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

Molecular design of microalgae as sustainable cell factories

Microalgae are regarded as sustainable and potent chassis for biotechnology. Their capacity for efficient photosynthesis fuels dynamic growth independent from organic carbon sources and converts atmospheric CO2 directly into various valuable hydrocarbon-based metabolites. However, approaches to gene expression and metabolic regulation have been inferior to those in more established heterotrophs (e.g., prokaryotes or yeast) since the genetic tools and insights in expression regulation have been distinctly less advanced. In recent years, however, these tools and their efficiency have dramatically improved. Various examples have demonstrated new trends in microalgal biotechnology and the potential of microalgae for the transition towards a sustainable bioeconomy.

Concurrent enhancement of biomass production and phycocyanin content in salt-stressed Arthrospira platensis: A glycine betaine- supplementation approach

In industrial-scale cultivation of microalgae, salinity stress often stimulates high-value metabolites production but decreases biomass yield. In this research, we present an extraordinary response of Arthrospira platensis to salinity stress. Specifically, we observed a significant increase in both biomass production (2.58 g L-1) and phycocyanin (PC) content (22.31%), which were enhanced by 1.26-fold and 2.62-fold, respectively, compared to the control, upon exposure to exogenous glycine betaine (GB). The biochemical analysis reveals a significant enhancement in carbonic anhydrase activity and chlorophyll a level, concurrent with reductions in carbohydrate content and reactive oxygen species (ROS) levels. Further, transcriptomic profiling indicates a downregulation of genes associated with the tricarboxylic acid (TCA) cycle and an upregulation of genes linked to nitrogen assimilation, hinting at a rebalanced carbon/nitrogen metabolism favoring PC accumulation. This work thus presents a promising strategy for simultaneous enhancement of biomass production and PC content in A. platensis and expands our understanding of PC biosynthesis and salinity stress responses in A. platensis.

Melatonin, a phytohormone for enhancing the accumulation of high-value metabolites and stress tolerance in microalgae: Applications, mechanisms, and challenges

High-value metabolites, such as carotenoids, lipids, and proteins, are synthesized by microalgae and find applications in various fields, including food, health supplements, and cosmetics. However, the potential of the microalgal industry to serve these sectors is constrained by low productivity and high energy consumption. Environmental stressors can not only stimulate the accumulation of secondary metabolites in microalgae but also induce oxidative stress, suppressing cell growth and activity, thereby resulting in a decrease in overall productivity. Using melatonin (MT) under stressful conditions is an effective approach to enhance the productivity of microalgal metabolites. This review underscores the role of MT in promoting the accumulation of high-value metabolites and enhancing stress resistance in microalgae under stressful and wastewater conditions. It discusses the underlying mechanisms whereby MT enhances metabolite synthesis and improves stress resistance. The review also offers new perspectives on utilizing MT to improve microalgal productivity and stress resistance in challenging environments.

Microbial Pigments: Major Groups and Industrial Applications

Microbial pigments have many structures and functions with excellent characteristics, such as being biodegradable, non-toxic, and ecologically friendly, constituting an important source of pigments. Industrial production presents a bottleneck in production cost that restricts large-scale commercialization. However, microbial pigments are progressively gaining popularity because of their health advantages. The development of metabolic engineering and cost reduction of the bioprocess using industry by-products opened possibilities for cost and quality improvements in all production phases. We are thus addressing several points related to microbial pigments, including the major classes and structures found, the advantages of use, the biotechnological applications in different industrial sectors, their characteristics, and their impacts on the environment and society.

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.

Isomerization of carotenoids in photosynthesis and metabolic adaptation

In nature, carotenoids are present as trans- and cis-isomers. Various physical and chemical factors like light, heat, acids, catalytic agents, and photosensitizers can contribute to the isomerization of carotenoids. Living organisms in the process of evolution have developed different mechanisms of adaptation to light stress, which can also involve isomeric forms of carotenoids. Particularly, light stress conditions can enhance isomerization processes. The purpose of this work is to review the recent studies on cis/trans isomerization of carotenoids as well as the role of carotenoid isomers for the light capture, energy transfer, photoprotection in light-harvesting complexes, and reaction centers of the photosynthetic apparatus of plants and other photosynthetic organisms. The review also presents recent studies of carotenoid isomers for the biomedical aspects, showing cis- and trans-isomers differ in bioavailability, antioxidant activity and biological activity, which can be used for therapeutic and prophylactic purposes.

Engineered Living Materials for Advanced Diseases Therapy

Natural living materials serving as biotherapeutics exhibit great potential for treating various diseases owing to their immunoactivity, tissue targeting, and other biological activities. In this review, the recent developments in engineered living materials, including mammalian cells, bacteria, viruses, fungi, microalgae, plants, and their active derivatives that are used for treating various diseases are summarized. Further, the future perspectives and challenges of such engineered living material-based biotherapeutics are discussed to provide considerations for future advances in biomedical applications.

Natural Carotenoids: Recent Advances on Separation from Microbial Biomass and Methods of Analysis

Biotechnologically produced carotenoids occupy an important place in the scientific research. Owing to their role as natural pigments and their high antioxidant properties, microbial carotenoids have been proposed as alternatives to their synthetic counterparts. To this end, many studies are focusing on their efficient and sustainable production from renewable substrates. Besides the development of an efficient upstream process, their separation and purification as well as their analysis from the microbial biomass confers another important aspect. Currently, the use of organic solvents constitutes the main extraction process; however, environmental concerns along with potential toxicity towards human health necessitate the employment of "greener" techniques. Hence, many research groups are focusing on applying emerging technologies such as ultrasounds, microwaves, ionic liquids or eutectic solvents for the separation of carotenoids from microbial cells. This review aims to summarize the progress on both the biotechnological production of carotenoids and the methods for their effective extraction. In the framework of circular economy and sustainability, the focus is given on green recovery methods targeting high-value applications such as novel functional foods and pharmaceuticals. Finally, methods for carotenoids identification and quantification are also discussed in order to create a roadmap for successful carotenoids analysis.

Recent advances in CO2 fixation by microalgae and its potential contribution to carbon neutrality

Many countries and regions have set their schedules to achieve the carbon neutrality between 2030 and 2070. Microalgae are capable of efficiently fixing CO₂ and simultaneously producing biomass for multiple applications, which is considered one of the most promising pathways for carbon capture and utilization. This work reviews the current research on microalgae CO₂ fixation technologies and the challenges faced by the related industries and government agencies. The technoeconomic analysis indicates that cultivation is the major cost factor. Use of waste resources such as wastewater and flue gas can significantly reduce the costs and carbon footprints. The life cycle assessment has identified fossil-based electricity use as the major contributor to the global warming potential of microalgae-based CO₂ fixation approach. Substantial efforts and investments are needed to identify and bridge the gaps among the microalgae strain development, cultivation conditions and systems, and use of renewable resources and energy.

Filamentous fungi for sustainable vegan food production systems within a circular economy: Present status and future prospects

Filamentous fungi serve as potential candidates in the production of different value-added products. In the context of food, there are several advantages of using filamentous fungi for food. Among the main advantages is that the fungal biomass used food not only meets basic nutritional requirements but that it is also rich in protein, low in fat, and free of cholesterol. This speaks to the potential of filamentous fungi in the production of food that can substitute animal-derived protein sources such as meat. Moreover, life-cycle analyses and techno-economic analyses reveal that fungal proteins perform better than animal-derived proteins in terms of land use efficiency as well as global warming. The present article provides an overview of the potential of filamentous fungi as a source of food and food supplements. The commercialization potential as well as social, legal and safety issues of fungi-based food products are discussed.

A Review on a Hidden Gem: Phycoerythrin from Blue-Green Algae

Phycoerythrin (PE) is a pink/red-colored pigment found in rhodophytes, cryptophytes, and blue-green algae (cyanobacteria). The interest in PE is emerging from its role in delivering health benefits. Unfortunately, the current cyanobacterial-PE (C-PE) knowledge is still in the infant stage. It is essential to acquire a more comprehensive understanding of C-PE. This study aimed to review the C-PE structure, up and downstream processes of C-PE, application of C-PE, and strategies to enhance its stability and market value. In addition, this study also presented a strengths, weaknesses, opportunities, and threats (SWOT) analysis on C-PE. Cyanobacteria appeared to be the more promising PE producers compared to rhodophytes, cryptophytes, and macroalgae. Green/blue light is preferred to accumulate higher PE content in cyanobacteria. Currently, the prominent C-PE extraction method is repeated freezing-thawing. A combination of precipitation and chromatography approaches is proposed to obtain greater purity of C-PE. C-PE has been widely exploited in various fields, such as nutraceuticals, pharmaceuticals, therapeutics, cosmetics, biotechnology, food, and feed, owing to its bioactivities and fluorescent properties. This review provides insight into the state-of-art nature of C-PE and advances a step further in commercializing this prospective pigment.

Mechanisms of promotion in the heterotrophic growth of Chlorella vulgaris by the combination of sodium acetate and hydrolysate of broken rice

In order to reduce the culture cost and increase the growth rate of heterotrophic Chlorella vulgaris, the effects of hydrolysate of broken rice (HBR) combined with sodium acetate on its growth were evaluated. Results showed that the addition of 0.4 g/L of sodium acetate could stabilize the pH of the medium via the co-metabolism of acetate, ammonia and nitrate by Chlorella vulgaris. Meanwhile, isocitrate lyase activity increased threefold, which further promoted the glyoxylate cycle and the citric acid cycle, which finally provided more energy and metabolic precursors for cell growth. The biomass production (5.04 g/L), biomass productivity (1.65 g/L/day) and protein content (64.14 %) were 1.56, 1.81 and 1.77 times higher than the glucose group. This study demonstrated that HBR combined with sodium acetate could effectively promote the heterotrophic metabolism of microalgae, which provided scientific basis and guidance for industrial production of high-value products using Chlorella vulgaris as a fermentation platform.

Pigment production by a newly isolated strain Pycnoporus sanguineus SYBC-L7 in solid-state fermentation

Natural pigments are playing important roles in our daily lives. They not only make products colorful but also provide various health benefits for humans. In addition, Pycnoporus genus, listed as food- and cosmetic-grade microorganism, is one of the promising organisms for developing natural pigments. In this study, a new fungal strain with high efficiency in producing intense orange pigments was isolated and identified as Pycnoporus sanguineus SYBC-L7. Different agro-industrial wastes were applied to evaluate the growth and pigment production of strain SYBC-L7. SYBC-L7 can grow rapidly and effectively produce pigments using wood chips as substrate in solid-state fermentation (SSF). Culture conditions were also optimized for value-added pigments production and the optimum production conditions were glucose as carbon source, ammonium tartrate as nitrogen source, initial pH 6.0, and relative humidity of 65%. Pigment components, cinnabarinic acid, tramesanguin, and 2-amino-9-formylphenoxazone-1-carbonic acid were confirmed by liquid chromatography–mass spectrometry. Meanwhile, an agar plate diffusion assay was performed to evaluate the antimicrobial activity of the pigment. These pigments showed more significant inhibition of Gram-positive than Gram-negative bacteria. The results showed that Pycnoporus sanguineus SYBC-L7 was able to cost-effectively produce intense natural orange pigments with antibacterial activity in SSF, which is the basis of their large-scale production and application.

Fungal Contamination in Microalgal Cultivation: Biological and Biotechnological Aspects of Fungi-Microalgae Interaction

In the last few decades, the increasing interest in microalgae as sources of new biomolecules and environmental remediators stimulated scientists’ investigations and industrial applications. Nowadays, microalgae are exploited in different fields such as cosmeceuticals, nutraceuticals and as human and animal food supplements. Microalgae can be grown using various cultivation systems depending on their final application. One of the main problems in microalgae cultivations is the possible presence of biological contaminants. Fungi, among the main contaminants in microalgal cultures, are able to influence the production and quality of biomass significantly. Here, we describe fungal contamination considering both shortcomings and benefits of fungi-microalgae interactions, highlighting the biological aspects of this interaction and the possible biotechnological applications.

Phycobiliproteins, the pigment-protein complex form of natural food colorants and bioactive ingredients

Currently, the use of synthetic pigments in foods is restricted since synthetic pigments are proven and suspected to be harmful to human health. Phycobiliproteins (PBPs), existed in phycobilisomes (PBSs) of algae, are a kind of pigment-proteins with intense color. The specific color of PBPs (red and blue) is given by the water-soluble open-chained tetrapyrrole chromophore (phycobilin) that covalently attaches to the apo-protein via thioether linkages to cysteine residues. According to the spectral characteristics of PBPs, they can be categorized as phycoerythrins (PEs), phycocyanins (PCs), allophycocyanins (APCs), and phycoerythrocyanins (PECs). PBPs can be used as natural food colorants, fluorescent substances, and bioactive ingredients in food applications owing to their color characteristics and physiological activities. This paper mainly summarizes the extraction and purification methods of the PBPs and reviews their characteristics and applications. Moreover, the use of several strategies such as additives, microencapsulation, electrospray, and cross-linking to improve the stability and bioavailability of PBPs as well as the future outlooks of PBPs as natural colorants in food commercialization are elucidated.

Optimized Protocol for Microalgae DNA Staining with SYTO9/SYBR Green I, Based on Flow Cytometry and RSM Methodology: Experimental Design, Impacts and Validation

Multiple fluorochromes are extensively used to investigate different microalgal aspects, such as viability and physiology. Some of them can be used to stain nucleic acids (DNA). Well-known examples are SYBR Green I and SYTO 9, the latter of which offers several advantages, especially when combined with flow cytometry (FCM)—a powerful method for studying microalgal population heterogeneity and analyzing their cell cycles. However, the effects of these dyes on the microalgae cell physiology have not been fully elucidated yet. A statistical experimental design, using response surface methodology (RSM) with FCM was applied in this study to optimize the DNA staining of a non-conventional microalgae, Chromochloris zofingiensis, with SYBR Green I and SYTO 9, and to optimize the variables affecting staining efficiency, i.e., the dye concentration, incubation time and staining temperature. We found that none of these factors affects the staining efficiency, which was not less than 99.65%. However, for both dyes, the dye concentration was shown to be the most significant factor causing cell damage (p-values: 0.0003; <0.0001) for SYBR Green I and SYTO 9, respectively. The staining temperature was only significant for SYTO 9 (p-value: 0.0082), and no significant effect was observed regarding the incubation time for both dyes. The values of the optimized parameters (0.5 µM, 05 min and 25 °C) for SYTO 9 and (0.5 X, 5 min and 25 °C) for SYBR Green I resulted in the maximum staining efficiency (99.8%; 99.6%), and the minimum damaging effects (12.86%; 13.75%) for SYTO 9 and SYBR Green I, respectively. These results offer new perspectives for improving the use of DNA staining fluorochromes and provides insights into their possible side effects on microalgae.

Natural Substrates and Culture Conditions to Produce Pigments from Potential Microbes in Submerged Fermentation

Pigments from bacteria, fungi, yeast, cyanobacteria, and microalgae have been gaining more demand in the food, leather, and textile industries due to their natural origin and effective bioactive functions. Mass production of microbial pigments using inexpensive and ecofriendly agro-industrial residues is gaining more demand in the current research due to their low cost, natural origin, waste utilization, and high pigment stimulating characteristics. A wide range of natural substrates has been employed in submerged fermentation as carbon and nitrogen sources to enhance the pigment production from these microorganisms to obtain the required quantity of pigments. Submerged fermentation is proven to yield more pigment when added with agro-waste residues. Hence, in this review, aspects of potential pigmented microbes such as diversity, natural substrates that stimulate more pigment production from bacteria, fungi, yeast, and a few microalgae under submerged culture conditions, pigment identification, and ecological functions are detailed for the benefit of industrial personnel, researchers, and other entrepreneurs to explore pigmented microbes for multifaceted applications. In addition, some important aspects of microbial pigments are covered herein to disseminate the knowledge.