Regulation and stimulation of photosynthesis of mixotrophically cultured Haematococcus pluvialis by ribose

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.

Instant Inhibition and Subsequent Self-Adaptation of Chlorella sp. Toward Free Ammonia Shock in Wastewater: Physiological and Genetic Responses

Free ammonia (FA) has been recently demonstrated as the primary stress factor suppressing microalgal activities in high-ammonium wastewater. However, its inhibition mechanism and microalgal self-adaptive regulations remain unknown. This study revealed an initial inhibition and subsequent self-adaptation of a wastewater-indigenous Chlorella sp. exposed to FA shock. Mutual physiological and transcriptome analysis indicated that genetic information processing, photosynthesis, and nutrient metabolism were the most influenced metabolic processes. Specifically, for the inhibition behavior, DNA damage was indicated by the significantly up-regulated related genes, leading to the activation of cell cycle checkpoints, programmed apoptosis, and suppressed microalgal growth; FA shock inhibited the photosynthetic activities including both light and dark reactions and photoprotection through non-photochemical quenching; ammonium uptake was also suppressed with the inhibited glutamine synthetase/glutamine α-oxoglutarate aminotransferase cycle and the inactivated glutamate dehydrogenase pathway. With respect to microalgal self-adaptation, DNA damage possibly enhanced overall cell viability through reprogramming the cell fate; recovered nutrient uptake provided substances for self-adaptation activities including amino acid biosynthesis, energy production and storage, and genetic information processing; elevated light reactions further promoted self-adaptation through photodamage repair, photoprotection, and antioxidant systems. These findings enrich our knowledge of microalgal molecular responses to FA shock, facilitating the development of engineering optimization strategies for the microalgal wastewater bioremediation system.

Coelastrella terrestris for Adonixanthin Production: Physiological Characterization and Evaluation of Secondary Carotenoid Productivity

A novel strain of Coelastrella terrestris (Chlorophyta) was collected from red mucilage in a glacier foreland in Iceland. Its morphology showed characteristic single, ellipsoidal cells with apical wart-like wall thickenings. Physiological characterization revealed the presence of the rare keto-carotenoid adonixanthin, as well as high levels of unsaturated fatty acids of up to 85%. Initial screening experiments with different carbon sources for accelerated mixotrophic biomass growth were done. Consequently, a scale up to 1.25 L stirred photobioreactor cultivations yielded a maximum of 1.96 mg·L⁻¹ adonixanthin in free and esterified forms. It could be shown that supplementing acetate to the medium increased the volumetric productivity after entering the nitrogen limitation phase compared to autotrophic control cultures. This study describes a promising way of biotechnological adonixanthin production using Coelastrella terrestris.

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.

Recent advancements in mixotrophic bioprocessing for production of high value microalgal products

Recently, microalgal biomass has become an attractive and sustainable feedstock for renewable production of various biochemicals and biofuels. However, attaining required productivity remains a key challenge to develop industrial applications. Fortunately, mixotrophic cultivation strategy (MCS) is leading to higher productivity due to the metabolic ability of some microalgal strain to utilise both photosynthesis and organic carbon compared to phototrophic or heterotrophic processes. The potential of MCS is being explored by researchers for maximized biochemicals and biofuels production however it requires further development yet to reach commercialization stage. In this review, recent developments in the MCS bioprocess for selective value-added (carotenoids) products have been reviewed; synergistic mechanism of carbon and energy was conferred. Moreover, the metabolic regulation of microalgae under MCS for utilized carbon forms and carbon recycling was demonstrated; Additionally, the opportunities and challenges of large-scale MCS have been discussed.

Recycling of Nutrients from Dairy Wastewater by Extremophilic Microalgae with High Ammonia Tolerance

This study explored the possibility of incorporating extremophilic algal cultivation into dairy wastewater treatment by characterizing a unique algal strain. Results showed that extremophilic microalgae Chlorella vulgaris CA1 newly isolated from dairy wastewater tolerated a high level of ammonia nitrogen (2.7 g/L), which was over 20 times the ammonia nitrogen that regular Chlorella sp. could tolerate. The isolate was mixotrophically cultured in dairy effluent treated by anaerobic digestion (AD) for recycling nutrients and polishing the wastewater. The highest biomass content of 13.3 g/L and protein content of 43.4% were achieved in the culture in AD effluent. Up to 96% of the total nitrogen and 79% of the total phosphorus were removed from the dairy AD effluent. The ability of the algae to tolerate a high level of ammonia nitrogen suggests the potential for direct nutrient recycling from dairy wastewater while producing algal biomass and high value bioproducts.

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.

Multilevel heuristic LED regime for stimulating lipid and bioproducts biosynthesis in Haematococcus pluvialis under mixotrophic conditions

Enhanced biosynthesis and overproduction of lipid and high-value bioproducts endows microalgae as a promising feedstock for the cost-effective development of advanced bioenergy currently. In this study, multilevel heuristic LED regimes, designed with the coordination of light, nitrogen and carbons in terms of cell physiology and morphology, have been implemented to significantly induce the growth and bioproducts accumulation of Haematococcus pluvialis under mixotrophic conditions. The white-red regime with C5 organic carbon showed a good potential for enhancing microalgal biomass and lipid synthesis, especially for saturated fatty acids. The astaxanthin biosynthesis has been significantly enhanced and the highest content of 3.3% was achieved with gluconate at the white-blue regime. Additionally, white-blue regime enhanced the protein content of H. pluvialis up to 45%, increased by 40% over the white-red culture. The multilevel LED protocol can be developed synergistically with the different adaptation and tolerance of algal cell physiology and morphology for target bioproducts.