitigation, and contributing to sustainable agricultural practises. Chlamydomonas-bacterial consortia play an important role in the production of high-value products, particularly in the production of biofuels and the enhancement of H2 production. This review aims to provide a comprehensive understanding of the potential of Chlamydomonas monoculture and its bacterial consortia to identify current applications and to propose new research and development directions to maximise their potential.

The Microalgae Chlamydomonas for Bioremediation and Bioproduct Production

The extensive metabolic diversity of microalgae, coupled with their rapid growth rates and cost-effective production, position these organisms as highly promising resources for a wide range of biotechnological applications. These characteristics allow microalgae to address crucial needs in the agricultural, medical, and industrial sectors. Microalgae are proving to be valuable in various fields, including the remediation of diverse wastewater types, the production of biofuels and biofertilizers, and the extraction of various products from their biomass. For decades, the microalga Chlamydomonas has been widely used as a fundamental research model organism in various areas such as photosynthesis, respiration, sulfur and phosphorus metabolism, nitrogen metabolism, and flagella synthesis, among others. However, in recent years, the potential of Chlamydomonas as a biotechnological tool for bioremediation, biofertilization, biomass, and bioproducts production has been increasingly recognized. Bioremediation of wastewater using Chlamydomonas presents significant potential for sustainable reduction in contaminants and facilitates resource recovery and valorization of microalgal biomass, offering important economic benefits. Chlamydomonas has also established itself as a platform for the production of a wide variety of biotechnologically interesting products, such as different types of biofuels, and high-value-added products. The aim of this review is to achieve a comprehensive understanding of the potential of Chlamydomonas in these aspects, and to explore their interrelationship, which would offer significant environmental and biotechnological advantages.

Antares I: a Modular Photobioreactor Suitable for Photosynthesis and Bioenergetics Research

Oxygenic photosynthesis is responsible for most of the fixation of atmospheric CO2. The microalgal community can transport atmospheric carbon into biological cycles in which no additional CO2 is created. This represents a resource to confront the actual climate change crisis. These organisms have evolved to adapt to several environments and different spectral distribution of light that may strongly influence their metabolism. Therefore, there is a need for development of photobioreactors specialized in addressing spectral optimization. Here, a multi-scale modular photobioreactor made from standard glass materials, ad hoc light circuits, and easily accessible, small commercial devices is described. The system is suitable to manage the principal culture variables of research in bioenergetics and photosynthesis. Its performance was tested by growing four evolutionary-distant microalgal species with different endosymbiotic scenarios: Chlamydomonas reinhardtii (Archaeplastida, green primary plastid), Polytomella parva (Archaeplastida, colorless plastid), Euglena gracilis (Discoba, green secondary plastid), and Phaeodactylum tricornutum (Stramenophiles, red secondary plastid). Our results show an improvement of biomass production, as compared to the traditional flask system. The modulation of the incident light spectra allowed us to observe a far-red adaptation in Euglena gracilis with a difference on paramylon production, and it also significantly increased the maximal cell density of the diatom species under green light. Together, these confirm that for photobioreactors with artificial light, manipulation of the light spectrum is a critical parameter for controlling the optimal performance, depending on the downstream goals.

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.

Pollutant Gases to Algal Animal Feed: Impacts of Poultry House Exhaust Air on Amino Acid Profile of Algae

Algae provide a rich source of proteins, lipids, vitamins, and minerals, making them valuable feed ingredients in animal nutrition. Beyond their nutritional benefits, algae have been recognized for their potential to mitigate the negative environmental impacts of poultry production. Poultry production is crucial for the global food supply but contributes to environmental concerns, particularly in terms of ammonia and carbon dioxide gas emissions. This study emphasizes the importance of reducing greenhouse gas and ammonia production in poultry operations by utilizing algae species suitable for animal consumption, highlighting the need for sustainable feed sources. This study investigated the effects of poultry exhaust air and culture conditions on the amino acid profiles of three microalgae species, namely, Scenedesmus sp. (AQUAMEB-60), Ankistrodesmus sp. (AQUAMEB-33), and Synechococcaceae (AQUAMEB 32). The experiments were conducted in a commercial broiler farm in Bursa, Turkey, focusing on reducing pollutant gas emissions and utilizing poultry exhaust air in algae cultivation. The highest protein content of 50.4% was observed in the biomass of Synechococcaceae with BBM and DI water. Scenedesmus sp. had the highest carbohydrate content of 33.4% cultivated with DI water. The algae biomass produced from Synechococcaceae growth with DI water was found to have the highest content of essential and nonessential amino acids, except for glutamic acid and glycine. The arsenic, cadmium, and mercury content showed variations within the following respective ranges: 1.076-3.500 mg/kg, 0.0127-0.1210 mg/kg, and 0.1330-0.0124 mg/kg. The overall operating costs for producing 1.0 g L-1 d-1 of dry algal biomass with the existing PBR system were $0.12-0.35 L-1 d-1, $0.10-0.26 L-1 d-1, and $0.11-0.24 L-1 d-1 for Scenedesmus sp., Ankistrodesmus sp., and Synechococcaceae, respectively. The operating cost of producing 1.0 g L-1 d-1 of protein was in the range of $0.25-0.88 L-1 d-1 for the three algae species. The results provide insights into the potential of algae as a sustainable feed ingredient in animal diets, emphasizing both environmental and economic considerations. The results demonstrated a considerable reduction in the production costs of dry biomass and protein when utilizing poultry house exhaust air, highlighting the economic viability and nutritional benefits of this cultivation method.

Influence of super-optimal light intensity on the acetic acid uptake and microalgal growth in mixotrophic culture of Chlorella sorokiniana in bubble-column photobioreactors

This study seeks to determine the influence of super-optimal light intensity on acetic acid uptake and its associated impact on the cellular composition of Chlorella sorokiniana in a semi-batch mixotrophic cultivation setup. Unicellular green microalga Chlorella sorokiniana is grown in a 1L bubble-column photobioreactor at light intensities from 6000 to 14,000 lx (≈81 to 189 µmol.photons.m-2.s-1). We find that microalgal acetic acid utilization reduces as illumination increases from an optimal 10,000 lx (≈135 µmol.photons.m-2.s-1) to a super-optimal zone (>10000 lx). This lowers microalgal growth (2.75 g/L) and acetic acid intake, which peak at 6 mL/L (10000 lx) and drop to 2 and 1 mL/L at 12,000 and 14,000 lx, respectively. Concurrently, the maximum lipid yield decreases from 0.66 g/L (10000 lx) to 0.54 g/L (12000 lx) and 0.42 g/L (14000 lx). Hence, super-optimal illumination not only disturbs phototrophy but also affects the heterotrophic component, creating an imbalance between the two.

Integrating wet stirred-bead milling for Tetraselmis suecica biorefinery: Operating parameters influence and specific energy efficiency

Stirred bead milling proved to be an efficient cell destruction technique in a biorefinery unit for the extraction of over 95 % of proteins and 60 % of carbohydrates from the green marine microalga Tetraselmis suecica. Optimum conditions, expressed in terms of metabolite yield and energy consumption, were found for average values of bead size and agitator rotation speed. The higher the microalgae concentration, up to 100 g.L-1, which is adequate for biofilm algae growth in an industrial unit, the more efficient the cell destruction process. Cell destruction rates and metabolite extraction yields are similar in pendular and recycling modes, but the pendular configuration reduces the residence time of the suspension in the grinding chamber, which is less costly. With regard to the cell destruction mechanism, it was concluded that bead shocks first damage cells by permeabilizing them, and that after a longer period, all cells are shredded and destroyed, forming elongated debris.

Regulation of microclimate and shading effects of microalgal photobioreactors on rooftops: Microalgae as a promising emergent for green roof technology

Urban areas remarkably affect global public health due to their emissions of greenhouse gases and poor air quality. Although urban areas only cover 2% of the Earth's surface, they are responsible for 80% of greenhouse gas emissions. Dense buildings limit vegetation, leading to increased air pollution and disruption of the local and regional carbon cycle. The substitution of urban gray roofs with microalgal green roofs has the potential to improve the carbon cycle by sequestering CO2 from the atmosphere. Microalgae can fix 15-50 times more CO2 than other types of vegetation. Advanced microalgal-based green roof technology may significantly accelerate the reduction of atmospheric CO2 in a more effective way. Microalgal green roofs also enhance air quality, oxygen production, acoustic isolation, sunlight absorption, and biomass production. This endeavor yields the advantage of simultaneously generating protein, lipids, vitamins, and a spectrum of valuable bioactive compounds, including astaxanthin, carotenoids, polysaccharides, and phycocyanin, thus contributing to a green economy. The primary focus of the current work is on analyzing the ecological advantages and CO2 bio-fixation efficiency attained through microalgal cultivation on urban rooftops. This study also briefly examines the idea of green roofs, clarifies the ecological benefits associated with them, discusses the practice of growing microalgae on rooftops, identifies the difficulties involved, and the positive aspects of this novel strategy.

Microalgal capture of carbon dioxide: A carbon sink or source?

The rapidly evolving global warming is triggering all levels of actions to reduce industrial carbon emissions, while capturing carbon dioxide of industrial origin via microalgae has attracted increasing attention. This article attempted to offer preliminary analysis on the carbon capture potential of microalgal cultivation. It was shown that the energy consumption-associated with operation and nutrient input could significantly contribute to indirect carbon emissions, making the microalgal capture of carbon dioxide much less effective. In fact, the current microalgae processes may not be environmentally sustainable and economically viable in the scenario where the carbon footprints of both upstream and downstream processing are considered. To address these challenging issues, renewable energy (e.g., solar energy) and cheap nutrient source (e.g., municipal wastewater) should be explored to cut off the indirect carbon emissions of microalgae cultivation, meanwhile produced microalgae, without further processing, should be ideally used as biofertilizer or aquafeeds for realizing complete nutrients recycling.

An optimized approach towards bio-capture and carbon dioxide sequestration with microalgae Phormidium valderianum using response surface methodology

As carbon dioxide emissions rise, there's need for alternative strategies, including microorganisms, to capture and mitigate them. The present study investigated on the capability and tolerance of microalgal strain, Phormidium valderianum to capture gaseous CO2 at varying levels (5-30 %). A biomass productivity of 0.0216 ± 0.027 gL-1day-1 and rate of CO2 fixation of 0.035 gL-1day-1 was obtained for 25 % CO2 concentration. From this study, it is evident that higher CO2 levels led to elevated carbohydrate concentration. In addition, protein concentration doubled with the introduction of 25 % CO2. In optimization studies, pH 10, 25 % CO2, and 200 mg/L of Ca(OH)2 concentration was found to be optimal for biomass growth. A higher rate of CO2 fixation of 0.315 gL-1day-1 was achieved at these optimum conditions using response surface methodology. Furthermore, the study demonstrated that microalgae, Phormidium valderianum has the potential to serve as a promising alternative for capturing CO2 emissions.

Enhancing the bio-prospective of microalgae by different light systems and photoperiods

Microalgae are a source of highly valuable bioactive metabolites and a high-potential feedstock for environmentally friendly and sustainable biofuel production. Recent research has shown that microalgae benefit the environment using less water than conventional crops while increasing oxygen production and lowering CO2 emissions. Microalgae are an excellent source of value-added compounds, such as proteins, pigments, lipids, and polysaccharides, as well as a high-potential feedstock for environmentally friendly and sustainable biofuel production. Various factors, such as nutrient concentration, temperature, light, pH, and cultivation method, effect the biomass cultivation and accumulation of high-value-added compounds in microalgae. Among the aforementioned factors, light is a key and essential factor for microalgae growth. Since photoautotrophic microalgae rely on light to absorb energy and transform it into chemical energy, light has a significant impact on algal growth. During micro-algal culture, spectral quality may be tailored to improve biomass composition for use in downstream bio-refineries and boost production. The light regime, which includes changes in intensity and photoperiod, has an impact on the growth and metabolic composition of microalgae. In this review, we investigate the effects of red, blue, and UV light wavelengths, different photoperiod, and different lighting systems on micro-algal growth and their valuable compounds. It also focuses on different micro-algal growth, photosynthesis systems, cultivation methods, and current market shares.

Removal and recovery of nutrients from septic tank wastewater using microalgae: Key factors and practical implications

Treatment of septic tank wastewater (STWW) with high concentrations of ammonium (NH4+) and total phosphorus (TP), is challenging in decentralized areas. Utilizing microalgae for STWW treatment can simultaneously recover nutrients in the form of high-value microalgal biomass. However, despite the potential benefits, microalgal treatment of STWW is rarely reported. Therefore, this work utilized bench-scale photobioreactors (PBR) to investigate different factors that could affect microalgal cultivation in STWW and treatment efficiency. Accordingly, it was observed that suspended solids present in STWW did not significantly affect the microalgae growth and nutrient removal efficiencies in bubble column PBR. On the other hand, the effect of endemic microorganism could not be verified in this study due to observed fungal contamination and change in nutrient profile of STWW after autoclave. Nevertheless, the highest microalgal growth and nutrient removal efficiencies of NH4+-N = 79.14% and TP = 41.11% were observed within 14 days of photoautotrophic cultivation in raw STWW. Further, 25 days of upscaled photoautotrophic cultivation in 4-L bubble column PBR was performed to study biomass yield, nutrient removal kinetics, and nutrient removal efficiency. Consequently, 0.75 g‧L-1 dry biomass was produced with improved removal efficiency of NH4+-N (96.16%), and TP (69.57%). Elemental analysis of biomass revealed that 62.99 ± 1.46 mg‧L-1 TN and 11.41 ± 1.42 mg‧L-1 TP were recovered. Further, 1.02 geq carbon dioxide (CO2) was bio-fixed with every liter of STWW treated. The findings of this study revealed that microalgae can be successfully utilized for the removal and recovery of nutrients from STWW.

Microalgae as a Sustainable Source of Antioxidants in Animal Nutrition, Health and Livestock Development

Microalgae are a renewable and sustainable source of bioactive compounds, such as essential amino acids, polyunsaturated fatty acids, and antioxidant compounds, that have been documented to have beneficial effects on nutrition and health. Among these natural products, the demand for natural antioxidants, as an alternative to synthetic antioxidants, has increased. The antioxidant activity of microalgae significantly varies between species and depends on growth conditions. In the last decade, microalgae have been explored in livestock animals as feed additives with the aim of improving both animals' health and performance as well as product quality and the environmental impact of livestock. These findings are highly dependent on the composition of microalgae strain and their amount in the diet. The use of carbohydrate-active enzymes can increase nutrient bioavailability as a consequence of recalcitrant microalgae cell wall degradation, making it a promising strategy for monogastric nutrition for improving livestock productivity. The use of microalgae as an alternative to conventional feedstuffs is becoming increasingly important due to food-feed competition, land degradation, water deprivation, and climate change. However, the cost-effective production and use of microalgae is a major challenge in the near future, and their cultivation technology should be improved by reducing production costs, thus increasing profitability.

Advancements of microalgal upstream technologies: Bioengineering and application aspects in the paradigm of circular bioeconomy

Sustainable energy transition has brought the attention towards microalgae utilization as potential feedstock due to its tremendous capabilities over its predecessors for generating more energy with reduced carbon footprint. However, the commercialization of microalgae feedstock remains debatable due to the various factors and considerations taken into scaling-up the conventional microalgal upstream processes. This review provides a state-of-the-art assessment over the recent developments of available and existing microalgal upstream cultivation systems catered for maximum biomass production. The key growth parameters and main cultivation modes necessary for optimized microalgal growth conditions along with the fundamental aspects were also reviewed and evaluated comprehensively. In addition, the advancements and strategies towards potential scale-up of the microalgal cultivation technologies were highlighted to provide insights for further development into the upstream processes aimed at sustainable circular bioeconomy.

Fundamental alteration of cellular biochemicals from attached microalgae onto palm kernel expeller waste upon optimizing the growth environment in forming adhesion complex

Changing the growth environment for microalgae can overall lead to the fundamental alteration in cellular biochemicals whilst attaching onto palm kernel expeller (PKE) waste to form adhesion complex in easing harvesting at stationary growth phase. This study had initially optimized the PKE dosage, light intensity and photoperiod in maximizing the attached microalgal productivity being attained at 0.72 g/g day. Lipid content increased progressively from pH 3 to pH 11, with the highest value observed at pH 11. Meanwhile, in terms of protein and carbohydrate contents, the highest values were obtained by cultivation medium of pH 5 with 9.92 g and 17.72 g, respectively followed by pH 7 with 9.16 g and 16.36 g, respectively. Moreover, the findings also suggested that the low pH mediums utilized polar interactions in the formation of complexes between PKE and microalgae, whereas at higher pH levels, the non-polar interactions became more significant. The work of attachment was thermodynamically favourable towards the attachment formation with values greater than zero which was also aligned with the microscopic surface topography, i.e., revealing a clustering pattern of microalgae colonizing the PKE surface. These findings contribute to comprehensive understanding of optimizing growth condition and harvesting strategy of attached microalgae in attaining the cellular biochemical components, facilitating the development of efficient and sustainable bioresource utilization.

Fabrication of MIL-101(Cr)/silver nanocomposites as SERS substrate for sensitive determination of malachite green and crystal violet in tilapia

Nanocomposites with multiple functions have attracted much attention in designing novel SERS substrates. In this report, the enrichment ability of MIL-101(Cr) and the local surface plasma resonance (LSPR) of silver nanoparticles are combined to fabricate a SERS substrate denoted as MIL-101-MA@Ag, which can simultaneously produce high-density and uniformly distributed hot spots. Moreover, the enrichment ability of MIL-101(Cr) can further improve the sensitivity by concentrating and transferring the analytes in the vicinity of hot spots. Under optimal conditions, MIL-101-MA@Ag showed good SERS activity for malachite green (MG) and crystal violet (CV), with detection limits as low as 9.5×10-11 M and 9.2×10-12 M at 1616 cm-1, respectively. The prepared substrate has been successfully applied to detect MG and CV in tilapia, the recovery rate of fish tissue extract was 86.4~102%, and the relative standard deviation (RSD) was 8.9~15%. The results demonstrate that MOF-based nanocomposites are expected to be useful SERS substrates and have a universal applicability for the detection of other hazardous molecules.

Enhanced production of bioactive compounds from marine microalgae Tetraselmis tetrathele under salinity and light stresses: A two-stage cultivation strategy

This study leveraged the salinity and light intensity stresses during the stationary phase for enhancing the pigment contents and antioxidant capacity of Tetraselmis tetrathele. The highest pigments content was obtained in cultures under salinity stress (40 g L^-1) illuminated using fluorescent light. Furthermore, the best inhibitory concentration (IC₅₀) for scavenging the 2, 2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radicals was found as 79.53 µg mL^-1 in ethanol extract and cultures under red LED light stress (300 µmol m^-2 s^-1). The highest antioxidant capacity in a ferric-reducing antioxidant power (FRAP) assay (1,778.6 µM Fe⁺²) was found in ethanol extract and cultures under salinity stress illuminated using fluorescent light. Maximum scavenging of the 2.2-diphenyl-1-picrylhydrazyl (DPPH) radical was found in ethyl acetate extracts under light and salinity stresses. These results indicated that abiotic stresses could enhance the pigment and antioxidant components of T. tetrathele, which are value-added compounds in the pharmaceutical, cosmetic, and food industries.

Microalga Coelastrella sp. Cultivation on Unhydrolyzed Molasses-Based Medium towards the Optimization of Conditions for Growth and Biomass Production under Mixotrophic Cultivation

Improving biomass production with the utilization of low-cost substrate is a crucial approach to overcome the hindrance of high cost in developing large-scale microalgae production. The microalga Coelastrella sp. KKU-P1 was mixotrophically cultivated using unhydrolyzed molasses as a carbon source, with the key environmental conditions being varied in order to maximize biomass production. The batch cultivation in flasks achieved the highest biomass production of 3.81 g/L, under an initial pH 5.0, a substrate to inoculum ratio of 100:3, an initial total sugar concentration of 10 g/L, and a sodium nitrate concentration of 1.5 g/L with continuous light illumination at 23.7 W/m². The photobioreactor cultivation results indicated that CO₂ supplementation did not improve biomass production. An ambient concentration of CO₂ was sufficient to promote the mixotrophic growth of the microalga as indicated by the highest biomass production of 4.28 g/L with 33.91% protein, 46.71% carbohydrate, and 15.10% lipid. The results of the biochemical composition analysis suggest that the microalgal biomass obtained is promising as a source of essential amino acids and pigments as well as saturated and monounsaturated fatty acids. This research highlights the potential for bioresource production via microalgal mixotrophic cultivation using untreated molasses as a low-cost raw material.

CO2 capture and lipid production performance of microalgae in the S-shaped photobioreactor under different culture modes

An S-shaped photobioreactor was designed by adding grooves and baffles in the traditional photobioreactor to improve the culture efficiency of microalgae. After that, the parameters of the characterization of the S-shaped photobioreactor, such as the mixing time, gas holdup, and gas-liquid mass transfer coefficient, were determined. The biomass, lipid production rate, and average CO₂ capture rate of microalgae were then analyzed under different culture modes. Finally, the feasibility of using digested piggery wastewater combined with simulated flue gas was explored as a culture mode for the microalgae and the lipid properties of the microalgae were analyzed. The results revealed that, at a flow rate of 0.08 vvm, the mixing time was reduced by 8.5 s, the gas hold-up increased by 44.6% and the gas-liquid mass transfer ability was also improved. Improvements were also observed in the biomass values, lipid production rate, and average CO₂ capture rate of the microalgae under different culture conditions, with respective values reaching 0.23 g·(L·d)^-1, 70.28 mg·(L·d)^-1, and 0.43 g·(L·d)^-1 under the mixotrophic mode. Additionally, digested piggery wastewater combined with the simulated microalgae flue gas culture was determined to be feasible. The biomass, lipid production rate, and the average CO₂ capture rate of microalgae, the values of which were 0.22 g·(L·d)^-1, 52.55 mg·(L·d)^-1, and 0.41 g·(L·d)^-1, respectively. Lipid was observed to have the potential to produce high-quality biofuel.

Mechanistic behaviour of Chlorella vulgaris biofilm formation onto waste organic solid support used to treat palm kernel expeller in the recent Anthropocene

The capacity to maximize the proliferation of microalgal cells by means of topologically textured organic solid surfaces under various pH gave rise to the fundamental biophysical analysis of cell-surface attachment in this study. The substrate used in analysis was palm kernel expeller (PKE) in which the microalgal cells had adhered onto its surface. The findings elucidated the relevance of surface properties in terms of surface wettability and surface energy in relation to the attached microalgal growth with pH as the limiting factor. The increase in hydrophobicity of PKE-microalgae attachment was able to facilitate the formation of biofilm better. The pH 5 and pH 11 were found to be the conditions with highest and lowest microalgal growths, respectively, which were in tandem with the highest contact angle value at pH 5 and conversely for pH 11. The work of attachment (Wcs) had supported the derived model with positive values being attained for all the pH conditions, corroborating the thermodynamic feasibility. Finally, this study had unveiled the mechanism of microalgal attachment onto the surface of PKE using the aid of extracellular polymeric surfaces (EPS) from microalgae. Also, the hydrophobic nature of PKE enabled excellent attachment alongside with nutrients for microalgae to grow and from layer-by-layer (LbL) assembly. This assembly was then isolated using organosolv method by means of biphasic solvents, namely, methanol and chloroform, to induce detachment.

Bioprocessing to Preserve and Improve Microalgae Nutritional and Functional Potential: Novel Insight and Perspectives

Microalgae are aquatic unicellular microorganisms and, although various species are approved for human consumption, Arthrospira and Chlorella are the most widespread. Several nutritional and functional properties have been bestowed to microalgae principal micro- and macro-nutrients, with antioxidant, immunomodulatory and anticancer being the most common. The many references to their potential as a food of the future is mainly ascribed to the high protein and essential amino acid content, but they are also a source of pigments, lipids, sterols, polysaccharides, vitamins, and phenolic compounds with positive effects on human health. Nevertheless, microalgae use is often hindered by unpleasant color and flavor and several strategies have been sought to minimize such challenges. This review provides an overview of the strategies so far proposed and the main nutritional and functional characteristic of microalgae and the foods made thereof. Processing treatments have been used to enrich microalgae-derived substrates in compounds with antioxidant, antimicrobial, and anti-hypertensive properties. Extraction, microencapsulation, enzymatic treatments, and fermentation are the most common, each with their own pros and cons. Yet, for microalgae to be the food of the future, more effort should be put into finding the right pre-treatments that can allow the use of the whole biomass and be cost-effective while bringing about features that go beyond the mere increase of proteins.

Recent Advances in Supercritical CO2 Extraction of Pigments, Lipids and Bioactive Compounds from Microalgae

Supercritical CO₂ extraction is a green method that combines economic and environmental benefits. Microalgae, on the other hand, is a biomass in abundance, capable of providing a vast variety of valuable compounds, finding applications in the food industry, cosmetics, pharmaceuticals and biofuels. An extensive study on the existing literature concerning supercritical fluid extraction (SFE) of microalgae has been carried out focusing on carotenoids, chlorophylls, lipids and fatty acids recovery, as well as the bioactivity of the extracts. Moreover, kinetic models used to describe SFE process and experimental design are included. Finally, biomass pretreatment processes applied prior to SFE are mentioned, and other extraction methods used as benchmarks are also presented.

DC-Dielectrophoretic Manipulation and Isolation of Microplastic Particle-Treated Microalgae Cells in Asymmetric-Orifice-Based Microfluidic Chip

A novel direct-current dielectrophoretic (DC-DEP) method is proposed for the manipulation and isolation of microplastic particle (MP)-treated microalgae cells according to their dielectric properties in a microfluidic chip. The lateral migration and trajectory of the microalgae cells were investigated. To induce stronger DC-DEP effects, a non-homogeneous electric-field gradient was generated by applying the DC electric voltages through triple pairs of asymmetric orifices with three small orifices and one large orifice located on the opposite microchannel wall across the whole channel, leading to the enhanced magnitude of the non-uniform electric-field gradient and effective dielectrophoretic area. The effects of the applied voltage, the polystyrene (PS) adsorption coverage, and thickness on the DC-DEP behaviors and migration were numerically investigated, and it was found that the effect of the PS adsorption thickness of the Chlorella cells on the DC-DEP behaviors can be neglected, but the effect on their trajectory shifts cannot. In this way, the separation of 3 µm and 6 µm Chlorella coated with 100% PS particles and the isolation of the Chlorella cells from those coated with various coverages and thicknesses of PS particles was successfully achieved, providing a promising method for the isolation of microalgae cells and the removal of undesired cells from a target suspension.

Improvement of phosphate solubilizing bacteria Paenibacillus xylanexedens on the growth of Chlorella pyrenoidosa and wastewater treatment in attached cultivation

A symbiotic system of algae-bacteria, and attached cultivation, are two ways to increase microalgae biomass, and beneficially effect wastewater treatment. However, the possible advantages of the algae-bacteria co-culture in attached cultivation, are still unclear. This paper investigates the effects of different morphologies of a phosphate solubilizing bacteria-Paenibacillus xylanexedens (bacteria supernatant, bacteria, broken bacteria), on the growth of microalgae-Chlorella pyrenoidosa and wastewater treatment in an attached co-culture system. The results show that the broken bacteria had the most significant effect, with the biomass and protein content of Chlorella pyrenoidosa increasing by 125.67% and 25.04%; and the removal rate of COD, NH₄⁺-N and PO₄^3- in wastewater increasing by 23.57%, 146.15% and 9.96% respectively. This indicates that the intracellular material of the Paenibacillus xylanexedens was more effective in promoting the biomass growth of Chlorella pyrenoidosa and the removal rates of COD, NH₄⁺-N and PO₄^3-, compared to the algae growing without the bacteria. The algae-bacteria symbiotic attached mode was superior to the suspended mode, in terms of both Chlorella pyrenoidosa biomass enhancement and effective wastewater treatment. The addition of different morphologies of Paenibacillus xylanexedens significantly enlarged the difference between the two culture modes. This study provides a new method for coupled algae-bacteria co-cultures for wastewater treatment, based on the symbiotic effect.

Enhancement of Arthrospira sp. culturing for sulfate removal and mining wastewater bioremediation

Sulfate content in mining wastewater can reach concentrations over 2,000 mg·L^-1, which is considered as a pollutant of concern. In this article, two cyanobacteria species were cultured using highly sulfated wastewater (3,000 mg·L^-1) as the culture medium. This investigation aimed to analyze the sulfate bioremediation potential of microalgae while enhancing the uptaking of this pollutant through the design of a novel nutritional medium. The results obtained show the suitability of Arthrospira maxima as a bioremediation organism of sulfated wastewater. The appropriateness of this organism is based on its great growth performance when cultured in this residue, 2.16 times higher than the initial value. Moreover, the initially obtained sulfate reduction, 23.3%, was significantly enhanced to a final removal of 73% (2,247 mg·L^-1). In addition, scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to evaluate sulfur crystallization. To the best of our knowledge, there are no previous works focused on microalgal sulfate removal that have reached such an uptaking rate. Accordingly, this study presents the highest performance on sulfate microalgal bioremediation published to date. Our findings suggest that A. maxima can be cultured for sulfated wastewater bioremediation while showing a removal yield that is theoretically sufficient for industrial applications.

Algal biorefinery culminating multiple value-added products: recent advances, emerging trends, opportunities, and challenges

Algal biorefinery is rising as a prominent solution to economically fulfill the escalating global requirement for nutrition, feed, fuel, and medicines. In recent years, scientific productiveness associated with microalgae-based studies has elaborated in multiplied aspects, while translation to the commercial level continues to be missing. The present microalgal biorefinery has a challenge in long-term viability due to escalated market price of algal-mediated biofuels and bioproducts. Advancements are required in a few aspects like improvement in algae processing, energy investment, and cost analysis of microalgae biorefinery. Therefore, it is essential to recognize the modern work by understanding the knowledge gaps and hotspots driving business scale up. The microalgae biorefinery integrated with energy-based products, bioactive and green compounds, focusing on a circular bioeconomy, is urgently needed. A detailed investigation of techno-economic analysis (TEA) and life cycle assessment (LCA) is important to increase the market value of algal products. This review discusses the valorization of algal biomass for the value-added application that holds a sustainable approach and cost-competitive algal biorefinery. The current industries, policies, technology transfer trends, challenges, and future economic outlook are discussed. This study is an overview through scientometric investigation attempt to describe the research development contributing to this rising field.

Microalgae as a potential therapeutic drug candidate for neurodegenerative diseases

Microalgae are highly photosynthetic unicellular organism that have increased demand in the recent days owing to the presence of valuable cellular metabolites. They are ubiquitous in terrestrial and aquatic habitats, rich in species diversity and are capable of generating significant biomass by efficiently using CO₂, light and other nutrients like nitrogen, phosphate etc., The microalgal biomass has upsurged in economic potential and is used as both food and feed in many countries across the world, accounting for more than 75 % of annual microalgal biomass production in the past decades. The microalgal cells are sustainable resource that synthesize various secondary metabolites such as carotenoids, polysaccharides, polyphenols, essential amino acids, sterols, and polyunsaturated fatty acids (PUFA). Microalgae and its derived compounds possess significant pharmacological and biological effects such as antioxidant, anti-inflammatory, anti-cancer, immunomodulatory and anti-obesity. Because of their potential health promoting properties, the utilization of microalgae and its derived substances in food, pharmaceutical and cosmetic industries has skyrocketed in recent years. In this context, the current review discusses about the benefits of microalgae and its bioactive compounds against several neurodegenerative disorders like Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS).

Update on the application of magnetic fields to microalgal cultures

Several studies have shown that any magnetic field (MF) applied to microalgae modifies its cultivation conditions and may favor biomolecule production since it interacts with the microorganisms and affect their growth. As a result, there are changes in concentrations and compositions of biomass and biomolecules. This review aims at updating MF applications to microalga cultures that were reported by studies conducted in the last 5 years. It shows the main studies that reached positive results of carbohydrate, lipid, protein and pigment production. Effects of MFs may be positive, negative or null, depending on some factors, such as intensity, exposure time, physiological state of cells and application devices. Therefore, this review details cultivation conditions used for reaching high concentration of biomolecules, explains the action of MFs on microalgae and describes their applicability to the biorefinery concept.

Impact of Various Visible Spectra on Attached Microalgal Growth on Palm Decanter Cake in Triggering Protein, Carbohydrate, and Lipid to Biodiesel Production

Attached microalgal growth of Chlorella vulgaris on palm decanter cake (PDC) under irradiation with various visible monochromatic and polychromatic spectra to produce biodiesel was studied in this work. The results demonstrated that the white spectrum cultivation exhibited the highest microalgal density of 1.13 g/g along with 1.213 g/L day of microalgal productivity. Correspondingly, the biodiesel obtained was comprised mainly of C16 and C18 fatty acids, possessing a high cetane number and oxidation stability from the high saturated fatty acid content (70.38%), which was appealing in terms of most biodiesel production requirements. Nevertheless, the highest lipid content (14.341%) and lipid productivity (93.428 mg/L per day) were discovered with green spectrum cultivation. Blue and white spectra led to similar protein contents (34%) as well as carbohydrate contents (61%), corroborating PDC as a feasible carbon and nutrient source for growing microalgae. Lastly, the energy feasibilities of growing the attached microalgae under visible spectra were investigated, with the highest net energy ratio (NER) of 0.302 found for the yellow spectrum. This value outweighed that in many other works which have used suspended growth systems to produce microalgal fuel feedstock. The microalgal growth attached to PDC is deemed to be a suitable alternative cultivation mode for producing sustainable microalgal feedstock for the biofuel industry.

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.

Water-plasma-enhanced and phase-separation-assisted extraction of microalgal lipid for biodiesel production

Traditional methods for lipid extraction from microalgal biomass usually involve harsh reaction conditions or the use of contaminant reagents, which lead to enormous energy consumption and wastage. Hence, a novel strategy was presented, which combined water-plasma and three-phase partitioning (TPP) techniques. Benefiting from its unique advantages such as rapid and low cost, water-plasma strategy can disrupt microalgal cell wall and can thus greatly affect lipid extraction. As a result, assisted with the TPP method, excellent performance lipid recovery (74.34%) was obtained at 200 W in 10 min. The performance was superior to that achieved through cell disruption via water-plasma pretreatment. Importantly, the whole process of lipid extraction prevented the drying of microalgal biomass, contributing to reduced energy consumption in large-scale biodiesel production. Moreover, the high fatty acids content suggested that the extracted lipids are great potential candidate for biodiesel production.

Industrial CO2 Capture by Algae: A Review and Recent Advances

The problem of global warming and the emission of greenhouse gases is already directly affecting the world’s energy. In the future, the impact of CO2 emissions on the world economy will constantly grow. In this paper, we review the available literature sources on the benefits of using algae cultivation for CO2 capture to decrease CO2 emission. CO2 emission accounts for about 77% of all greenhouse gases, and the calculation of greenhouse gas emissions is 56% of all CO2 imports. As a result of the study of various types of algae, it was concluded that Chlorella sp. is the best at capturing CO2. Various methods of cultivating microalgae were also considered and it was found that vertical tubular bioreactors are emerging. Moreover, for energy purposes, thermochemical methods for processing algae that absorb CO2 from flue gases were considered. Of all five types of thermochemical processes for producing synthesis gas, the most preferred method is the method of supercritical gasification of algae. In addition, attention is paid to the drying and flocculation of biofuels. Several different experiments were also reviewed on the use of flue gases through the cultivation of algae biomass. Based on this literature review, it can be concluded that microalgae are a third generation biofuel. With the absorption of greenhouse gases, the growth of microalgae cultures is accelerated. When a large mass of microalgae appears, it can be used for energy purposes. In the results, we present a plan for further studies of microalgae cultivation, a thermodynamic analysis of gasification and pyrolysis, and a comparison of the results with other biofuels and other algae cultures.

Sustainable production of EPA-rich oil from microalgae: Towards an algal biorefinery

Utilization of sustainable natural resources such as microalgae has been considered for the production of biofuels, aquaculture feed, high-value bioactives such as omega-3 fatty acids, carotenoids, etc. Eicosapentaenoic acid (EPA) is an omega-3 fatty acid present in fish oil, which is of physiological importance to both humans and fishes. Marine microalgae are sustainable sources of lipid rich in EPA and different species have been explored for the production of EPA as a single product. There has been a rising interest in the concept of a multi-product biorefinery, focusing on maximum valorization of the algal biomass. Targeting one or more value-added compounds in a biorefinery scenario can improve the commercial viability of low-value products like triglycerides for biofuel. This approach has been viewed by technologists and experts as a sustainable and economically feasible possibility for the large-scale production of microalgae for its potential applications in biodiesel and jet fuel production, nutraceuticals, animal and aquaculture feeds, etc. In this review paper, we describe the recent developments in the production of high-value EPA-rich oil from microalgae, emphasizing on the upstream and downstream bioprocess techniques, and the advantages of considering an EPA-rich oil based biorefinery.