Carotenoid and lipid production by the autotrophic microalga Chlorella protothecoides under nutritional, salinity, and luminosity stress conditions

Identification and optimization of the key growth parameters involved in carotenoids production of the marine microalga Pavlova gyrans

In this work, a multivariate analysis was carried out, using a Plackett-Burman (PB) design involving seventeen growth parameters, on carotenoids production of Pavlova gyrans (p < 0.10). Each assay was analysed regarding its content (mg g-1) of fucoxanthin (Fx), diatoxanthin, diadinoxanthin, β-carotene (βCar), α-carotene, and the sum of all carotenoids analysed individually (TCar). According to the statistical analysis, modified medium formulations were developed for the particular cases of Fx, βCar, and TCar. The study showed that Fx content was positively affected by nitrogen supplementation and lower light intensities. Higher concentrations of nitrogen and iron increased the final content of βCar as well. Similarly, salinity, light intensity, nitrogen, iron, and cobalt were identified as key factors in TCar production. The PB-based formulations showed significant improvements (p < 0.05) for TCar (11.794 mg g-1) and Fx (6.153 mg g-1) when compared to the control conditions (Walne's medium-2.010 mg g-1). Furthermore, effective control of key variables (e.g., light intensity) throughout P. gyrans growth proved successful (p < 0.05), increasing the productivity of Fx (0.759 mg L-1 d-1) and TCar (1.615 mg L-1 d-1).

Introducing of high rich lysine, arginine, and unsaturated fatty acids microalga as a food supplement

Microalgae are powerful source for nutritionally valuable components as proteins, carbohydrates and especially unsaturated fatty acids. Microalgae may be employed in pharmaceutical, food, cosmetic, health industries, and biofuels. In this study for looking at high-level unsaturated fatty acids species, from 31 strains, by comparing growth curves, the best strain with a high growth rate and lipid content was selected by red Nile staining. It was determined by molecular identification that this strain belongs to the genus Chlorella sp. and is deposited into the Agricultural Biotechnology Research Institute of Iran Culture collection with culture collection number ABRIICC 30,041. Biomass analysis after growth optimization by response surface methodology showed that the selected strain had a specific growth rate of 0.216 ± 0.008 d-1, biomass productivity of 142.58 ± 4.41 mg/Ld, and lipid content of 13.9 ± 0.26% with a high level of unsaturated fatty acids of 53.15%. It also included 51.3 ± 0.53% protein with a very high quality essential amino acids of 40.36%, the most lysine (8.77%) and arginine (13.31%) has been reported until now, and 26.9 ± 0.23% carbohydrates in photoautotroph condition. By MTT assay, there is no effect of cytotoxicity. This research introduces a potent native strain comparable with commercial strains that can be a hopeful source for food supplements and valuable bioactive ingredients in functional foods.

Strategy Development for Microalgae Spirulina platensis Biomass Cultivation in a Bubble Photobioreactor to Promote High Carbohydrate Content

As a counter to climate change, energy crises, and global warming, microalgal biomass has gained a lot of interest as a sustainable and environmentally favorable biofuel feedstock. Microalgal carbohydrate is considered one of the promising feedstocks for biofuel produced via the bioconversion route under a biorefinery system. However, the present culture technique, which uses a commercial medium, has poor biomass and carbohydrate productivity, creating a bottleneck for long-term microalgal-carbohydrate-based biofuel generation. This current investigation aims toward the simultaneous increase in biomass and carbohydrate accumulation of Spirulina platensis by formulating an optimal growth condition under different concentrations of nitrogen and phosphorous in flasks and a bubble photobioreactor. For this purpose, the lack of nitrogen (NaNO3) and phosphorous (K2HPO4) in the culture medium resulted in an enhanced Spirulina platensis biomass and total carbohydrate 0.93 ± 0.00 g/L and 74.44% (w/w), respectively. This research is a significant step in defining culture conditions that might be used to tune the carbohydrate content of Spirulina.

Low Indirect Land Use Change (ILUC) Energy Crops to Bioenergy and Biofuels—A Review

Energy crops are dedicated cultures directed for biofuels, electricity, and heat production. Due to their tolerance to contaminated lands, they can alleviate and remediate land pollution by the disposal of toxic elements and polymetallic agents. Moreover, these crops are suitable to be exploited in marginal soils (e.g., saline), and, therefore, the risk of land-use conflicts due to competition for food, feed, and fuel is reduced, contributing positively to economic growth, and bringing additional revenue to landowners. Therefore, further study and investment in R&D is required to link energy crops to the implementation of biorefineries. The main objective of this study is to present a review of the potential of selected energy crops for bioenergy and biofuels production, when cultivated in marginal/degraded/contaminated (MDC) soils (not competing with agriculture), contributing to avoiding Indirect Land Use Change (ILUC) burdens. The selected energy crops are Cynara cardunculus, Arundo donax, Cannabis sativa, Helianthus tuberosus, Linum usitatissimum, Miscanthus × giganteus, Sorghum bicolor, Panicum virgatum, Acacia dealbata, Pinus pinaster, Paulownia tomentosa, Populus alba, Populus nigra, Salix viminalis, and microalgae cultures. This article is useful for researchers or entrepreneurs who want to know what kind of crops can produce which biofuels in MDC soils.

Biosynthesis and biotechnological interventions for commercial production of microalgal pigments: A review

Microalgae are photosynthetic eukaryotes that serve as microbial cell factories for the production of useful biochemicals, including pigments. These pigments are eco-friendly alternatives to synthetic dyes and reduce environmental and health risks. They also exhibit excellent anti-oxidative properties, making them a useful commodity in the nutrition and pharmaceutical industries. Light-harvesting pigments such as chlorophylls and phycobilins, and photoprotective carotenoids are some of the most common microalgal pigments. The increasing demand for these pigments in industrial applications has prompted a need to improve their metabolic yield in microalgal cells. So far, expensive cultivation methods and sensitivity to microbial contamination remain the main obstacles to the large-scale production of these pigments. This review highlights current issues and future prospects related to the production of microalgal pigments. The review also emphasizes the use of engineering approaches such as genetic engineering, and optimization of media components and physical parameters to increase their commercial-scale production.

Emerging microalgae-based technologies in biorefinery and risk assessment issues: Bioeconomy for sustainable development

Industrial wastewater treatment is of paramount importance considering the safety of the aquatic ecosystem and its associated health risk to humankind inhabiting near the water bodies. Microalgae-based technologies for remediation of environmental pollutants present avenues for bioenergy applications and production of value-added biochemicals having pharmaceutical, nutraceutical, antioxidants, carbohydrate, phenolics, long-chain multi-faceted fatty acids, enzymes, and proteins which are considered healthy supplements for human health. Such a wide range of products put up a good case for the biorefinery concept. Microalgae play a pivotal role in degrading complex pollutants, such as organic and inorganic contaminants thereby efficiently removing them from the environment. In addition, microalgal species, such as Botryococcus braunii, Tetraselmis suecica, Phaeodactylum tricornutum, Neochloris oleoabundans, Chlorella vulgaris, Arthrospira, Chlorella, and Tetraselmis sp., etc., are also reported for generation of value-added products. This review presents a holistic view of microalgae based biorefinery starting from cultivation and harvesting of microalgae, the potential for remediation of environmental pollutants, bioenergy application, and production of value-added biomolecules. Further, it summarizes the current understanding of microalgae-based technologies and discusses the risks involved, potential for bioeconomy, and outlines future research directions.

Optimization of Extraction Conditions of Carotenoids from Dunaliella parva by Response Surface Methodology

Extraction conditions can exert a remarkable influence on extraction efficiency. The aim of this study was to improve the extraction efficiency of carotenoids from Dunaliella parva (D. parva). Dimethyl sulfoxide (DMSO) and 95% ethanol were used as the extraction solvents. The extraction time, extraction temperature and the proportions of mixed solvent were taken as influencing factors, and the experimental scheme was determined by Central Composite Design (CCD) of Design Expert 10.0.4.0 to optimize the extraction process of carotenoids from D. parva. The absorbance values of the extract at 665 nm, 649 nm and 480 nm were determined by a microplate spectrophotometer, and the extraction efficiency of carotenoids was calculated. Analyses of the model fitting degree, variance and interaction term 3D surface were performed by response surface analysis. The optimal extraction conditions were as follows: extraction time of 20 min, extraction temperature of 40 °C, and a mixed solvent ratio (DMSO: 95% ethanol) of 3.64:1. Under the optimal conditions, the actual extraction efficiency of carotenoids was 0.0464%, which was increased by 18.19% (the initial extraction efficiency of 0.03926%) with a lower extraction temperature (i.e., lower energy consumption) compared to the standard protocol.

Microalgal Systems for Wastewater Treatment: Technological Trends and Challenges towards Waste Recovery

Wastewater (WW) treatment using microalgae has become a growing trend due the economic and environmental benefits of the process. As microalgae need CO2, nitrogen, and phosphorus to grow, they remove these potential pollutants from wastewaters, making them able to replace energetically expensive treatment steps in conventional WW treatment. Unlike traditional sludge, biomass can be used to produce biofuels, biofertilizers, high value chemicals, and even next-generation growth media for “organically” grown microalgal biomass targeting zero-waste policies and contributing to a more sustainable circular bioeconomy. The main challenge in this technology is the techno-economic feasibility of the system. Alternatives such as the isolation of novel strains, the use of native consortia, and the design of new bioreactors have been studied to overcome this and aid the scale-up of microalgal systems. This review focuses on the treatment of urban, industrial, and agricultural wastewaters by microalgae and their ability to not only remove, but also promote the reuse, of those pollutants. Opportunities and future prospects are discussed, including the upgrading of the produced biomass into valuable compounds, mainly biofuels.

Channeling of Carbon Flux Towards Carotenogenesis in Botryococcus braunii: A Media Engineering Perspective

Microalgae, due to their unique properties, gained attention for producing promising feedstocks having high contents of proteins, antioxidants, carotenoids, and terpenoids for applications in nutraceutical and pharmaceutical industries. Optimizing production of the high-value renewables (HVRs) in microalgae requires an in-depth understanding of their functional relationship of the genes involved in these metabolic pathways. In the present study, bioinformatic tools were employed for characterization of the protein-encoding genes of methyl erythritol phosphate (MEP) pathway involved in carotenoid and squalene biosynthesis based upon their conserved motif/domain organization. Our analysis demonstrates nearly 200 putative genes showing a conservation pattern within divergent microalgal lineages. Furthermore, phylogenomic studies confirm the close evolutionary proximity among these microalgal strains in the carotenoid and squalene biosynthetic pathways. Further analysis employing STRING predicts interactions among two rate-limiting genes, i.e., phytoene synthase (PSY) and farnesyl diphosphate farnesyl synthase (FPPS), which are specifically involved in the synthesis of carotenoids and squalene. Experimentally, to understand the carbon flux of these rate-limiting genes involved in carotenogenesis, an industrial potential strain, namely, Botryococcus braunii, was selected in this study for improved biomass productivity (i.e., 100 mg L^-1 D^-1) along with enhanced carotenoid content [0.18% dry cell weight (DCW)] when subjected to carbon supplementation. In conclusion, our approach of media engineering demonstrates that the channeling of carbon flux favors carotenogenesis rather than squalene synthesis. Henceforth, employing omics perspectives will further provide us with new insights for engineering regulatory networks for enhanced production of high-value carbon biorenewables without compromising growth.

Transcription activation of β-carotene biosynthetic genes at the initial stage of stresses as an indicator of the increased β-carotene accumulation in isolated Dunaliella salina strain GY-H13

β-carotene is an efficient antioxidant and its accumulation is an oxidative response to stressors. Dunaliella salina strain GY-H13 is rich in β-carotene under environmental stresses, which was selected as material to understand the molecular mechanism underlying β-carotene biosynthesis. Seven full length cDNA sequences in β-carotene biosynthesis pathway were cloned, including geranylgeranyl pyrophosphate synthase (GGPS), phytoene synthase (PSY), phytoene desaturase (PDS), 15-cis-zeta-carotene isomerase (ZISO), zeta-carotene desaturase (ZDS), prolycopene isomerase (CRTISO), lycopene beta-cyclase (LCYb). The seven protein sequences from the strain GY-H13 showed the highest similarity with other D. salina strains. Especially, PSY, PDS and LCYb protein sequences shared 100 % identity. Phylogenetic analysis indicated all proteins from GY-H13 firstly clustered with those from other D. salina strains with a bootstrap of 100 %. Multiple alignment indicated several distinct conserved motifs such as aspartate-rich domain (ARD), dinucleotide binding domain (DBD), and carotene binding domain (CBD). These motifs are located near ligand-binding pocket, which may be required for the activity of enzyme. Expression levels of these genes and β-carotene content were measured over 24-h cycle, showing clear daily dynamics. All genes were dramatically up-regulated in the morning but the highest accumulation of β-carotene was observed at noon, suggesting a lag-effect between gene transcription and biological response. Furthermore, the accumulation of β-carotene increased under nitrogen deficiency, Cd exposure and high light and decreased under high salinity in a time-dependent manner. No gene of β-carotene biosynthesis was up-regulated by high salinity while most genes were activated by the other stresses at the beginning stage of exposure. Growth inhibition and oxidative damage were also observed under high salinity. Overall, transcription activation of β-carotene biosynthetic genes at the initial stage of stress exposure is a determinant of the increased accumulation of β-carotene in microalgae, which help their survive under harsh environments. The newly isolated D. salina strain GY-H13 would be a promising microalgae model for investigating the molecular mechanism of stress-induced β-carotene biosynthesis.

Biological Activity of Hydrophilic Extract of Chlorella vulgaris Grown on Post-Fermentation Leachate from a Biogas Plant Supplied with Stillage and Maize Silage

Algae are employed commonly in cosmetics, food and pharmaceuticals, as well as in feed production and biorefinery processes. In this study, post-fermentation leachate from a biogas plant which exploits stillage and maize silage was utilized as a culture medium for Chlorella vulgaris. The content of polyphenols in hydrophilic extracts of the Chlorella vulgaris biomass was determined, and the extracts were evaluated for their antioxidant activity (DPPH assay), antibacterial activity (against Escherichia coli, Lactobacillusplantarum, Staphylococcus aureus, Staphylococcus epidermidis) and antifungal activity (against Aspergillus niger, Candida albicans, Saccharomyces cerevisiae). The use of the post-fermentation leachate was not found to affect the biological activity of the microalgae. The aqueous extract of Chlorella vulgaris biomass was also observed to exhibit activity against nematodes. The results of this study suggest that Chlorella vulgaris biomass cultured on post-fermentation leachate from a biogas plant can be successfully employed as a source of natural antioxidants, food supplements, feed, natural antibacterial and antifungal compounds, as well as in natural methods of plant protection.

A novel alkalophilic Trebouxiophyte: Identification and its capability for CO2 capture and biomass production in high bicarbonate-based cultivation

Aiming to evaluate the capability for CO₂ capture and valuable biomass production potential from a novel alkalophilic Trebouxiophyte domesticated by sodium bicarbonate gradients, the strain was cultivated in a 2 L flat plate photobioreactor with high bicarbonate medium and controlled pH by CO₂ supplementation. The results indicated that the strain had a higher maximum quantum efficiency (F_v/F_m, 0.71) and biomass yield (1.42 g L^-1) at pH 8.3 under 25.2 g L^-1 NaHCO₃ compared to pH 7.3 or 9.3. Higher contents of fatty acids (21.72%) and carbohydrates (20.85%) were attained at pH 8.3, while a higher protein content (ca. 46%) was attained at pH 7.3 and 9.3. The results demonstrated that this strain, with a high growth rate and high biomass yield, has great potential to extend to the application for CO₂ capture and utilization through highly efficient photosynthesis in alkaline environments.

The growth and lutein accumulation in heterotrophic Chlorella protothecoides provoked by waste Monascus fermentation broth feeding

Although the potential of heterotrophic microalgae served as a sustainable source for lutein, it was still crucial to formulate a suitable medium to offset the cost involved in algal biomass cultivation while improve inherent lutein productivity. The objective of this study was to investigate the feasibilities of waste Monascus fermentation broth medium (MFBM) toward heterotrophic Chlorella protothecoides-enriched lutein. The results indicated that C. protothecoides subjected to MFBM batch feeding achieved 7.1 g/L biomass and 7.27 mg/g lutein. The resulting lutein productivity (7.34 mg/L/day) represented 1.54-fold more than that of frequently used Basal medium. Concurrently, the effective metabolism and absorption of carbon, nitrogen, and phosphorus in MFBM by C. subellipsoidea cultivation make it easily complied with the permissible dischargeable limits for fermentation broth. When response to fed-batch culture mode, the biomass and lutein productivity peaked 20.4 g/L and 9.11 mg/L/day with concentrated MFBM feeding. Transcriptomics data hinted that MFBM feeding manipulated lutein biosynthesis key checkpoints (e.g., lycopene β-cyclase and lycopene ε-cyclase) while accelerated energy pathways (e.g., glycolysis and TCA cycle) to contribute such high lutein productivity in C. protothecoides. These encouraging findings not only provided indications in applying nutrient-rich fermentation broth for affordable microalgae cultivation but also presented possibilities in linking algal high value-added products like lutein with high-efficient biological nutrition removal from industrial fermentation processing.

Cultivation of Chlorella protothecoides under different growth modes and its utilisation in oil/water emulsions

Microalgae can be incorporated in different bio-based products; however, the green colour is a barrier for a successful integration. This study aims to overcome this barrier by growing microalgae in different cultivation modes. Mixotrophic cultivation of Chlorella protothecoides resulted in the highest biomass production after 5 days (5.56 ± 0.09 g/L), followed by heterotrophic and photoautotrophic cultivation (4.33 ± 0.15 and 1.80 ± 0.05 g/L, respectively). Mixotrophically and heterotrophically produced biomass presented a reduced greenish colouration compared to photoautotrophically produced biomass. Chlorophyll content resulted in 1.46 ± 0.21 and 0.95 ± 0.28 mg/g dry weight (DW) in mixotrophic and heterotrophic cultures, respectively, and 25.98 ± 1.28 mg/g DW in photoautotrophic cultures. In contrast, the fraction of carotenoids in the total pigments was much higher. With the whole microalgae fractions after cell disruption as ingredients, stable emulsions containing 50% oil could be produced. No syneresis with serum separation was observed 24 h after preparation.

Enhanced Lipid Production and Molecular Dynamics under Salinity Stress in Green Microalga Chlamydomonas reinhardtii (137C)

Microalgal lipids are a source of valuable nutritional ingredients in biotechnological industries, and are precursors to biodiesel production. Here, the effects of salt-induced stresses, including NaCl, KCl, and LiCl stresses, on the production of lipid in green microalga Chlamydomonas reinhardtii (137c) were investigated. NaCl stress dramatically increased saturated fatty acids (SFAs), which accounted for 70.2% of the fatty acid methyl ester (FAMEs) under stress. In contrary, KCl stress led to a slight increase in SFAs (47.05%) with the remaining being polyunsaturated fatty acids (PUFAs) (45.77%). RT-PCR analysis revealed that the genes involved in FA biosynthesis, such as PDH2, ACCase, MAT and KAS2, were up-regulated by NaCl-induced stress. Conversely, the genes responsible for the Kennedy pathway were suppressed. The alteration of FA homeostasis was further assessed by overexpressing MAT, the enzyme responsible for the production of malonyl-ACP, a key building block for FA biosynthesis, in the cyanobacterium Synechococcus elongatus PCC 7942. Intracellular FA composition was affected, with a predominant synthesis of SFAs in transformed cells. Owing to the diversity and relative abundance of SFAs, monounsaturated fatty acid (MUFAs) and PUFAs enable the feasibility of using microorganisms as a source of microalgal lipids or valuable nutritional ingredients; salt-induced stress and expression of MAT are useful in providing precursors for enhanced lipid production.

Growth performance, biochemical composition and sedimentation velocity of Tetraselmis sp. CTP4 under different salinities using low-cost lab- and pilot-scale systems

Biomass harvesting is one of the most expensive steps of the whole microalgal production pipeline. Therefore, the present work aimed to understand the effect of salinity on the growth performance, biochemical composition and sedimentation velocity of Tetraselmis sp. CTP4, in order to establish an effective low-cost pilot-scale harvesting system for this strain. At lab scale, similar growth performance was obtained in cultures grown at salinities of 5, 10 and 20 g L^-1 NaCl. In addition, identical settling velocities (2.4-3.6 cm h^-1) were observed on all salinities under study, regardless of the growth stage. However, higher salinities (20 g L^-1) promoted a significant increase in lipid contents in this strain compared to when this microalga was cultivated at 5 or 10 g L^-1 NaCl. At pilot-scale, cultures were cultivated semi-continuously in 2.5-m³ tubular photobioreactors, fed every four days, and stored in a 1-m³ harvesting tank. Upon a 24-hour settling step, natural sedimentation of the microalgal cells resulted in the removal of 93% of the culture medium in the form of a clear liquid containing only vestigial amounts of biomass (0.07 ± 0.02 g L^-1 dry weight; DW). The remaining culture was recovered as a highly concentrated culture (19.53 ± 4.83 g L^-1 DW) and wet microalgal paste (272.7 ± 18.5 g L^-1 DW). Overall, this method provided an effective recovery of 97% of the total biomass, decreasing significantly the harvesting costs.

Microbial platforms to produce commercially vital carotenoids at industrial scale: an updated review of critical issues

Carotenoids are a diverse group of isoprenoid pigments that play crucial roles in plants, animals, and microorganisms, including body pigmentation, bio-communication, precursors for vitamin A, and potent antioxidant activities. With their potent antioxidant activities, carotenoids are emerging as molecules of vital importance in protecting against chronic degenerative disease, such as aging, cancer, cataract, cardiovascular, and neurodegenerative diseases. Due to countless functions in the cellular system, carotenoids are extensively used in dietary supplements, food colorants, aquaculture and poultry feed, nutraceuticals, and cosmetics. Moreover, the emerging demand for carotenoids in these vast areas has triggered their industrial-scale production. Currently, 80%-90% of carotenoids are produced synthetically by chemical synthesis. However, the demand for naturally produced carotenoids is increasing due to the health concern of synthetic counterparts. This article presents a review of the industrial production of carotenoids utilizing a number of diverse microbes, including microalgae, bacteria, and fungi, some of which have been genetically engineered to improve production titers.

Characterization of the responses to saline stress in the symbiotic green microalga Trebouxia sp. TR9

For the first time we provide a study on the physiological, ultrastructural and molecular effects of salt stress on a terrestrial symbiotic green microalga, Trebouxia sp. TR9. Although tolerance to saline conditions has been thoroughly studied in plants and, to an extent, free-living microalgae, scientific data regarding salt stress on symbiotic lichen microalgae is scarce to non-existent. Since lichen phycobionts are capable of enduring harsh, restrictive and rapidly changing environments, it is interesting to study the metabolic machinery operating under these extreme conditions. We aim to determine the effects of prolonged exposure to high salt concentrations on the symbiotic phycobiont Trebouxia sp. TR9, isolated from the lichen Ramalina farinacea. Our results suggest that, when this alga is confronted with extreme saline conditions, the cellular structures are affected to an extent, with limited chlorophyll content loss and photosynthetic activity remaining after 72 h of exposure to 5 M NaCl. Furthermore, this organism displays a rather different molecular response compared to land plants and free-living halophile microalgae, with no noticeable increase in ABA levels and ABA-related gene expression until the external NaCl concentration is raised to 3 M NaCl. Despite this, the ABA transduction pathway seems functional, since the ABA-related genes tested are responsive to exogenous ABA. These observations could suggest that this symbiotic green alga may have developed alternative molecular pathways to cope with highly saline environments.

Scale-up and large-scale production of Tetraselmis sp. CTP4 (Chlorophyta) for CO2 mitigation: from an agar plate to 100-m3 industrial photobioreactors

Industrial production of novel microalgal isolates is key to improving the current portfolio of available strains that are able to grow in large-scale production systems for different biotechnological applications, including carbon mitigation. In this context, Tetraselmis sp. CTP4 was successfully scaled up from an agar plate to 35- and 100-m³ industrial scale tubular photobioreactors (PBR). Growth was performed semi-continuously for 60 days in the autumn-winter season (17^th October – 14^th December). Optimisation of tubular PBR operations showed that improved productivities were obtained at a culture velocity of 0.65–1.35 m s⁻¹ and a pH set-point for CO₂ injection of 8.0. Highest volumetric (0.08 ± 0.01 g L⁻¹ d⁻¹) and areal (20.3 ± 3.2 g m⁻² d⁻¹) biomass productivities were attained in the 100-m³ PBR compared to those of the 35-m³ PBR (0.05 ± 0.02 g L⁻¹ d⁻¹ and 13.5 ± 4.3 g m⁻² d⁻¹, respectively). Lipid contents were similar in both PBRs (9–10% of ash free dry weight). CO₂ sequestration was followed in the 100-m³ PBR, revealing a mean CO₂ mitigation efficiency of 65% and a biomass to carbon ratio of 1.80. Tetraselmis sp. CTP4 is thus a robust candidate for industrial-scale production with promising biomass productivities and photosynthetic efficiencies up to 3.5% of total solar irradiance.

Industrial potential of carotenoid pigments from microalgae: Current trends and future prospects

Microalgae are rich source of various bioactive molecules such as carotenoids, lipids, fatty acids, hydrocarbons, proteins, carbohydrates, amino acids, etc. and in recent Years carotenoids from algae gained commercial recognition in the global market for food and cosmeceutical applications. However, the production of carotenoids from algae is not yet fully cost effective to compete with synthetic ones. In this context the present review examines the technologies/methods in relation to mass production of algae, cell harvesting for extraction of carotenoids, optimizing extraction methods etc. Research studies from different microalgal species such as Spirulina platensis, Haematococcus pluvialis, Dunaliella salina, Chlorella sps., Nannochloropsis sps., Scenedesmus sps., Chlorococcum sps., Botryococcus braunii and Diatoms in relation to carotenoid content, chemical structure, extraction and processing of carotenoids are discussed. Further these carotenoid pigments, are useful in various health applications and their use in food, feed, nutraceutical, pharmaceutical and cosmeceutical industries was briefly touched upon. The commercial value of algal carotenoids has also been discussed in this review. Possible recommendations for future research studies are proposed.

Microbial consortia: a critical look at microalgae co-cultures for enhanced biomanufacturing

Monocultures have been the preferred production route in the bio-industry, where contamination has been a major bottleneck. In nature, microorganisms usually exist as part of organized communities and consortia, gaining benefits from co-habitation, keeping invaders at bay. There is increasing interest in the use of co-cultures to tackle contamination issues, and simultaneously increase productivity and product diversity. The feasibility of extending the natural phenomenon of co-habitation to the biomanufacturing industry in the form of co-cultures requires careful and systematic consideration of several aspects. This article will critically examine and review current work on microbial co-cultures, with the intent of examining the concept and proposing a design pipeline that can be developed in a biomanufacturing context.

Abiotic stresses as tools for metabolites in microalgae

Microalgae, due to various environmental stresses, constantly tune their cellular mechanisms to cope with them. The accumulation of the stress metabolites is closely related to the changes occurring in their metabolic pathways. The biosynthesis of metabolites can be triggered by a number of abiotic stresses like temperature, salinity, UV- radiation and nutrient deprivation. Although, microalgae have been considered as an alternative sustainable source for nutraceutical products like pigments and omega-3 polyunsaturated fatty acids (PUFAs) to cater the requirement of emerging human population but inadequate biomass generation makes the process economically impractical. The stress metabolism for carotenoid regulation in green algae is a 2-step metabolism. There are a few major stresses which can influence the formation of phycobiliprotein in cyanobacteria. This review would primarily focus on the cellular level changes under stress conditions and their corresponding effects on lipids (including omega-3 PUFAs), pigments and polymers.

Manipulating environmental stresses and stress tolerance of microalgae for enhanced production of lipids and value-added products-A review

Microalgae have promising potential to produce lipids and a variety of high-value chemicals. Suitable stress conditions such as nitrogen starvation and high salinity could stimulate synthesis and accumulation of lipids and high-value products by microalgae, therefore, various stress-modification strategies were developed to manipulate and optimize cultivation processes to enhance bioproduction efficiency. On the other hand, advancements in omics-based technologies have boosted the research to globally understand microalgal gene regulation under stress conditions, which enable further improvement of production efficiency via genetic engineering. Moreover, integration of multi-omics data, synthetic biology design, and genetic engineering manipulations exhibits a tremendous potential in the betterment of microalgal biorefinery. This review discusses the process manipulation strategies and omics studies on understanding the regulation of metabolite biosynthesis under various stressful conditions, and proposes genetic engineering of microalgae to improve bioproduction via manipulating stress tolerance.

CO2 utilization in the production of biomass and biocompounds by three different microalgae

The atmospheric CO₂ increase is considered the main cause of global warming. Microalgae are photosynthetic microorganisms that can help in CO₂ mitigation and at the same time produce value-added compounds. In this study, Scenedesmus obliquus, Chlorella vulgaris, and Chlorella protothecoides were cultivated under 0.035 (air), 5 and 10% (v/v) of CO₂ concentrations in air to evaluate the performance of the microalgae in terms of kinetic growth parameters, theoretical CO₂ biofixation rate, and biomass composition. Among the microalgae studied, S. obliquus presented the highest values of specific growth rate (μ = 1.28 d^-1), maximum productivities (P _max = 0.28 g L^-1d^-1), and theoretical CO₂ biofixation rates (0.56 g L^-1d^-1) at 10% CO₂. The highest oil content was found at 5% CO₂, and the fatty acid profile was not influenced by the concentration of CO₂ in the inflow gas mixture and was in compliance with EN 14214, being suitable for biodiesel purposes. The impact of the CO₂ on S. obliquus cells' viability/cell membrane integrity evaluated by the in-line flow cytometry is quite innovative and fast, and revealed that 86.4% of the cells were damaged/permeabilized in cultures without the addition of CO₂.

Effects of stepwise nitrogen depletion on carotenoid content, fluorescence parameters and the cellular stoichiometry of Chlorella vulgaris

Stressful conditions can stimulate the accumulation of carotenoids in some microalgae. To obtain more knowledge of the stress response, we studied the effects of different N concentrations on unicellular content of carotenoids using Raman spectroscopic technique; cellular stoichiometric changes and the fluorescence parameters of Chlorella vulgaris were concomitantly studied. Initially, we optimized the Raman scattering conditions and demonstrated the feasibility of unicellular carotenoid analysis by Raman spectroscopic technique. The results showed that an integration time of 10 s, laser power at 0.1mW and an accumulation time of 1 were the optimum conditions, and the peak height at 1523cm^-1 scaled linearly with the carotenoid content in the range of 0.625-1440mg/L with a recovery rate of 97%~103%. In the experiment, seven different nitrogen levels ranging from 0 to 2.48×10⁵μg/L were imposed. Samples were taken at the start, exponential phase and end of the experiment. The results showed that nitrogen stress can facilitate the synthesis of carotenoids, while at the same time, excessive nitrogen stress led to lower proliferative and photosynthetic activity. Compared with carotenoids, chlorophylls were more sensitive to nitrogen stress; it declined dramatically as stress processed. There existed no significant differences for F_v/Fm among different nitrogen levels during the exponential phase, while in the end, it declined and a significant difference appeared between cells in 2.48×10⁵μg/L N and other experimental levels. Photosynthetic efficiency, namely the C/N mole ratio in algal cells, didnot significantly change during the exponential phase; however, apparent increases ultimately occurred, except for the stable C/N in BG11 medium. This increase matched well with the carotenoid decline, indicating that an increasing cellular C/N mole ratio can be used as an indicator of excessive stress in carotenoid production. Besides, there also existed an inverse correlation with ETR_max.

Growth and biochemical characteristics of an indigenous freshwater microalga, Scenedesmus obtusus, cultivated in an airlift photobioreactor: effect of reactor hydrodynamics, light intensity, and photoperiod

The freshwater green algae, Scenedesmus obtusus, was cultivated in a 3.4 L airlift photobioreactor. The hydrodynamic parameters were estimated at different inlet gas flow rates (1, 2, 3, and 4 LPM) and their subsequent impact on the growth and biochemical characteristics of microalgae was studied. The biomass concentration and productivity increased with an increase in flow rates from 1 to 4 LPM. A maximum of 0.07 g L^-1 day^-1 productivity of biomass was attained at 3 LPM. An increase of total carbohydrate content from 19.6 to 26.4% was noticed with increment in the inlet flow rate of gas from 1 to 4 LPM. Major variations in total fatty acid content were not observed. The impact of light irradiance on growth and biochemical characteristics of S. obtusus was also evaluated. A maximum biomass productivity of 0.103 g L^-1 day^-1 was attained at an illumination of 150 μmol m^-2 s^-1 under continuous light. The major fatty acids reported were palmitic acid (C16:0), α-linolenic acid (C18:3), linoleic acid (C18:2), and oleic acid (C18:1). Biodiesel properties of the microalgae were estimated under various culture conditions. The light profile inside the airlift reactor was experimentally measured and the predictive modelling of light profile was also attempted.

Effect of salinity stress on growth, lipid productivity, fatty acid composition, and biodiesel properties in Acutodesmus obliquus and Chlorella vulgaris

Two microalgae strains including Chlorella vulgaris and Acutodesmus obliquus were grown on BG11 medium with salinity stress ranging from 0.06 to 0.4 M NaCl. Highest lipid content in C. vulgaris and A. obliquus was 49 and 43% in BG11 amended with 0.4 M NaCl. The microalgal strains C. vulgaris and A. obliquus grow better at 0.06 M NaCl concentration than control condition. At 0.06 M NaCl, improved dry biomass content in C. vulgaris and A. obliquus was 0.92 and 0.68 gL^-1, respectively. Stress biomarkers like reactive oxygen species, antioxidant enzyme catalase, and ascorbate peroxidase were also lowest at 0.06 M NaCl concentration revealing that both the microalgal strains are well acclimatized at 0.06 M NaCl concentration. The fatty acid composition of the investigated microalgal strains was also improved by increased NaCl concentration. At 0.4 M NaCl, palmitic acid (37%), oleic acid (15.5%), and linoleic acid (20%) were the dominant fatty acids in C. vulgaris while palmitic acid (54%) and stearic acid (26.6%) were major fatty acids found in A. obliquus. Fatty acid profiling of C. vulgaris and A. obliquus significantly varied with salinity concentration. Therefore, the study showed that salt stress is an effective stress that could increase not only the lipid content but also improved the fatty acid composition which could make C. vulgaris and A. obliquus potential strains for biodiesel production.

Isolation of a euryhaline microalgal strain, Tetraselmis sp. CTP4, as a robust feedstock for biodiesel production

Bioprospecting for novel microalgal strains is key to improving the feasibility of microalgae-derived biodiesel production. Tetraselmis sp. CTP4 (Chlorophyta, Chlorodendrophyceae) was isolated using fluorescence activated cell sorting (FACS) in order to screen novel lipid-rich microalgae. CTP4 is a robust, euryhaline strain able to grow in seawater growth medium as well as in non-sterile urban wastewater. Because of its large cell size (9–22 μm), CTP4 settles down after a six-hour sedimentation step. This leads to a medium removal efficiency of 80%, allowing a significant decrease of biomass dewatering costs. Using a two-stage system, a 3-fold increase in lipid content (up to 33% of DW) and a 2-fold enhancement in lipid productivity (up to 52.1 mg L⁻¹ d⁻¹) were observed upon exposure to nutrient depletion for 7 days. The biodiesel synthesized from the lipids of CTP4 contained high levels of oleic acid (25.67% of total fatty acids content) and minor amounts of polyunsaturated fatty acids with ≥4 double bonds (<1%). As a result, this biofuel complies with most of the European (EN14214) and American (ASTM D6751) specifications, which commonly used microalgal feedstocks are usually unable to meet. In conclusion, Tetraselmis sp. CTP4 displays promising features as feedstock with lower downstream processing costs for biomass dewatering and biodiesel refining.

A Review on the Assessment of Stress Conditions for Simultaneous Production of Microalgal Lipids and Carotenoids

Microalgal species are potential resource of both biofuels and high-value metabolites, and their production is growth dependent. Growth parameters can be screened for the selection of novel microalgal species that produce molecules of interest. In this context our review confirms that, autotrophic and heterotrophic organisms have demonstrated a dual potential, namely the ability to produce lipids as well as value-added products (particularly carotenoids) under influence of various physico-chemical stresses on microalgae. Some species of microalgae can synthesize, besides some pigments, very-long-chain polyunsaturated fatty acids (VL-PUFA,>20C) such as docosahexaenoic acid and eicosapentaenoic acid, those have significant applications in food and health. Producing value-added by-products in addition to biofuels, fatty acid methyl esters (FAME), and lipids has the potential to improve microalgae-based biorefineries by employing either the autotrophic or the heterotrophic mode, which could be an offshoot of biotechnology. The review considers the potential of microalgae to produce a range of products and indicates future directions for developing suitable criteria for choosing novel isolates through bioprospecting large gene pool of microalga obtained from various habitats and climatic conditions.

Use of De Novo Transcriptome Libraries to Characterize a Novel Oleaginous Marine Chlorella Species during the Accumulation of Triacylglycerols

Marine chlorophytes of the genus Chlorella are unicellular algae capable of accumulating a high proportion of cellular lipids that can be used for biodiesel production. In this study, we examined the broad physiological capabilities of a subtropical strain (C596) of Chlorella sp. “SAG-211-18” including its heterotrophic growth and tolerance to low salt. We found that the alga replicates more slowly at diluted salt concentrations and can grow on a wide range of carbon substrates in the dark. We then sequenced the RNA of Chlorella strain C596 to elucidate key metabolic genes and investigate the transcriptomic response of the organism when transitioning from a nutrient-replete to a nutrient-deficient condition when neutral lipids accumulate. Specific transcripts encoding for enzymes involved in both starch and lipid biosynthesis, among others, were up-regulated as the cultures transitioned into a lipid-accumulating state whereas photosynthesis-related genes were down-regulated. Transcripts encoding for two of the up-regulated enzymes—a galactoglycerolipid lipase and a diacylglyceride acyltransferase—were also monitored by reverse transcription quantitative polymerase chain reaction assays. The results of these assays confirmed the transcriptome-sequencing data. The present transcriptomic study will assist in the greater understanding, more effective application, and efficient design of Chlorella-based biofuel production systems.

Carotenoids in Microalgae

Carotenoids are a class of isoprenoids synthesized by all photosynthetic organisms as well as by some non-photosynthetic bacteria and fungi with broad applications in food, feed and cosmetics, and also in the nutraceutical and pharmaceutical industries. Microalgae represent an important source of high-value products, which include carotenoids, among others. Carotenoids play key roles in light harvesting and energy transfer during photosynthesis and in the protection of the photosynthetic apparatus against photooxidative damage. Carotenoids are generally divided into carotenes and xanthophyls, but accumulation in microalgae can also be classified as primary (essential for survival) and secondary (by exposure to specific stimuli).In this chapter, we outline the high value carotenoids produced by commercially important microalgae, their production pathways, the improved production rates that can be achieved by genetic engineering as well as their biotechnological applications.