Chlorella zofingiensis as a promising strain in wastewater treatment

Selecting a preculture strategy for improving biomass and astaxanthin productivity of Chromochloris zofingiensis

Chromochloris zofingiensis is a potential source of natural astaxanthin; however, its rapid growth and astaxanthin enrichment cannot be achieved simultaneously. This study established autotrophic, mixotrophic, and heterotrophic preculture patterns to assess their ameliorative effect on the C. zofingiensis heterotrophic growth state. In comparison, mixotrophic preculture (MP) exhibited the best improving effect on heterotrophic biomass concentration of C. zofingiensis (up to 121.5 g L-1) in a 20 L fermenter, reaching the global leading level. The astaxanthin productivity achieved 111 mg L-1 day-1, 7.4-fold higher than the best record. The transcriptome and 13C tracer-based metabolic flux analysis were used for mechanism inquiry. The results revealed that MP promoted carotenoid and lipid synthesis, and supported synthesis preference of low unsaturated fatty acids represented by C18:1 and C16:0. The MP group maintained the best astaxanthin productivity via mastering the balance between increasing glucose metabolism and inhibition of carotenoid synthesis. The MP strategy optimized the physiological state of C. zofingiensis and realized its heterotrophic high-density growth for an excellent astaxanthin yield on a pilot scale. This strategy exhibits great application potential in the microalgae-related industry. KEY POINTS: • Preculture strategies changed carbon flux and gene expression in C. zofingiensis • C. zofingiensis realized a high-density culture with MP and fed-batch culture (FBC) • Astaxanthin productivity achieved 0.111 g L-1 day-1 with MP and FBC.

Microalgal metabolic engineering facilitates precision nutrition and dietary regulation

Microalgae have gained considerable attention as promising candidates for precision nutrition and dietary regulation due to their versatile metabolic capabilities. This review innovatively applies system metabolic engineering to utilize microalgae for precision nutrition and sustainable diets, encompassing the construction of microalgal cell factories, cell cultivation and practical application of microalgae. Manipulating the metabolic pathways and key metabolites of microalgae through multi-omics analysis and employing advanced metabolic engineering strategies, including ZFNs, TALENs, and the CRISPR/Cas system, enhances the production of valuable bioactive compounds, such as omega-3 fatty acids, antioxidants, and essential amino acids. This work begins by providing an overview of the metabolic diversity of microalgae and their ability to thrive in diverse environmental conditions. It then delves into the principles and strategies of metabolic engineering, emphasizing the genetic modifications employed to optimize microalgal strains for enhanced nutritional content. Enhancing PSY, BKT, and CHYB benefits carotenoid synthesis, whereas boosting ACCase, fatty acid desaturases, and elongases promotes polyunsaturated fatty acid production. Here, advancements in synthetic biology, evolutionary biology and machine learning are discussed, offering insights into the precision and efficiency of metabolic pathway manipulation. Also, this review highlights the potential impact of microalgal precision nutrition on human health and aquaculture. The optimized microalgal strains could serve as sustainable and cost-effective sources of nutrition for both human consumption and aquaculture feed, addressing the growing demand for functional foods and environmentally friendly feed alternatives. The tailored microalgal strains are anticipated to play a crucial role in meeting the nutritional needs of diverse populations and contributing to sustainable food production systems.

A comprehensive review on the heterotrophic production of bioactive compounds by microalgae

Bioactive compounds derived from microalgae have garnered considerable attention as valuable resources for drugs, functional foods, and cosmetics. Among these compounds, photosynthetic pigments and polyunsaturated fatty acids (PUFAs) have gained increasing interest due to their numerous beneficial properties, including anti-oxidant, anti-viral, anti-bacterial, anti-fungal, anti-inflammatory, and anti-tumor effects. Several microalgae species have been identified as rich sources of bioactive compounds, including the Chlorophyceae Dunaliella and Haematococcus, the Bacillariophyta Phaeodactylum and Nitzschia, and the dinoflagellate Crypthecodinium cohnii. However, most of the reported microalgae species primarily grow through autotrophic mechanisms, resulting in low yields and high production costs of bioactive compounds. Consequently, the utilization of heterotrophic microalgae, such as Chromochloris zofingiensis and Nitzschia laevis, has shown significant advantages in the production of astaxanthin and eicosapentaenoic acid (EPA), respectively. These heterotrophic microalgae exhibit superior capabilities in synthesizing target compounds. This comprehensive review provides a thorough examination of the heterotrophic production of bioactive compounds by microalgae. It covers key aspects, including the metabolic pathways involved, the impact of cultivation conditions, and the practical applications of these compounds. The review discusses how heterotrophic cultivation strategies can be optimized to enhance bioactive compound yields, shedding light on the potential of microalgae as a valuable resource for high-value product development.

Realization process of microalgal biorefinery: The optional approach toward carbon net-zero emission

Increasing carbon dioxide (CO2) emission has already become a dire threat to the human race and Earth's ecology. Microalgae are recommended to be engineered as CO2 fixers in biorefinery, which play crucial roles in responding climate change and accelerating the transition to a sustainable future. This review sorted through each segment of microalgal biorefinery to explore the potential for its practical implementation and commercialization, offering valuable insights into research trends and identifies challenges that needed to be addressed in the development process. Firstly, the known mechanisms of microalgal photosynthetic CO2 fixation and the approaches for strain improvement were summarized. The significance of process regulation for strengthening fixation efficiency and augmenting competitiveness was emphasized, with a specific focus on CO2 and light optimization strategies. Thereafter, the massive potential of microalgal refineries for various bioresource production was discussed in detail, and the integration with contaminant reclamation was mentioned for economic and ecological benefits. Subsequently, economic and environmental impacts of microalgal biorefinery were evaluated via life cycle assessment (LCA) and techno-economic analysis (TEA) to lit up commercial feasibility. Finally, the current obstacles and future perspectives were discussed objectively to offer an impartial reference for future researchers and investors.

Comparison of Production and Fluorescence Characteristics of Phycoerythrin from Three Strains of Porphyridium

Phycoerythrin, a special photosynthetic pigment, is widely used as fluorescent dye and has lots of underlying beneficial effects on health. A marine red microalga Porphyridium is considered as the potential feedstock for phycoerythrin production. However, the phycoerythrin-related properties of Porphyridium have not been systematically evaluated, especially between the species of P. cruentum and P. purpureum. The present study aimed to evaluate the production and fluorescence characteristics of phycoerythrin of three strains of Porphyridium. The results showed that P. purpureum SCS-02 presented the highest biomass, phycoerythrin content and yield were 6.43 g L⁻¹, 9.18% DW and 0.288 g L⁻¹, respectively. There was no significant difference between P. purpureum and P. cruentum in α and β subunits amino acid sequences of phycoerythrin and in fluorescence characteristics. The high gene expression level of the key enzymes in phycoerythrobilin synthesis (porphobilinogen synthase and oxygen-dependent coproporphyrinogen-III oxidase) could be related to the high phycoerythrin content of Porphyridium. Based on systematic evaluation, P. purpureum SCS-02 was selected due to its high biomass and phycoerythrin yield. P. purpureum and P. cruentum were highly similar in the phylogenetic tree, as well as in fluorescence characteristics; therefore, it was speculated that they might be the same Porphyridium species.

Effective bioremediation of tobacco wastewater by microalgae at acidic pH for synergistic biomass and lipid accumulation

Tobacco wastewater is too difficult to decontaminate which poses a significant environmental problem due to the harmful and toxic components. Chlorella pyrenoidosa is a typical microalgal species with potential in removal of organic/inorganic pollutants and proves to be an ideal algal-based system for wastewater treatment. However, the strategy of tobacco related wastewater treatment using microalgae is in urgent need of development. In this study, C. pyrenoidosa was used to evaluate the removal efficiency of artificial tobacco wastewater. Under various solid-to-liquid (g/L) ratios, 1:1 ratio and acidic pH 5.0 were optimal for C. pyrenoidosa to grow with high performance of removal capacity to toxic pollutants (such as COD, NH3-N, nicotine, nitrosamines and heavy metals) with the alleviation of oxidative damage. Algal biomass could reach up to 540.24 mg/L. Furthermore, carbon flux of C. pyrenoidosa was reallocated from carbohydrate and protein biosynthesis to lipogenesis with a high lipid content of 268.60 mg/L at pH 5.0. Overall, this study demonstrates an efficient and sustainable strategy for tobacco wastewater treatment at acidic pH with the production of valuable microalgal products, which provides a promising biorefinery strategy for microalgal-based wastewater bioremediation.

Nutraceutical productions from microalgal derived compounds via circular bioeconomy perspective

Circular bioeconomy has become a sustainable business model for commercial production that promises to reuse, recycle & recover while considering less environmental footprints in nutraceutical industries. Microalgae biotechnology has the synergy to bioremediate waste stream while generating high-value-added compounds such as astaxanthin, protein and polyunsaturated fatty acids that are potential compounds used in various industries, thus, the integration of this approach provides economic advantages. However, since the industrial production of these compounds is costly and affected byunstable climate in the Nordic regions such as low temperature, light intensity, and polar circle, the focus of biosynthesis has shifted from less tolerant commercial strains towards indigenous strains. Nutraceutical productions such as polyunsaturated fatty acids and protein can now be synthesized at low temperatures which significantly improve the industry's economy. In this review, the above-mentioned compounds with potential strains were discussed based on a Nordic region's perspective.

Wastewater-based microalgal biorefineries for the production of astaxanthin and co-products: Current status, challenges and future perspectives

The freshwater microalgae Haematococcus pluvialis and Chlorella zofingiensis are attractive biorefinery feedstocks in view of their ability to simultaneously synthesize astaxanthin and other valuable metabolites. Nonetheless, there are concerns regarding the sustainability of such biorefineries due to the high freshwater footprint of microalgae cultivation. The integration of wastewater as an alternative growth media is a promising approach to reduce freshwater demand. Wastewater-based cultivation enables the recovery of essential nutrients required for microalgae growth and consequently results in phycoremediation of wastewater, thus promoting the concept of a circular economy and further enhancing the sustainability of the process. In this review, recent developments in wastewater-integrated cultivation of H. pluvialis and C. zofingiensis for astaxanthin production are discussed. Furthermore, prospective strategies for overcoming the inherent challenges of wastewater-based cultivation are reviewed. Moreover, the biorefinery potential of wastewater-grown H. pluvialis and C. zofingiensis is delineated and future perspectives of wastewater-based biorefineries are outlined.

Small-scale phyco-mitigation of raw urban wastewater integrated with biodiesel production and its utilization for aquaculture

A low-cost small-scale high-rate algal pond (HRAP) was constructed to investigate the synergistic potential of a novel oleaginous microalga, Chlorella sorokiniana for phyco-mitigation, and biodiesel production using raw urban wastewater. An enhanced nutrient removal (97%), total organic carbon (74%), alkalinity (70%) and hardness (75%) from the wastewater was obtained. The microalga dominated in the HRAP as ~90% increase in the dissolved oxygen with high biomass (1.13 g/L) was noted. The microalga biomass showed sufficient lipid content (~31% of dry cell weight) as compared to control (Bold's Basal media). The total lipid profiling of the microalga cultivated in wastewater showed augmentation in the levels of both storage and neutral lipids with good quality fatty acids composition. Moreover, the sucker fishes grew healthy in the treated wastewater with an increase in body weight.

Cost-effective wastewater treatment in a continuous manner by a novel bio-photoelectrolysis cell (BPE) system

Approaches to improve wastewater treatment by microalgae have objectives of greater culture control, efficient nutrient removal and increased lipid content. This work designed a bio-photoelectrolysis cell (BPE) system to modulate wastewater treatment by electric current. The electric current had the capacity to enrich entrapped cell weight with a 0.72-fold increase, which resulted in high daily nutrient removal, with 6.78 mg/L/d for nitrogen and 2.14 mg/L/d for phosphorus at 0.6 A/m². As the nutrient removal was mostly dependent on cell growth, the 1.17-fold increase of lipid productivity was achieved. The harvesting at 6 A/m² required lower energy input of 1.77 KWh/kg. For the recyclability of treatment, BPE system could continuously treat the fresh wastewater for at least three cycles with biomass and lipid productivities of 68.67 and 22.04 mg/L/d, respectively. The nitrogen removal model of C_st = 45.52-5.52exp(0.45 t) and phosphorus removal model of C_st = 12.54-1.48exp(0.45 t) were established to evaluate the stability of BPE system.