A novel strategy for the hyper-production of astaxanthin from the newly isolated microalga Haematococcus pluvialis JNU35

Thraustochytrids as a promising source of fatty acids, carotenoids, and sterols: bioactive compound biosynthesis, and modern biotechnology

Thraustochytrids are eukaryotes and obligate marine protists. They are increasingly considered to be a promising feed additive because of their superior and sustainable application in the production of health-benefiting bioactive compounds, such as fatty acids, carotenoids, and sterols. Moreover, the increasing demand makes it critical to rationally design the targeted products by engineering industrial strains. In this review, bioactive compounds accumulated in thraustochytrids were comprehensively evaluated according to their chemical structure, properties, and physiological function. Metabolic networks and biosynthetic pathways of fatty acids, carotenoids, and sterols were methodically summarized. Further, stress-based strategies used in thraustochytrids were reviewed to explore the potential methodologies for enhancing specific product yields. There are internal relationships between the biosynthesis of fatty acids, carotenoids, and sterols in thraustochytrids since they share some branches of the synthetic routes with some intermediate substrates in common. Although there are classic synthesis pathways presented in the previous research, the metabolic flow of how these compounds are being synthesized in thraustochytrids still remains uncovered. Further, combined with omics technologies to deeply understand the mechanism and effects of different stresses is necessary, which could provide guidance for genetic engineering. While gene-editing technology has allowed targeted gene knock-in and knock-outs in thraustochytrids, efficient gene editing is still required. This critical review will provide comprehensive information to benefit boosting the commercial productivity of specific bioactive substances by thraustochytrids.

Extraction and separation of astaxanthin with the help of pre-treatment of Haematococcus pluvialis microalgae biomass using aqueous two-phase systems based on deep eutectic solvents

The microalgae Haematococcus pluvialis are the main source of the natural antioxidant astaxanthin. However, the effective extraction of astaxanthin from these microalgae remains a significant challenge due to the rigid, non-hydrolyzable cell walls. Energy savings and high-efficiency cell disruption are essential steps in the recovery of the antioxidant astaxanthin from the cysts of H. pluvialis. In the present study, H. pluvialis microalgae were first cultured in Bold's Basal medium under certain conditions to reach the maximum biomass concentration, and then light shock was applied for astaxanthin accumulation. The cells were initially green and oval, with two flagella. As the induction time increases, the motile cells lose their flagellum and become red cysts with thick cell walls. Pre-treatment of aqueous two-phase systems based on deep eutectic solvents was used to decompose the cell wall. These systems included dipotassium hydrogen phosphate salt, water, and two types of deep eutectic solvents (choline chloride-urea and choline chloride-glucose). The results of pre-treatment of Haematococcus cells by the studied systems showed that intact, healthy cysts were significantly ruptured, disrupted, and facilitated the release of cytoplasmic components, thus facilitating the subsequent separation of astaxanthin by liquid-liquid extraction. The system containing the deep eutectic solvent of choline chloride-urea was the most effective system for cell wall degradation, which resulted in the highest ability to extract astaxanthin. More than 99% of astaxanthin was extracted from Haematococcus under mild conditions (35% deep eutectic solvent, 30% dipotassium hydrogen phosphate at 50 °C, pH = 7.5, followed by liquid-liquid extraction at 25 °C). The present study shows that the pre-treatment of two-phase systems based on deep eutectic solvent and, thus, liquid-liquid extraction is an efficient and environmentally friendly process to improve astaxanthin from the microalgae H. pluvialis.

Advancement of Carotenogenesis of Astaxanthin from Haematococcus pluvialis: Recent Insight and Way Forward

The demand for astaxanthin has been increasing for many health applications ranging from pharmaceuticals, food, cosmetics, and aquaculture due to its bioactive properties. Haematococcus pluvialis is widely recognized as the microalgae species with the highest natural accumulation of astaxanthin, which has made it a valuable source for industrial production. Astaxanthin produced by other sources such as chemical synthesis or fermentation are often produced in the cis configuration, which has been shown to have lower bioactivity. Additionally, some sources of astaxanthin, such as shrimp, may denature or degrade when exposed to high temperatures, which can result in a loss of bioactivity. Producing natural astaxanthin through the cultivation of H. pluvialis is presently a demanding and time-consuming task, which incurs high expenses and restricts the cost-effective industrial production of this valuable substance. The production of astaxanthin occurs through two distinct pathways, namely the cytosolic mevalonate pathway and the chloroplast methylerythritol phosphate (MEP) pathway. The latest advancements in enhancing product quality and extracting techniques at a reasonable cost are emphasized in this review. The comparative of specific extraction processes of H. pluvialis biological astaxanthin production that may be applied to large-scale industries were assessed. The article covers a contemporary approach to optimizing microalgae culture for increased astaxanthin content, as well as obtaining preliminary data on the sustainability of astaxanthin production and astaxanthin marketing information.

On-Chip Photoacoustics-Activated Cell Sorting (PA-ACS) for Label-Free and High-Throughput Detection and Screening of Microalgal Cells

Microalgae play a crucial role in global carbon cycling as they convert carbon dioxide into various valuable macromolecules. Among them, Haematococcus pluvialis (H. pluvialis) is the richest natural source of astaxanthin (AXT), which is a valuable antioxidant, anti-inflammatory, and antiapoptosis agent. These benefits make AXT highly commercially valuable in pharmaceuticals, cosmetics, and nutritional industries. However, intrinsic genetic characteristics and extrinsic cultivation conditions influence biomass gains, leading to low productivity and extraction as the main techno-economic bottlenecks in this industry. Thus, detecting AXT in H. pluvialis is essential to determine the influence of multiple parameters on biocompound accumulation, enabling optimization of cultivation and enrichment of AXT-rich H. pluvialis cells. This work developed an opto-acousto-fluidic microplatform for detection, analysis, and sorting of microalgae. Via label-free monitoring and extraction of sample-induced ultrasonic signals, a photoacoustic microscopic system was proposed to provide a full-field visualization of AXT's content and distribution inside H. pluvialis cells. When employed as on-chip image-based flow cytometry, our microplatform can also offer high-throughput measurements of intracellular AXT in real time, which demonstrates similar results to conventional spectrophotometry methods and further reveals the heterogeneity of AXT content at the single-cell level. In addition, a solenoid valve-pump dual-mode cell sorter was integrated for effective sorting of cells with a maximum working frequency of 0.77 Hz, reducing the fluid response time by 50% in rising and 40-fold in recovery. The H. pluvialis cells which have more AXT accumulation (>30 μm in diameter) were 4.38-fold enriched with almost no dead empty and small green cells. According to the results, automated and reliable photoacoustics-activated cell sorting (PA-ACS) can screen AXT-rich cells and remove impurities at the terminal stage of cultivation, thereby increasing the effectiveness and purity of AXT extraction. The proposed system can be further adopted to enrich strains and mutants for the production of biofuels or other rare organic substances such as β-carotene and lutein.

Biotechnological strategies overcoming limitations to H. pluvialis-derived astaxanthin production and Morocco's potential

Haematococcus pluvialis is the richest source of natural astaxanthin, but the production of H. pluvialis-derived astaxanthin is usually limited by its slow cell proliferation and astaxanthin accumulation. Efforts to enhance biomass productivity, astaxanthin accumulation, and extraction are ongoing. This review highlights different approaches that have previously been studied in microalgal species for enhanced biomass productivity, as well as optimized methods for astaxanthin accumulation and extraction, and how these methods could be combined to bypass the challenges limiting natural astaxanthin production, particularly in H. pluvialis, at all stages (biomass production, and astaxanthin accumulation and extraction). Biotechnological approaches, such as overexpressing low CO2 inducible genes, utilizing complementary carbon sources, CRISPR-Cas9 bioengineering, and the use of active compounds, for biomass productivity are outlined. Direct astaxanthin extraction from H. pluvialis zoospores and Morocco's potential for microalgal-based astaxanthin production are equally discussed. This review emphasizes the need to engineer an optimized H. pluvialis-derived astaxanthin production system combining two or more of these strategies for increased growth, and astaxanthin productivity, to compete in the larger, lower-priced market in aquaculture and nutraceutical sectors.

Metabolic engineering and synthetic biology strategies for producing high-value natural pigments in Microalgae

Microalgae are microorganisms capable of producing bioactive compounds using photosynthesis. Microalgae contain a variety of high value-added natural pigments such as carotenoids, phycobilins, and chlorophylls. These pigments play an important role in many areas such as food, pharmaceuticals, and cosmetics. Natural pigments have a health value that is unmatched by synthetic pigments. However, the current commercial production of natural pigments from microalgae is not able to meet the growing market demand. The use of metabolic engineering and synthetic biological strategies to improve the production performance of microalgal cell factories is essential to promote the large-scale production of high-value pigments from microalgae. This paper reviews the health and economic values, the applications, and the synthesis pathways of microalgal pigments. Overall, this review aims to highlight the latest research progress in metabolic engineering and synthetic biology in constructing engineered strains of microalgae with high-value pigments and the application of CRISPR technology and multi-omics in this context. Finally, we conclude with a discussion on the bottlenecks and challenges of microalgal pigment production and their future development prospects.

A crosswalk on the genetic and conventional strategies for enhancing astaxanthin production in Haematococcus pluvialis

Astaxanthin is a naturally occurring xanthophyll with powerful: antioxidant, antitumor, and antibacterial properties that are widely employed in food, feed, medicinal and nutraceutical industries. Currently, chemical synthesis dominates the world's astaxanthin market, but the increasing demand for natural products is shifting the market for natural astaxanthin. Haematococcus pluvialis (H. pluvialis) is the factory source of natural astaxanthin when grown in optimal conditions. Currently, various strategies for the production of astaxanthin have been proposed or are being developed in order to meet its market demand. This up-to-date review scrutinized the current approaches or strategies that aim to increase astaxanthin yield from H. pluvialis. We have emphasized the genetic and environmental parameters that increase astaxanthin yield. We also looked at the transcriptomic dynamics caused by environmental factors (phytohormones induction, light, salt, temperature, and nutrient starvation) on astaxanthin synthesizing genes and other metabolic changes. Genetic engineering and culture optimization (environmental factors) are effective approaches to producing more astaxanthin for commercial purposes. Genetic engineering, in particular, is accurate, specific, potent, and safer than conventional random mutagenesis approaches. New technologies, such as CRISPR-Cas9 coupled with omics and emerging computational tools, may be the principal strategies in the future to attain strains that can produce more astaxanthin. This review provides accessible data on the strategies to increase astaxanthin accumulation natively. Also, this review can be a starting point for new scholars interested in H. pluvialis research.

Promoting cell growth and characterizing partial symbiotic relationships in the co-cultivation of green alga Chlamydomonas reinhardtii and Escherichia coli

BACKGROUND

By co-culturing selected microalgae and heterotrophic microorganisms, the growth rate of microalgae can be improved even under atmospheric conditions with a low CO₂ concentration. However, the detailed mechanism of improvement of proliferative capacity by co-culture has not been elucidated. In this study, we investigated changes in the proliferative capacity of the green alga Chlamydomonas reinhardtii by co-culturing with Escherichia coli.

MAIN METHODS AND MAJOR RESULTS

In the co-culture, the number of C. reinhardtii cells reached 2.22 × 10¹⁰  cell/L on day 14 of culture. This was about 1.9 times the number of cells (1.16 × 10¹⁰  cell/L) on day 14 compared to C. reinhardtii cells in monoculture. The starch content per cell in the co-culture of C. reinhardtii and E. coli on the 14th day (2.09 × 10^-11  g/cell) was 1.3 times higher than that in the C. reinhardtii monoculture (1.59 × 10^-11  g/cell), and the starch content per culture medium improved 2.5 times with co-cultivation. By analyzing the gene transcription profiles and key media components, we clarified that E. coli produced CO₂ from the organic carbon in the medium and the organic carbon produced by photosynthesis of C. reinhardtii, and this CO₂ likely enhanced the growth of C. reinhardtii.

CONCLUSIONS

Consequently, E. coli plays a key role in promoting the growth of C. reinhardtii as well as the accumulation of starch which is a valuable intermediate for the production of a range of useful chemicals from CO₂ .

Comparison of the Retention Rates of Synthetic and Natural Astaxanthin in Feeds and Their Effects on Pigmentation, Growth, and Health in Rainbow Trout (Oncorhynchus mykiss)

The coloring efficiency and physiological function of astaxanthin in fish vary with its regions. The aim of this study was to compare the retention rates of dietary astaxanthin from different sources and its effects on growth, pigmentation, and physiological function in Oncorhynchus mykiss. Fish were fed astaxanthin-supplemented diets (LP: 0.1% Lucantin^® Pink CWD; CP: 0.1% Carophyll^® Pink; EP: 0.1% Essention^® Pink; PR: 1% Phaffia rhodozyma; HP: 1% Haematococcus pluvialis), or a diet without astaxanthin supplementation, for 56 days. Dietary astaxanthin enhanced pigmentation as well as the growth of the fish. The intestinal morphology of fish was improved, and the crude protein content of dorsal muscle significantly increased in fish fed with astaxanthin. Moreover, astaxanthin led to a decrease in total cholesterol levels and alanine aminotransferase and aspartate aminotransferase activity in plasma. Fish fed on the CP diet also produced the highest level of umami amino acids (aspartic acid and glutamic acid). Regarding antioxidant capacity, astaxanthin increased Nrf2/HO-1 signaling and antioxidant enzyme activity. Innate immune responses, including lysozyme and complement systems, were also stimulated by astaxanthin. Lucantin^® Pink CWD had the highest stability in feed and achieved the best pigmentation, Essention^® Pink performed best in growth promotion and Carophyll^® Pink resulted in the best flesh quality. H. pluvialis was the astaxanthin source for achieving the best antioxidant properties and immunity of O. mykiss.

Increasing production and bio-accessibility of natural astaxanthin in Haematococcus pluvialis by screening and culturing red motile cells under high light condition

The thick cell wall and low astaxanthin productivity were two important bottlenecks limiting industrial production of astaxanthin via Haematococcus pluvialis. This study reports a strategy for increasing production and bio-accessibility of astaxanthin in H. pluvialis by screening and culturing red motile cells under high light condition. Compared with the original strain NBU489, the biomass of the novel isolated strain RMS10 increased by 31.9% under low light condition, and the astaxanthin content (44.6 mg/g) increased by 53.3% after 9-day high light induction, which were readily extracted and digested without cell disruption. Subsequent transcriptomic analysis confirmed the accumulation of astaxanthin and lipids in RMS10 cells as expression of genes associated with biosynthesis of fatty acid and astaxanthin were up-regulated, while those involved in thick cell wall biosynthesis and reactive oxygen species scavenging were down-regulated in RMS10. Collectively, this study provides a simple and effective method for economical production of natural astaxanthin.

Bioenergy, Biofuels, Lipids and Pigments—Research Trends in the Use of Microalgae Grown in Photobioreactors

This scientometric review and bibliometric analysis aimed to characterize trends in scientific research related to algae, photobioreactors and astaxanthin. Scientific articles published between 1995 and 2020 in the Web of Science and Scopus bibliographic databases were analyzed. The article presents the number of scientific articles in particular years and according to the publication type (e.g., articles, reviews and books). The most productive authors were selected in terms of the number of publications, the number of citations, the impact factor, affiliated research units and individual countries. Based on the number of keyword occurrences and a content analysis of 367 publications, seven leading areas of scientific interest (clusters) were identified: (1) techno-economic profitability of biofuels, bioenergy and pigment production in microalgae biorefineries, (2) the impact of the construction of photobioreactors and process parameters on the efficiency of microalgae cultivation, (3) strategies for increasing the amount of obtained lipids and obtaining biodiesel in Chlorella microalgae cultivation, (4) the production of astaxanthin on an industrial scale using Haematococcus microalgae, (5) the productivity of biomass and the use of alternative carbon sources in microalgae culture, (6) the effect of light and carbon dioxide conversion on biomass yield and (7) heterotrophy. Analysis revealed that topics closely related to bioenergy production and biofuels played a dominant role in scientific research. This publication indicates the directions and topics for future scientific research that should be carried out to successfully implement economically viable technology based on microalgae on an industrial scale.

Beneficial Changes in Growth Performance, Antioxidant Capacity, Immune Response, Hepatic Health, and Flesh Quality of Trachinotus ovatus Fed With Oedocladium carolinianum

The purpose of this study is to assess the feasibility of astaxanthin-rich Oedocladium carolinianum as an immunostimulant in the diet for Trachinotus ovatus. Three experimental diets containing 0% (OC0), 1% (OC1), and 5% (OC5) O. carolinianum powder were formulated for 6-week feeding trials. The results indicated that the OC5 diet boosted the growth performance through decreasing the feed conversion ratio and increasing digestive enzyme activities and intestinal villus length. Meanwhile, fish fed with the OC5 diet promoted antioxidant ability via stimulating the Nrf2-ARE signal pathway and enhancing antioxidant enzyme activities. Furthermore, the OC5 diet exerted hepatoprotective effects by suppressing the lipid deposition and inflammation response and enhancing the transport capacity of cholesterol. Besides, the OC5 diet improved the non-specific immunity by activating the lysozyme and complement system and increasing the nitric oxide content and total nitric oxide synthase activity. Dietary O. carolinianum supplementation promoted the deposition of astaxanthin in the whole body. Therefore, a diet supplemented with 5% O. carolinianum is recommended to boost the growth, antioxidant capacity, immune response, and flesh quality of T. ovatus.

Effects of Barranca yajiagengensis Powder in the Diet of Trachinotus ovatus on the Growth Performance, Antioxidant Capacity, Immunity and Morphology of the Liver and Intestine

Barranca yajiagengensis, a novel filamentous microalga, can accumulate lutein under high-light and low-nitrogen conditions. It is well known that lutein has antioxidant, anti-inflammatory and immune-modulating properties. The purpose of this study is to evaluate the effects of including lutein-rich B. yajiagengensis powder in the diet of Trachinotus ovatus on the growth performance, antioxidant capacity, immunity, liver, and intestinal morphology. For this aim, three experimental diets containing 0% (BY0), 1% (BY1), and 5% (BY5) B. yajiagengensis powder were formulated for six-week feeding trials. The results indicated that growth performance, feed utilization, and intestinal morphology were not affected by different diet treatments. Fish fed with the BY5 diet promoted antioxidant ability by activating the Nrf2-ARE signal pathway and enhancing antioxidant enzymes activities. Furthermore, the BY5 diet improved non-specific immunity and antibacterial ability by activating lysozymes and the complement system and increasing the nitric oxide (NO) content and total nitric oxide synthase activity. Dietary B. yajiagengensis supplementation improved the liver morphology and exerted hepatoprotective effects. Therefore, as a natural source of lutein, B. yajiagengensis has the potential as a safe and non-toxic immunostimulant for T. ovatus. A diet supplemented with 5% B. yajiagengensis is recommended to improve the growth, antioxidant capacity, immune response, and liver health of T. ovatus.

Identification and Characterization of a New Microalga Dysmorphococcus globosus-HI from the Himalayan Region as a Potential Source of Natural Astaxanthin

Synthesized astaxanthin (ASX), stereoisomers of 3S,3′R, 3R,3′R, and 3S,3′S, have over 95% market share and have relatively poor antioxidant and bioactivity properties, with persistent issues in terms of biological functions, health benefits, and biosafety if compared to natural ASX. Bioprospecting of new microalgal strains could be vital for a new source of powerful antioxidant (ASX). In this study, a new algal strain was isolated from the Indian foothills of the Himalayas. Its identity was discerned by morphological and DNA barcode studies. It is a unicellular spheroidal cell-shaped alga with 100–200 μm diameter. The isolate has 93.4% similarity to Dysmorphococcus globosus species based on 18S-rDNA phylogenetic analysis and named as D. globosus-HI (HI stands for Himalayan India). Its growth and major cellular components (carotenoids, carbohydrates, protein, lipids, fatty acid profile, and ASX) were optimized using the seven different culture media. The highest biomass (1.14 g L−1) was observed in the MBBM medium, with a specific growth rate (0.087 day−1), division/day (0.125), and cellular yield (6.16 x 106 cells/mL). The highest carotenoids (1.56 mg g−1), lipids (32.5 mg L−1), and carbohydrates (135.62 mg L−1) were recorded in the 3N-BBM medium. The maximum ω3-FAs (17.78%), ω6-FAs (23.11%), and ω9-FAs (7.06%) were observed in MBBM, JW, and BG-11 medium respectively. The highest amount of antioxidant ASX was accumulated in the 3N-BBM medium (391 mg L−1). It is more than any other known algal species used in the production of natural ASX. The optimized biochemical studies on the D. globosus-HI strain should fulfill the increasing demand for natural ASX for commercial application.

A Newly Isolated Strain of Haematococcus pluvialis GXU-A23 Improves the Growth Performance, Antioxidant and Anti-Inflammatory Status, Metabolic Capacity and Mid-intestine Morphology of Juvenile Litopenaeus vannamei

Haematococcus pluvialis can be used as a green additive in aquafeeds due to it contains rich astaxanthin and polyunsaturated fatty acid. In the present study, a newly strain of H. pluvialis GXU-A23 with high concentration of astaxanthin was firstly isolated by a newly culture strategy in our laboratory. In addition, H. pluvialis GXU-A23 was applied in the Litopenaeus vannamei feed for determining whether it has positive effects on the growth performance, antioxidant and anti-inflammatory status, metabolic capacity and mid-intestine morphology of juvenile L. vannamei. Shrimp with 0.63 g approximately initial body weight were fed diets supplemented with/without 50 g/kg H. pluvialis GXU-A23. After 8 weeks feeding intervention, significantly higher growth performance of L. vannamei was obtained in the H. pluvialis GXU-A23 treatment group compared to the control group (p < 0.05). At the same time, L. vannamei fed with H. pluvialis GXU-A23 acquired significantly better antioxidant and anti-inflammatory status than the control group (p < 0.05). In addition, higher RNA expression level of hepatopancreas digestive enzyme, hepatopancreas lipid and glucose metabolic enzymes as well as better mid-intestine morphology were found in the H. pluvialis GXU-A23 treatment group than the control group (p < 0.05). These results indicated that 50 g/kg H. pluvialis GXU-A23 was suitable for the L. vannamei feed, which could improve the growth performance, antioxidant and anti-inflammatory status, metabolic capacity and mid-intestine morphology of juvenile L. vannamei.

Growth Characteristic Analysis of Haematococcus pluvialis in a Microfluidic Chip Using Digital in-Line Holographic Flow Cytometry

In order to obtain high yield of astaxanthin, a high-value compound with ultrastrong antioxidant capacity, it is necessary to identify the growth characteristics (biomass, morphology, and size) of Haematococcus pluvialis. The current detection methods have the disadvantages of labor-consuming operation or complicated measurement system. It is an urgent need to explore a simple and cost-effective method for the detection of H. pluvialis with large size distribution during its growth period. In this work, a digital in-line holographic flow cytometry using a linear array sensor is proposed to measure the growth characteristics of H. pluvialis in a two-dimensional (2-D) hydrodynamic focusing microfluidic chip. Based on the modified angular spectrum method, the distorting holograms caused by the asynchrony of sample flow velocity and acquisition speed of the linear array sensor were rectified and reconstructed. In addition, the depth-of-focus of the imaging system were digitally extended to cover the entire depth of the microfluidic channel for optimized imaging quality. We have utilized the proposed method to statistically investigate the biomass, morphology and size of H. pluvialis under different culture conditions and growth durations.

Enhancement of astaxanthin production in Haematococcus pluvialis using zinc oxide nanoparticles

Today, there is a great interest in using astaxanthin due to its potential health advantages. Application of different types of nanoparticles (NPs) as stress agents to enhance astaxanthin production in Haematococcus pluvialis, a microalgae strain, has been reported in the literature. In this study, the effect of different concentrations of zinc oxide (ZnO) NPs on the enhancement of astaxanthin production in H. pluvialis was investigated. First, ZnO NPs were synthesized from zinc nitrate as the precursor and sodium hydroxide (chemical method), and peel extract of pomegranate (green method) as reducing agents. To study the cell viability and stimulate the astaxanthin production, H. pluvialis cells were exposed to the different concentrations (i.e. 50, 100, 200, and 400 μg.ml^-1) of ZnO NPs. The synthesized powders were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and dynamic light scattering (DLS) methods. The characterization results showed that the pure ZnO NPs were successfully synthesized via both methods with uniform particle size distribution. But, the average particle size of the green synthesized ZnO NPs (about 30 nm) was smaller than that of the chemically synthesized ones (about 80 nm). Maximum astaxanthin production (~ 20 mg.g^-1 of dry biomass of H. pluvialis) was achieved at 100 μg.ml^-1 of green synthesized ZnO NPs exposure to the H. pluvialis in comparison with the control culture after 15 days. However, ZnO NPs concentration above 200 μg.ml^-1 was toxic to the microalgae. From these results, it can be concluded that a specific amount of ZnO NPs could be considered as a worthy candidate for the enhancement of astaxanthin production in H. pluvialis.

Influence of Light Conditions on Microalgae Growth and Content of Lipids, Carotenoids, and Fatty Acid Composition

Microalgae are a valuable natural resource for a variety of value-added products. The growth of microalgae is determined by the impact of many factors, but, from the point of view of the implementation of autotrophic growth, light is of primary importance. This work presents an overview of the influence of light conditions on the growth of microalgae, the content of lipids, carotenoids, and the composition of fatty acids in their biomass, taking into account parameters such as the intensity, duration of lighting, and use of rays of different spectral composition. The optimal light intensity for the growth of microalgae lies in the following range: 26-400 µmol photons m-2 s-1. An increase in light intensity leads to an activation of lipid synthesis. For maximum lipid productivity, various microalgae species and strains need lighting of different intensities: from 60 to 700 µmol photons m-2 s-1. Strong light preferentially increases the triacylglyceride content. The intensity of lighting has a regulating effect on the synthesis of fatty acids, carotenoids, including β-carotene, lutein and astaxanthin. In intense lighting conditions, saturated fatty acids usually accumulate, as well as monounsaturated ones, and the number of polyunsaturated fatty acids decreases. Red as well as blue LED lighting improves the biomass productivity of microalgae of various taxonomic groups. Changing the duration of the photoperiod, the use of pulsed light can stimulate microalgae growth, the production of lipids, and carotenoids. The simultaneous use of light and other stresses contributes to a stronger effect on the productivity of algae.

Carotenoids from fungi and microalgae: A review on their recent production, extraction, and developments

The demand for carotenoids from natural sources obtained by biological extraction methods is increasing with the development of biotechnology and the continued awareness of food safety. Natural plant-derived carotenoids have a relatively high production cost and are affected by the season, while microbial-derived carotenoids are favored due to their natural, high-efficiency, low production cost, and ease of industrialization. This article reviewed the following aspects of natural carotenoids derived from microorganisms: (1) the structures and properties of main carotenoids; (2) fungal and microalgal sources of the main carotenoids; (3) influencing factors and modes of improvement for carotenoids production; (4) efficient extraction methods for carotenoids; and (5) the commercial value of carotenoids. This review provided a reference and guidance for the development of natural carotenoids derived from microorganisms.

A Review on Haematococcus pluvialis Bioprocess Optimization of Green and Red Stage Culture Conditions for the Production of Natural Astaxanthin

As the most recognizable natural secondary carotenoid astaxanthin producer, the green microalga Haematococcus pluvialis cultivation is performed via a two-stage process. The first is dedicated to biomass accumulation under growth-favoring conditions (green stage), and the second stage is for astaxanthin evolution under various stress conditions (red stage). This mini-review discusses the further improvement made on astaxanthin production by providing an overview of recent works on H. pluvialis, including the valuable ideas for bioprocess optimization on cell growth, and the current stress-exerting strategies for astaxanthin pigment production. The effects of nutrient constituents, especially nitrogen and carbon sources, and illumination intensity are emphasized during the green stage. On the other hand, the significance of the nitrogen depletion strategy and other exogenous factors comprising salinity, illumination, and temperature are considered for the astaxanthin inducement during the red stage. In short, any factor that interferes with the cellular processes that limit the growth or photosynthesis in the green stage could trigger the encystment process and astaxanthin formation during the red stage. This review provides an insight regarding the parameters involved in bioprocess optimization for high-value astaxanthin biosynthesis from H. pluvialis.

Reprogramming microorganisms for the biosynthesis of astaxanthin via metabolic engineering

There is an increasing demand for astaxanthin in food, feed, cosmetics and pharmaceutical applications because of its superior anti-oxidative and coloring properties. However, naturally produced astaxanthin is expensive, mainly due to low productivity and limited sources. Reprogramming of microorganisms for astaxanthin production via metabolic engineering is a promising strategy. We primarily focus on the application of synthetic biology, enzyme engineering and metabolic engineering in enhancing the synthesis and accumulation of astaxanthin in microorganisms in this review. We also discuss the biosynthetic pathways of astaxanthin within natural producers, and summarize the achievements and challenges in reprogramming microorganisms for enhancing astaxanthin production. This review illuminates recent biotechnological advances in microbial production of astaxanthin. Future perspectives on utilization of new technologies for boosting microbial astaxanthin production are also discussed.

Stresses as First-Line Tools for Enhancing Lipid and Carotenoid Production in Microalgae

Microalgae can produce high-value-added products such as lipids and carotenoids using light or sugars, and their biosynthesis mechanism can be triggered by various stress conditions. Under nutrient deprivation or environmental stresses, microalgal cells accumulate lipids as an energy-rich carbon storage battery and generate additional amounts of carotenoids to alleviate the oxidative damage induced by stress conditions. Though stressful conditions are unfavorable for biomass accumulation and can induce oxidative damage, stress-based strategies are widely used in this field due to their effectiveness and economy. For the overproduction of different target products, it is required and meaningful to deeply understand the effects and mechanisms of various stress conditions so as to provide guidance on choosing the appropriate stress conditions. Moreover, the underlying molecular mechanisms under stress conditions can be clarified by omics technologies, which exhibit enormous potential in guiding rational genetic engineering for improving lipid and carotenoid biosynthesis.

Evaluation and Transcriptome Analysis of the Novel Oleaginous Microalga Lobosphaera bisecta (Trebouxiophyceae, Chlorophyta) for Arachidonic Acid Production

Arachidonic acid (AA) is an omega-6 long-chain polyunsaturated fatty acid and is important for human health. The coccoid green microalga Lobosphaera bisecta has been reported to be able to accumulate high AA content under certain conditions. Nutrient management and light intensity had significant effects on the biomass and accumulation of lipids and AA in L. bisecta SAG2043. Both a high nitrogen concentration (18 mM) and high light intensity (bilateral light-300 μmol m⁻² s⁻¹) were beneficial to the growth of L. bisecta, and the replacement of culture medium further enhanced the biomass, which reached 8.9 g L⁻¹. Low nitrogen concentration (3.6 mM) and high light significantly promoted the accumulation of lipids and AA. The highest lipid and AA content reached 54.0% and 10.8% of dry weight, respectively. Lipid compositions analysis showed that 88.2% of AA was distributed within the neutral lipids. We then reconstructed the lipid metabolic pathways of L. bisecta for the first time, and demonstrated that the upregulation of a key desaturase and elongase in the Δ6 pathway was conducive to the accumulation of fatty acids toward AA synthesis. L. bisecta SAG2043 exhibits high biomass, lipid and AA production. It may be a potential candidate for AA production.

Integrated biorefinery strategy for tofu wastewater biotransformation and biomass valorization with the filamentous microalga Tribonema minus

This study focused on the feasibility of using different concentrations of tofu wastewater (TW) as alternative media for Tribonema minus cultures to produce valuable biorefinery feedstock. T. minus grew mixotrophically in 100% TW with larger carbohydrate (30.99% of dry weight (DW)), protein (15.56% of DW) and chrysolaminarin (6.93% of DW) accumulations than that of in mBG-11 medium. The highest biomass concentration, 7.77 g/L, was achieved in 100% TW, and nutrient removal efficiencies of T. minus at this concentration ranged from 60.49% to 93.60%. Although smaller neutral lipid and palmitoleic acid amounts were detected in 100% TW, their productivities reached 133.77 and 67.19 mg/L/d, respectively, due to the largest biomass yield contribution, which were comparable to those in mBG-11 medium. These findings demonstrated that TW is a promising alternative medium, and an integrated TW biotransformation and biomass valorization process is proposed to achieve better economic performance and environmental sustainability.