Effects of CO2 enrichment on primary photochemistry, growth and astaxanthin accumulation in the chlorophyte Haematococcus pluvialis

Modeling of astaxanthin biosynthesis via machine learning, mathematical and metabolic network modeling

Natural astaxanthin is synthesized by diverse organisms including: bacteria, fungi, microalgae, and plants involving complex cellular processes, which depend on numerous interrelated parameters. Nonetheless, existing knowledge regarding astaxanthin biosynthesis and the conditions influencing astaxanthin accumulation is fairly limited. Thus, manipulation of the growth conditions to achieve desired biomass and astaxanthin yields can be a complicated process requiring cost-intensive and time-consuming experiment-based research. As a potential solution, modeling and simulation of biological systems have recently emerged, allowing researchers to predict/estimate astaxanthin production dynamics in selected organisms. Moreover, mathematical modeling techniques would enable further optimization of astaxanthin synthesis in a shorter period of time, ultimately contributing to a notable reduction in production costs. Thus, the present review comprehensively discusses existing mathematical modeling techniques which simulate the bioaccumulation of astaxanthin in diverse organisms. Associated challenges, solutions, and future perspectives are critically analyzed and presented.

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.

Excessive composite pollution carbon sources enhance the bio-fertilizer efficiency of Tetradesmus obliquus: focused on cultivation period

Microalgae can use carbon sources in sludge extract prepared from sludge. Moreover, the high concentration of CO2 and the large number of carbon sources in the liquid phase will promote microalgae growth and metabolism. In this experiment, Tetradesmus obliquus was cultivated with sludge extract at 30% CO2. Algae liquid (the name used to describe the fertilizer made in this research) was further prepared as lettuce fertilizer. The effect of different times of microalgae culture (10, 15, 20, 25, and 30 days) on the fertilizer efficiency of the algae liquid was evaluated by lettuce hydroponic experiments. The findings indicate that lettuce cultivated in algae liquid collected on the 15th and 30th days exhibited superior performance in terms of growth, antioxidant capacity, and nutritional quality. We analyzed the experimental results in the context of microalgae metabolic mechanisms, aiming to contribute experience and data essential for the development of industrial microalgae fertilizer production.

Effects of Four Organic Carbon Sources on the Growth and Astaxanthin Accumulation of Haematococcus lacustris

The microalga Haematococcus lacustris has a complex life cycle and a slow growth rate, hampering its mass cultivation. Culture of microalgae with organic carbon sources can increase the growth rate. Few studies have evaluated the effects of organic carbon sources on H. lacustris. We compared the vegetative and inductive stages of H. lacustris under autotrophic and mixotrophic conditions using four organic carbon sources: sodium acetate, glycerol, sodium gluconate, and ribose, each at various concentrations (0.325, 0.65, 1.3, and 2.6 g/L). The cell density was increased by 1.3 g/L of glycerol in the vegetative stage. The rapid transition to the inductive stage under nitrogen-depletion conditions caused by 1.3 or 2.6 g/L sodium acetate promoted the accumulation of astaxanthin. The production of astaxanthin by H. lacustris in mass culture using organic carbon sources could increase profitability.

Enhancement of astaxanthin production, recovery, and bio-accessibility in Haematococcus pluvialis through taurine-mediated inhibition of secondary cell wall formation under high light conditions

This study explored the use of taurine in enhancing the production and bio-accessibility of astaxanthin in Haematococcus pluvialis, which typically forms a secondary cell wall hindering astaxanthin extraction. The biomass of taurine-treated group significantly increased by 18%, and astaxanthin yield surged by 34% in comparison to the control group. Without cell disruption, astaxanthin recovery from thin-walled cells in the taurine-treated group, using dimethyl sulfoxide and ethanol as extraction reagents, was 97% and 75%, respectively, which were 30-fold higher than those of thick-walled cells in the control group. Additionally, the cell fragmentation rate increased by 86% in taurine-treated group relative to the control group. Comparative transcriptome analysis identified taurine-induced upregulation of genes involved in the astaxanthin biosynthesis pathway and downregulation of those associated with secondary cell wall synthesis. This study thus offers an innovative taurine-based strategy to enhance astaxanthin production and bio-accessibility while shedding light on the mechanisms driving this process.

Exploring adequate CO2 elevation for optimum tomato growth and yield under protected cultivation

The adequate elevation of CO2 concentrations (e [CO2]) could not be assessed by constrained analysis of comparative experimental study for optimum plant growth and yield with improved fruit quality owing to the lack of conjunctive investigation of plant parametric responses. Instead, the principal component analysis (PCA) and technique for order preference by similarity to ideal solution (TOPSIS) assessed and quantified the parametric plant responses to identify the adequate level of e [CO2] for optimum plant growth and yield. In this study, tomato plants were grown under an ambient CO2 (a [CO2], 500 μmol mol-1) and three e [CO2] (700, 850 and 1000 μmol mol-1): named EC700, EC850 and EC1000, respectively, in autumn-winter (AW) 2020 and spring summer (SS) 2021 growing seasons to investigate and evaluate the plant parametric responses under e [CO2]. The tomato plant's response with maximum transportability of biomass to fruits was observed under 700 μmol mol-1. The plant height, stem diameter and LAI were enhanced compared to a [CO2] at the optimum level under 1000 μmol mol-1 (by 50.53, 20.98 and 44.44%) and 700 μmol mol-1 (by 22.41, 12.09 and 26.88%) in Aw 2020; Ss 2021, respectively. The optimum yield was increased under 700 μmol mol-1 by 73.95% and 55.58% in Aw 2020; Ss 2021, respectively. EC700 was ranked as a priority by TOPSIS with 0.632 and 0.694 plant response performance index in Aw 2020; Ss 2021, respectively, to get optimum tomato growth, yield, water use efficiency and fruit quality. The results of this study are beneficial for commercial greenhouse crop production by fumigating the adequate level of e [CO2], to reduce the cost of CO2 fertigation, enhance the yield and save the water quantity.

Astaxanthin: Past, Present, and Future

Astaxanthin (AX), a lipid-soluble pigment belonging to the xanthophyll carotenoids family, has recently garnered significant attention due to its unique physical properties, biochemical attributes, and physiological effects. Originally recognized primarily for its role in imparting the characteristic red-pink color to various organisms, AX is currently experiencing a surge in interest and research. The growing body of literature in this field predominantly focuses on AXs distinctive bioactivities and properties. However, the potential of algae-derived AX as a solution to various global environmental and societal challenges that threaten life on our planet has not received extensive attention. Furthermore, the historical context and the role of AX in nature, as well as its significance in diverse cultures and traditional health practices, have not been comprehensively explored in previous works. This review article embarks on a comprehensive journey through the history leading up to the present, offering insights into the discovery of AX, its chemical and physical attributes, distribution in organisms, and biosynthesis. Additionally, it delves into the intricate realm of health benefits, biofunctional characteristics, and the current market status of AX. By encompassing these multifaceted aspects, this review aims to provide readers with a more profound understanding and a robust foundation for future scientific endeavors directed at addressing societal needs for sustainable nutritional and medicinal solutions. An updated summary of AXs health benefits, its present market status, and potential future applications are also included for a well-rounded perspective.

Regulatory mechanism of high-concentration CO2 on polysaccharide accumulation in Tetradesmus obliquus cultured in sludge extract

Microalgae such as Tetradesmus obliquus have great potential in immobilizing high-concentration CO₂ and removing highly toxic organic matters, which could be produced from coal chemical industry and coal chemical wastewater biological treatment process. In this study, Tetradesmus obliquus was cultured in sludge extract and high-concentration CO₂ was added. The maximum cell density and dry weight were respectively (111.46 ± 4.87) × 10⁶ cell/mL and 3.365 ± 0.168 g/L under 30% CO₂. Tetradesmus obliquus accumulated the most polysaccharides (629.60 ± 31.48 mg/L) on the 30th day under 30% CO₂. The results of proteomic showed that the upregulation of A0A2Z4THB7 and A0A383VAT1 promoted polysaccharide accumulation. Polysaccharide was mainly formed at the stable phase instead of the log-growth phase due to the abiotic stress caused by high TOC at the log-growth phase. Collectively, this study revealed the regulatory mechanism of high-concentration CO₂ on the toxicity removal and accumulation of polysaccharides in Tetradesmus obliquus.

Development of a bench-scale photobioreactor with a novel recirculation system for continuous cultivation of microalgae

Microalgae cultivation can be used to increase the sustainability of carbon emitting processes, converting the CO₂ from exhaust gases into fuels, food and chemicals. Many of the carbon emitting industries operate in a continuous manner, for periods that can span days or months, resulting in a continuous stream of gas emissions. Biogenic CO₂ from industrial microbiological processes is one example, since in many cases it becomes unsustainable to stop these processes on a daily or weekly basis. To correctly sequester these emissions, microalgae systems must be operated under continuous constant conditions, requiring photobioreactors (PBRs) that can act as chemostats for long periods of time. However, in order to optimize culture parameters or study metabolic responses, bench-scale setups are necessary. Currently there is a lack of studies and design alternatives using chemostat, since most works focus on batch assays or semi-continuous cultures. Therefore, this work focused on the development of a continuous bench-scale PBR, which combines a retention vessel, a photocollector and a degasser, with an innovative recirculation system, that allows it to operate as an autotrophic chemostat, to study carbon sequestration from a biogenic CO₂-rich constant air stream. To assess its applicability, the PBR was used to cultivate the green microalga Haematococcus pluvialis using as sole carbon source the CO₂ produced by a coupled heterotrophic bacterial chemostat. An air stream containing ≈0.35 vol% of CO_2, was fed to the system, and it was evaluated in terms of stability, carbon fixation and biomass productivity, for dilution rates ranging from 0.1 to 0.5 d^-1. The PBR was able to operate under chemostat conditions for more than 100 days, producing a stable culture that generated proportional responses to the stimuli it was subjected to, attaining a maximum biomass productivity of 183 mg/L/d with a carbon fixation efficiency of ≈39% at 0.3 d^-1. These results reinforce the effectiveness of the developed PBR system, making it suitable for laboratory-scale studies of continuous photoautotrophic microalgae cultivation.

Analysis of photosynthetic pigments pathway produced by CO2-toxicity-induced Scenedesmus obliquus

The coal-to-gas process produces carbon dioxide, which increases global warming, and its wastewater treatment generates sludge with high organic toxicity. Scenedesmus obliquus is a potential solution to such environmental problems, and photosynthetic pigments are the focus of this study. The optimal concentration of CO₂ for the growth of Scenedesmus obliquus was found to be 30 % after increasing the concentration of CO₂ (0.05 %-100 %). The accumulation of photosynthetic pigments during cultivation could reach 31.74 ± 1.33 mg/L, 11.21 ± 0.42 mg/L, and 5.59 ± 0.19 mg/L respectively, and the organic toxicity of sludge extract could be reduced by 44.97 %. Upregulation of A0A383VSL5, A0A383WMQ3, and A0A2Z4THB7 as photo systemic oxygen release proteins and propylene phosphate isomerase resulted in oxygen-evolving proteins in photosystem II, electron transport in photosystem I, and intermediates in carbon fixation. This is achieved by increasing the intracellular antennae protein and carbon fixation pathway, allowing Scenedesmus obliquus to both tolerate and fix CO₂ and reduce the organic toxicity of sludge. These findings provide insights into the innovative strategy underlining the fixation of CO₂, treatment and disposal of industrial residual sludge, and the enhancement of microalgal biomass production.

The effects of acetate and glucose on carbon fixation and carbon utilization in mixotrophy of Haematococcus pluvialis

The culture method using sodium acetate and glucose, widely used as organic carbon sources in the mixotrophy of Haematococcus pluvialis, was compared with its autotrophy. In the 12-day culture, mixotrophy using sodium acetate and glucose increased by 40.4% and 77.1%, respectively, compared to autotrophy, but the mechanisms for the increasing biomass were different. The analysis of the mechanism was divided into autotrophic and heterotrophic metabolism. The mixotrophy with glucose increased the biomass by directly supplying the substrate and ATP to the TCA cycle while inhibiting photosynthesis. Gene expressions related to glycolysis and carbon fixation pathway were confirmed in autotrophy and mixotrophy with glucose and acetate. The metabolism predicted in the mixotrophy with acetate and glucose was proposed via autotrophic and heterotrophic metabolism analysis. The mechanism of Haematococcus pluvialis under mixotrophic conditions with high CO₂ concentration was confirmed through this study.

Biofixation of Air Emissions and Biomass Valorization-Evaluation of Microalgal Biotechnology

This research appraised the simultaneous biofixation, that is not quite common in scientific literature, of carbon dioxide (CO₂) and nitric oxides (NO_x) by microalgae species Chlorella vulgaris, Haematococcus pluvialis, and Scenedesmus subspicatus. The experimental design was established by five treatments with gas concentrations between control-0.04% of CO₂, 5 to 15% of CO₂, and 30 to 100 ppm of NOx. Parameters such as pH, growth, productivity, lipids, protein, carbon/ nitrogen ratio, and astaxanthin were evaluated. For all species, the maximal growth and productivity were achieved with 5% of CO₂ and 30 ppm of NO_x. Regarding protein content, for all the three species, better results were obtained at higher concentrations of CO₂ and NOx. These results prove the microalgae capacity for CO₂ and NO_x biofixation and reuse of biomass as a source of high value-added products, such as lipids, proteins, and astaxanthin. These findings support the indication of these species for flue gas treatment process and use in biorefineries systems.

Antioxidant Activity and Carotenoid Content Responses of Three Haematococcus sp. (Chlorophyta) Strains Exposed to Multiple Stressors

There has been increasing demands worldwide for bioactive compounds of natural origins, especially for the nutraceutical and food-supplement sectors. In this context, microalgae are viewed as sustainable sources of molecules with an array of health benefits. For instance, astaxanthin is a xanthophyll pigment with powerful antioxidant capacity produced by microalgae such as the chlorophyte Haematococcus sp., which is regarded as the most suitable organism for the mass production of this pigment. In this study, three Haematococcus sp. strains were cultivated using a batch mode under favourable conditions to promote vegetative growth. Their environment was altered in a second phase using a higher and constant illumination regime combined with either exposure to blue LED light, an osmotic shock (with NaCl addition) or supplementation with a phytohormone (gibberellic acid, GA3), a plant extract (ginger), an herbicide (molinate) or an oxidant reagent (hydrogen peroxide). The effects of these stressors were evaluated in terms of antioxidant response and astaxanthin and β-carotene accumulation. Overall, strain CCAP 34/7 returned the highest Trolox Equivalent Antioxidant Capacity (TEAC) response (14.1-49.1 µmoL Trolox eq. g^- 1 of DW), while the highest antioxidant response with the Folin-Ciocalteu (FC) was obtained for strain RPFW01 (62.5-155 µmoL Trolox eq. g^- 1 of DW). The highest β-β-carotene content was found in strain LAFW15 when supplemented with the ginger extract (4.8 mg. g^- 1). Strain RPFW01 exposed to blue light returned the highest astaxanthin yield (2.8 mg. g^- 1), 5-fold that of strain CCAP 34/7 on average. This study documents the importance of screening several strains when prospecting for species with potential to produce high-value metabolites. It highlights that strain-specific responses can ensue from exposure of cells to a variety of stressors, which is important for the adequate tailoring of a biorefinery pipeline.

Supplementation with rac-GR24 Facilitates the Accumulation of Biomass and Astaxanthin in Two Successive Stages of Haematococcus pluvialis Cultivation

The unicellular freshwater green alga Haematococcus pluvialis has attracted much research attention due to its biosynthetic ability for large amounts of astaxanthin, a blood-red ketocarotenoid that is used in cosmetics, nutraceuticals, and pharmaceuticals. Recently, numerous studies have investigated the functions of natural astaxanthin; however, the high cost of the production of astaxanthin from H. pluvialis cultures restricts its commercial viability. There is an urgent need to fulfill commercial demands by increasing astaxanthin accumulation from H. pluvialis cultures. In this study, we discovered that treatment of H. pluvialis cultures at the beginning of the macrozooid stage (day 0) with 1 μM rac-GR24, a synthetic analogue of strigolactones (a class of phytohormones), led to significant increases in biomass [up to a maximum dry cell weight (DCW) of 0.53 g/L] during the macrozooid stage and astaxanthin (from 0.63 to 5.32% of DCW) during the hematocyst stage. We elucidated that this enhancement of biomass accumulation during the macrozooid stage by rac-GR24 is due to its increasing CO₂ utilization efficiency in photosynthesis and carbohydrate biosynthesis. We also found that rac-GR24 stimulated the overproduction of nicotinamide adenine dinucleotide phosphate (NADPH) and antioxidant enzymes in H. pluvialis cultures, which alleviated the oxidative damage caused by reactive oxygen species generated during the hematocyst stage due to the exhaustion of nitrogen supplies. Moreover, rac-GR24 treatment of H. pluvialis synergistically altered the activity of the pathways of fatty acid biosynthesis and astaxanthin esterification, which resulted in larger amounts of astaxanthin being generated by rac-GR24-treated cultures than by controls. In summary, we have developed a feasible and economic rac-GR24-assisted strategy that increases the amounts of biomass and astaxanthin generated by H. pluvialis cultures, and have provided novel insights into the mechanistic roles of rac-GR24 to achieve these effects.

Chlorophyll fluorescence as a valuable multitool for microalgal biotechnology

Variable fluorescence of chlorophyll (CF) of the photosynthetic apparatus is an ample source of valuable information on physiological condition of photosynthetic organisms. Currently, the most widespread CF-based technique is represented by recording pulse-amplitude modulated (PAM) induction of CF by saturating light. The CF-based monitoring techniques are increasingly employed for characterization of performance and stress resilience of microalgae in microalgal biotechnology. Analysis of CF induction curves reveals the fate of light energy absorbed by photosynthetic apparatus, the proportions of the energy that have been utilized for photochemistry (culture growth), and heat dissipated by photoprotective mechanisms. Hence CF and its derived parameters are an accurate proxy of the metabolic activity of the photosynthetic cell and the engagement of photoprotective mechanisms. This information is a solid foundation for making decisions on the microalgal culture management during the lab-scale and industrial-scale cultivation. Applications of CF and PAM include the monitoring of stressor (high light, nutrient deprivation, extreme temperatures, etc.) effects for assessment of the culture robustness. It also serves as a non-invasive express test for gauging the effect of assorted toxicants in microalgae. This approach is becoming widespread in ecological toxicology and environmental biotechnology, particularly for bioprospecting strains capable of the destruction of dangerous pollutants such as pharmaceuticals. In the review, we discuss the advantages and drawbacks of using CF-based methods for assessment of the culture conditions. Special attention is paid to the potential caveats and applicability of different variations of CF and PAM measurements for solving problems of microalgal biotechnology.

A Fast-Growing Oleaginous Strain of Coelastrella Capable of Astaxanthin and Canthaxanthin Accumulation in Phototrophy and Heterotrophy

Considering the importance of microalgae as a promising feedstock for the production of both low- and high-value products, such as lipids and pigments, it is desirable to isolate strains which simultaneously accumulate these two types of products and grow in various conditions in order to widen their biotechnological applicability. A novel freshwater strain from the genus Coelastrella was isolated in Belgium. Compared to other Coelastrella species, the isolate presented rapid growth in phototrophy, dividing 3.5 times per day at a light intensity of 400 µmol·m⁻²·s⁻¹ and 5% CO₂. In addition, nitrogen depletion was associated with the accumulation of astaxanthin, canthaxanthin, and fatty acids, which reached ~30% of dry weight, and a majority of SFAs and MUFAs, which are good precursors for biodiesel. This strain also accumulated astaxanthin and canthaxanthin in heterotrophy. Although the content was very low in this latter condition, it is an interesting feature considering the biotechnological potential of the microalgal heterotrophic growth. Thus, due to its rapid growth in the light, its carotenogenesis, and its fatty acids characteristics, the newly identified Coelastrella strain could be considered as a potential candidate for biorefinery purposes of both low- and high-values products.

A strategy for promoting carbon flux into fatty acid and astaxanthin biosynthesis by inhibiting the alternative oxidase respiratory pathway in Haematococcus pluvialis

In the present study, the mechanisms for facilitating fatty acid and astaxanthin biosynthesis-related processes by inhibiting the alternative oxidase (AOX) respiratory pathway in Haematococcus pluvialis was investigated. The restriction of the AOX pathway induced the accumulation of reactive oxygen species, NAD(P)H and its substrates (acetyl-CoA, pyruvate and glyceraldehy-3-phosphate), which are required for fatty acid and astaxanthin production, thereby promoting the carbon flux into fatty acid and astaxanthin biosynthesis. During a 9-day incubation period, the fatty acid and astaxanthin contents increased by 20.6% and 20.7%, respectively, when the AOX pathway was inhibited approximately 37.7%. The AOX pathway may be inhibited by nutrient (nitrogen and phosphorus) removal, inhibitor addition and air/CO₂ aeration adjustments in the large-scale cultivation of H. pluvialis. Therefore, the current study provides a useful enhancement strategy for fatty acid and astaxanthin coproduction and elucidates the roles of the AOX pathway in regulating fatty acid and astaxanthin biosynthesis.

Techno-economic analysis of a new downstream process for the production of astaxanthin from the microalgae Haematococcus pluvialis

The biotechnological production of the carotenoid astaxanthin is done with the microalgae Haematococcus pluvialis (H. pluvialis). Under nutrient deficiency and light stress, H. pluvialis accumulates astaxanthin intracellularly and forms a resistant cyst cell wall that impedes direct astaxanthin extraction. Therefore, a complex downstream process is required, including centrifugation, mechanical cell wall disruption, drying, and supercritical extraction of astaxanthin with CO2. In this work, an alternative downstream process based on the direct extraction of astaxanthin from the algal broth into ethyl acetate using a centrifugal partition extractor (CPE) was developed. A mechanical cell wall disruption or germination of the cysts was carried out to make astaxanthin accessible to the solvent. Zoospores containing astaxanthin are released when growth conditions are applied to cyst cells, from which astaxanthin can directly be extracted into ethyl acetate. Energy-intensive unit operations such as spray-drying and extraction with supercritical CO2 can be replaced by directly extracting astaxanthin into ethyl acetate. Extraction yields of 85% were reached, and 3.5 g of oleoresin could be extracted from 7.85 g homogenised H. pluvialis biomass using a CPE unit with 244 mL column volume. A techno-economic analysis was done for a hypothetical H. pluvialis production facility with an annual biomass output of 8910 kg. Four downstream scenarios were examined, comparing the novel process of astaxanthin extraction from homogenised cyst cells and germinated zoospores via CPE extraction with the conventional industrial process using in-house or supercritical CO2 extraction via an external service provider. After 10 years of operation, the highest net present value (NPV) was determined for the CPE extraction from germinated zoospores.

Sequential Continuous Mixotrophic and Phototrophic Cultivation Might Be a Cost-Effective Strategy for Astaxanthin Production From the Microalga Haematococcus lacustris

Although Haematococcus lacustris has been developed for astaxanthin production for decades, the production cost is still high. In order to modify the production processes, we proposed a novel strategy of cultivation, featured by sequential indoor continuous mixotrophic cultivation for the production of green cells followed by outdoor phototrophic induction for astaxanthin accumulation. The continuous mixotrophic cultivation was first optimized indoor, and then the seed culture of mixotrophic cultivation was inoculated into outdoor open raceway ponds for photoinduction. The results showed that mixotrophically grown cultures could efficiently grow without losing their photosynthetic efficiency and yielded higher biomass concentration (0.655 g L⁻¹) and astaxanthin content (2.2% DW), compared to phototrophically grown seed culture controls. This novel strategy might be a promising alternative to the current approaches to advance the production technology of astaxanthin from microalgae.

Microalgae as Sustainable Biofactories to Produce High-Value Lipids: Biodiversity, Exploitation, and Biotechnological Applications

Microalgae are often called “sustainable biofactories” due to their dual potential to mitigate atmospheric carbon dioxide and produce a great diversity of high-value compounds. Nevertheless, the successful exploitation of microalgae as biofactories for industrial scale is dependent on choosing the right microalga and optimum growth conditions. Due to the rich biodiversity of microalgae, a screening pipeline should be developed to perform microalgal strain selection exploring their growth, robustness, and metabolite production. Current prospects in microalgal biotechnology are turning their focus to high-value lipids for pharmaceutic, nutraceutic, and cosmetic products. Within microalgal lipid fraction, polyunsaturated fatty acids and carotenoids are broadly recognized for their vital functions in human organisms. Microalgal-derived phytosterols are still an underexploited lipid resource despite presenting promising biological activities, including neuroprotective, anti-inflammatory, anti-cancer, neuromodulatory, immunomodulatory, and apoptosis inductive effects. To modulate microalgal biochemical composition, according to the intended field of application, it is important to know the contribution of each cultivation factor, or their combined effects, for the wanted product accumulation. Microalgae have a vital role to play in future low-carbon economy. Since microalgal biodiesel is still costly, it is desirable to explore the potential of oleaginous species for its high-value lipids which present great global market prospects.

Examination of Photo-, Mixo-, and Heterotrophic Cultivation Conditions on Haematococcus pluvialis Cyst Cell Germination

The microalgae Haematococcus pluvialis is used for the biotechnological production of astaxanthin. The red carotenoid accumulates in the cytoplasm under unfavorable conditions. Astaxanthin synthesis is associated with the transformation of motile vegetative cells into non-motile cyst cells. In the industrial process, after harvesting, the cyst cells are mechanically disrupted, dried, and finally, astaxanthin is extracted with supercritical CO2. The germination of the cyst cells represents an interesting alternative, replacing the mechanical cyst cell wall disruption. When cyst cells are exposed to favorable growth conditions, germination of the cyst cells occurs and zoospores are released after a certain time. These zoospores show a much weaker cell matrix compared to cyst cells. In this study, germination under phototrophic, mixotrophic, and heterotrophic conditions was examined. Glucose was used as the carbon source for mixotrophic and heterotrophic germination. Applying heterotrophic conditions, up to 80% of the cells were in the zoospore stage 49 h after the start of germination, and extraction yields of up to 50% were achieved using the solvent ethyl acetate for the extraction of astaxanthin from the algal broth containing zoospores. An extraction yield of up to 64% could be achieved by doubling the nitrate concentration and combining mixotrophic and heterotrophic cultivation.

Carbonate assisted lipid extraction and biodiesel production from wet microalgal biomass and recycling waste carbonate for CO2 supply in microalgae cultivation

High cost of microalgal biofuel is caused by all the steps in current technology, including cultivation, harvesting, lipid extraction, biofuel processing and wastewater and waste treatment. This study aims to systematically reduce these costs with one integrated process, in which carbonate is used for cell rupture, lipid extraction and biodiesel processing, and then recycled for CO₂ absorption and carbon supply for a new round of algae cultivation. To reach this goal, carbonate-heating treatment with N, N' - dibutylurea which can enhance cell disruption were used for cell-wall breaking of wet Neochloris oleoabundans (UTEX 1185) biomass. Lipid extraction was fulfilled with carbonate/ethanol aqueous two phase extraction method and residual carbonate with wastewater from bottom phase was recycled to absorb CO₂ to generate bicarbonate for algal cultivation with fresh medium. Taking into comprehensive consideration of cell disruption efficiency, partition coefficient, and lipid recovery, the condition of cell disruption and lipid extraction was set at 90 °C, 100 min reaction time, 1:7.5 DBU:H₂O (w/w) ratio, 1:3 Na₂CO₃:H₂O (w/w) ratio, and 9% (w/w_T) ethanol concentration. The results showed that carbonate-heating treatment of wet N. oleoabundans biomass resulted in up to 90.7% cell disruption efficiency. The lipid recovery rate in carbonate/ethanol system was up to 97.9%, and the final biodiesel production was 1.30 times of that with Soxhlet method. Utilization of the waste broth after CO₂ absorption with the content of 4% (v/v_T) in the medium for new batch of algae cultivation resulted in biomass concentration of 1.68 g/L. The corresponding total fatty acids production was 0.35 g/L, which was 1.63 fold of that with fresh medium. This study firstly proved the feasibility of using carbonate for lipid extraction and biodiesel production and recycle waste carbonate for carbon re-supply during algae cultivation.

Combined Production of Astaxanthin and β-Carotene in a New Strain of the Microalga Bracteacoccus aggregatus BM5/15 (IPPAS C-2045) Cultivated in Photobioreactor

Carotenoids astaxanthin and β-carotene are widely used natural antioxidants. They are key components of functional food, cosmetics, drugs and animal feeding. They hold leader positions on the world carotenoid market. In current work, we characterize the new strain of the green microalga Bracteacoccus aggregatus BM5/15 and propose the method of its culturing in a bubble-column photobioreactor for simultaneous production of astaxanthin and β-carotene. Culture was monitored by light microscopy and pigment kinetics. Fatty acid profile was evaluated by tandem gas-chromatography-mass spectrometry. Pigments were obtained by the classical two-stage scheme of autotrophic cultivation. At the first, vegetative, stage biomass accumulation occurred. Maximum specific growth rate and culture productivity at this stage were 100-200 mg∙L^-1∙day^-1, and 0.33 day^-1, respectively. At the second, inductive, stage carotenoid synthesis was promoted. Maximal carotenoid fraction in the biomass was 2.2-2.4%. Based on chromatography data, astaxanthin and β-carotene constituted 48 and 13% of total carotenoid mass, respectively. Possible pathways of astaxanthin synthesis are proposed based on carotenoid composition. Collectively, a new strain B. aggregatus BM5/15 is a potential biotechnological source of two natural antioxidants, astaxanthin and β-carotene. The results give the rise for further works on optimization of B. aggregatus cultivation on an industrial scale.

Extracellular carotenoid production and fatty acids profile of Parachlorella kessleri under increased CO2 concentrations

Large-scale cultivations of photoautotrophic microorganisms represent a very promising and potentially cost-effective alternative for climate change mitigation, when associated to the co-production of high value bioproducts, such as fatty acids and carotenoids, considering the growing demand for natural products. During microalgae cultivation, CO₂ enrichment is a requirement to reach high productivities, although high CO₂ levels are normally stressful to microalgae. On the other hand, cellular stress is a well reported strategy to induce carotenoid and fatty acids production. This work evaluated extracellular carotenoid production from the mangrove-isolated microalga Parachlorella kessleri cultivated under 5, 15 and 30% CO₂ in stirred tank photobioreactors. In the 10th day of cultivation, CO₂ supply was interrupted until the end of the cultivation (14th day), causing a stressful and imperative condition for microalgae cells to release the red pigment. Growth kinetics, physiological parameters and bioproducts production were evaluated. Growth kinetics were similar under all tested conditions and differences were not statistically significant, with the highest values of μ_max, biomass concentration, lipid content and CO₂ fixation rate of 0.77 d^-1, 1.24 g L^-1, 241 mg g^-1 (dw) and 165 mg L^-1 d^-1, respectively. In contrast, total carotenoid concentrations varied significantly (p < 0.01), with the highest concentration of 0.030 μg mL^-1 under 5% CO₂. The produced red pigment presented antioxidant activity and characteristics of carotenoids confirmed by UV-vis and tandem mass spectrometry (MS/MS). The fatty acid profiles in the biomass varied in response to CO₂ levels in the cultivations. In general, higher CO₂ concentrations (15 and 30%) favored the production of saturated and mono-unsaturated fatty acids, suitable as biodiesel feedstock, while drastically decreased the production of the polyunsaturated.

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.

The Dynamics of the Bacterial Community of the Photobioreactor-Cultivated Green Microalga Haematococcus lacustris during Stress-Induced Astaxanthin Accumulation

Simple Summary

The microalga Haematococcus lacustris is a source of the natural colorant astaxanthin, a powerful antioxidant and key component of cosmetics and animal feed. Haematococcus is cultivated in photobioreactors. It can obtain energy just from a light illuminating photobioreactor and uses inorganic salts and CO₂ as sources for chemical elements. The most widespread approach for Haematococcus cultivation is the two stage scheme. At the first stage, biomass accumulation under favorable growth conditions occurs. At the second stage, the cells are subjected to stress inducing astaxanthin synthesis. Generally, the culture of Haematococcus is not axenic. It exists in the form of a community with bacteria constituting its microbiome. The information on photobioreactor-cultivated Haematococcus microbiome is scarce. We analyzed its dynamic during astaxanthin production by DNA metabarcoding and microscopic observations. The main results of the work include the characterization of the daily dynamic of this microbiome and the revealing of contact between microalgae and bacteria. These findings are of potential significance for biotechnology. On one hand, they provide an insight into possible bacterial contamination of the harvested algal biomass. On the other hand, they reveal the presence of a core microbiome or bacteria essential for the growth of the microalga existing in all Haematococcus cultures.

Abstract

Haematococcus lacustris is a natural source of a valuable ketocarotenoid astaxanthin. Under autotrophic growth conditions, it exists in the form of a community with bacteria. The close coexistence of these microorganisms raises two questions: how broad their diversity is and how they interact with the microalga. Despite the importance these issues, little is known about microorganisms existing in Haematococcus cultures. For the first time, we characterize the dynamic of the H. lacustris microbiome of the microbiome of Haematococcus (a changeover of the bacterial associated species as function of the time) cultivated autotrophically in a photobioreactor based on 16S rRNA metabarcoding data. We found that Proteobacteria and Bacteroidetes are predominant phyla in the community. The Caulobacter bacterium became abundant during astaxanthin accumulation. These data were supported by microscopy. We discuss possible roles and interactions of the community members. These findings are of potential significance for biotechnology. They provide an insight into possible bacterial contamination in algal biomass and reveal the presence of bacteria essential for the algal growth.

Novel Insights into the Biotechnological Production of Haematococcus pluvialis-Derived Astaxanthin: Advances and Key Challenges to Allow Its Industrial Use as Novel Food Ingredient

Astaxanthin shows many biological activities. It has acquired a high economic potential and its current market is dominated by its synthetic form. However, due to the increase of the health and environmental concerns from consumers, natural forms are now preferred for human consumption. Haematococcus pluvialis is artificially cultured at an industrial scale to produce astaxanthin used as a dietary supplement. However, due to the high cost of its cultivation and its relatively low biomass and pigment productivities, the astaxanthin extracted from this microalga remains expensive and this has probably the consequence of slowing down its economic development in the lower added-value market such as food ingredient. In this review, we first aim to provide an overview of the chemical and biochemical properties of astaxanthin, as well as of its natural sources. We discuss its bioavailability, metabolism, and biological activities. We present a state-of-the-art of the biology and physiology of H. pluvialis, and highlight novel insights into the biotechnological processes which allow optimizing the biomass and astaxanthin productivities. We are trying to identify some lines of research that would improve the industrial sustainability and economic viability of this bio-production and to broaden the commercial potential of astaxanthin produced from H. pluvialis.

A novel CO2 steady feeding based on the pH steady strategy data in the Haematococcus pluvialis cultivation to maximize the cell growth and carbon bio-sequestration

Two common strategies to feed the CO₂ are pH steady (PS) and CO₂ steady (CS). An innovative strategy called ''CSBPS (CS feeding based on PS data)" in comparison to the 0.04% CS, 5% CS, and PS approaches improved the Haematococcus pluvialis growth and carbon bio-sequestration. The optimum concentration of CO₂ was estimated based on the PS cultivation data and fed to culture media using the CS approach with no buffering agent. The biomass productivity, CO₂ bio-fixation rate, and rubisco activity under CSBPS strategy were 127, 121, and 65% higher than 0.04% CS strategy, respectively. The DIC concentration 177-230 (mg/L) and C/N ratio 0.48-0.76 were found promising for cell growth through increasing the rubisco activities under CSBPS strategy by 65, 54 and, 4% higher than 0.04% CS, 5% CS and PS strategies, respectively. The presented strategy provides a promising eco-friendly approach to reduce the CO₂ losses and the production cost.

Biotechnological production of astaxanthin from the microalga Haematococcus pluvialis

Although biotechnologies for astaxanthin production from Haematococcus pluvialis have been developed for decades and many production facilities have been established throughout the world, the production cost is still high. This paper is to evaluate the current production processes and production facilities, to analyze the R&D strategies for process improvement, and to review the recent research advances shedding light on production cost reduction. With these efforts being made, we intent to conclude that the production cost of astaxanthin from Haematococcus might be substantially reduced to the levels comparable to that of chemical astaxanthin through further R&D and the future research might need to focus on strain selection and improvement, cultivation process optimization, innovation of cultivation methodologies, and revolution of extraction technologies.

β-Carotene Production from Dunaliella salina Cultivated with Bicarbonate as Carbon Source

Bicarbonate has been considered as a better approach for supplying CO₂ to microalgae cells microenvironments than gas bubbling owing t°Cost-effectiveness and easy operation. However, the β-carotene production was too low in Dunaliella salina cultivated with bicarbonate in previous studies. Also, the difference in photosynthetic efficiency between these tw°Carbon sources (bicarbonate and CO₂) has seldom been discussed. In this study, the culture conditions, including NaHCO₃, Ca²⁺, Mg²⁺ and microelement concentrations, were optimized when bicarbonate was used as carbon source. Under optimized condition, a maximum biomass concentration of 0.71 g/l and corresponding β-carotene content of 4.76% were obtained, with β-carotene yield of 32.0 mg/l, much higher than previous studies with NaHCO₃. Finally, these optimized conditions with bicarbonate were compared with CO₂ bubbling by online monitoring. There was a notable difference in F_v/F_m value between cultivations with bicarbonate and CO₂, but there was no difference in the F_v/F_m periodic changing patterns. This indicates that the high concentration of NaHCO₃ used in this study served as a stress factor for β-carotene accumulation, although high productivity of biomass was still obtained.

Natural Communities of Carotenogenic Chlorophyte Haematococcus lacustris and Bacteria from the White Sea Coastal Rock Ponds

Haematococcus lacustris is a biotechnologically important green unicellular alga producing widely used keta-karotenoid astaxanthin. In natural habitats, it exists in the form of algal-bacterial community, and under laboratory conditions, it is also accompanied by bacteria. The issue of the bacterial composition of industrial algal cultures is widely recognized as important. However, there is a dearth of information about bacterial composition of H. lacustris communities. In current work, we analyze the composition of natural H. lacustris communities from the White Sea coastal temporal rock ponds. For the first time, a 16S rRNA gene-based metagenome of natural H. lacustris bacterial communities has been generated. Main results of its analysis are as follow. Bacterial families Comamonadaceae, Cytophagaceae, Xanthomonadaceae, Acetobacteraceae, Rhodobacteraceae, and Rhodocyclaceae were observed in all studied H. lacustris natural communities. They also contained genera Hydrogenophaga and Cytophaga. Bacteria from the Hydrogenophaga genus were present in H. lacustris cultures after their isolation under the conditions of laboratory cultivation. Similar to other planktonic microalgae, H. lacustris forms a phycosphere around the cells. In this zone, bacteria attached to the algal surface. The contact between H. lacustris and bacteria is maintained even after sample drying. The study provides information about possible members of H. lacustris core microbiome, which can be presented in the industrial and laboratory cultures of the microalga.

High irradiance compensated with CO2 enhances the efficiency of Haematococcus lacustris growth

Haematococcus lacustris (H. lacustris), a promising source for astaxanthin production, is a light-sensitive microalga that is prone to sluggish growth when subjected to high levels of irradiance. A challenge in H. lacustris culture is to find a way to efficiently use illumination to maintain vigorous growth and harvest dense biomass, which is essential for further exploiting the potential for astaxanthin production. Previous studies have shown that in addition to illumination, carbon supply in culture is a key limitation for algae growth. Here, we investigated a combined culture approach involving high light intensity (110 μmol m^-2s^-1) and injection of a 1% (v/v) CO₂ air-gas mixture which provided an effective method for H. lacustris culture to achieve both a high growth rate and high cell density. The cell number in the group with high light exposure combined with CO₂ enrichment was increased almost four-fold compared with a high light group (110 μmol m^-2s^-1 without CO₂ injection). Additional experiments suggested a possible mechanism in which elevated CO₂ increases the electron sink capacity, thus alleviating photoinhibition and oxidative damage. The scaled-up photobioreactor demonstrated much better performance, with growth rates improved by 50-350 %, providing further evidence that this new method can improve algal cell production. Overall, our work provides an efficient way for H. lacustris culture and manufacture, with potential industrial applications.

Integration of anaerobic digestion and microalgal cultivation for digestate bioremediation and biogas upgrading

Biogas is the gaseous byproduct obtained during anaerobic digestion which is rich in methane, along with a significant amount of other gases like CO₂. The removal of CO₂ is essential to upgrade the biogas to biomethane (>95% methane content). High CO₂ tolerant microalgae can be employed as a biological CO₂ scrubbing agent for biogas upgrading. Many microalgal strains tolerant to the levels of CO₂ and CH₄ seen in biogas have been reported. A CO₂ removal efficiency of 50-99% can be attained based on the microalgae used and the cultivation conditions applied. Nutrient-rich liquid digestate obtained from anaerobic digestion can also be used as the cultivation medium for microalgae, performing biogas upgrading and digestate bioremediation simultaneously. Mixotrophic cultivation enables microalgae to utilize the organic carbon present in the liquid digestate along with nitrogen and phosphorus. Microalgae appears to be a potential biological CO₂ scrubbing agent for efficient biogas upgrading.

Non-photochemical quenching in the cells of the carotenogenic chlorophyte Haematococcus lacustris under favorable conditions and under stress

The microalga Haematococcus lacustris (formerly H. pluvialis) is the richest source of the valuable pigment astaxanthin, accumulated in red aplanospores (haematocysts). In this work, we report on the photoprotective mechanisms in H. lacustris, conveying this microalga its ability to cope with a wide range of adverse conditions, with special emphasis put on non-photochemical quenching (NPQ) of the excited chlorophyll states. We studied the changes in the primary photochemistry of the photosystems (PS) as a function of irradiance and the physiological state. We leveraged the transcriptomic data to gain a deeper insight into possible NPQ mechanisms in this microalga. Peculiar to H. lacustris is a bi-phasic pattern of changes in photoprotection during haematocyst formation. The first phase coincides with a transient rise of photosynthetic activity. Based on transcriptomic data, high NPQ level in the first phase is maintained predominantly by the expression of PsbS and LhcsR proteins. Then, (in mature haematocysts), stress tolerance is achieved by optical shielding by astaxanthin and dramatic reduction of photosynthetic apparatus. In contrast to many microalgae, shielding plays an important role in H. lacistris haematocysts, whereas regulated NPQ is suppressed. Astaxanthin is decoupled from the PS, hence the light energy is not transferred to reaction centers and dissipates as heat. It allows to retain a higher photochemical yield in haematocysts comparing to vegetative cells. The ability of H. lacustris to substitute the "classical" active photoprotective mechanisms such as NPQ with optic shielding and general metabolism quiescence makes this organism a useful model to reveal photoprotection mechanisms.

Enhanced astaxanthin accumulation in Haematococcus pluvialis using high carbon dioxide concentration and light illumination

In this study, an economical two-stage method was proposed for the production of natural astaxanthin from Haematococcus pluvialis without a medium replacement step. In stage 1, H. pluvialis were grown under low light illumination until they reached optimal biomass. In stage 2, cells were switched to astaxanthin induction conditions utilizing the combination of high light illumination and elevated carbon dioxide levels (5 or 15%). The introduction of CO₂ altered the C/N balance creating a nutrient deficiency without a change of media. The resulting astaxanthin yield was 2-3 times that of using either stressor alone. This astaxanthin induction method has many advantages over current methods including no medium replacement and a short induction time of less than four days.