Mixotrophic culture of bait microalgae for biomass and nutrients accumulation and their synergistic carbon metabolism

Carbon distribution and metabolism mechanism of a novel mixotrophic Chlorella in municipal wastewater

Conventional wastewater treatment technologies primarily convert complex organic matter into dissolved inorganic carbon (DIC) and a more difficult gaseous state CO2. Most microalgae species can photosynthetically assimilate above inorganic carbon, but their heterotrophic metabolic processes often dominate in glucose-mediated mixotrophy. Herein, we investigated the carbon-fixing metabolic pathways of Chlorella sp. MIHQ61 in municipal wastewater containing complex carbon sources. The total carbon removal (73.0 %) peaked on the 6th day, and DIC removal exceeded 50.0 % as the carbon migrating amount from municipal wastewater into the microalgal cells peaked. The glucose and NaHCO3 combination promoted both autotrophic and heterotrophic metabolism. Headspace CO2 emission, enzyme activity and central carbon metabolism results implied heterotrophic metabolism occurred more actively in the early stage and autotrophic metabolism dominated late stage. Redefined mixotrophic carbon allocation by revealing time-dependent autotrophic/heterotrophic interplay. Carbon distribution and mixotrophic mechanism provided new thinking on how to utilize microalgae and wastewater resource.

Mining marine dinoflagellate Amphidinium carterae for peridinin production by optimizing light conditions and mixotrophy strategy

As a promising marine bioresource, Amphidinium carterae represents a significant source of peridinin, a light-harvesting carotenoid exclusive to dinoflagellates. However, its low cultivation density has hindered its exploitation, particularly for efficient peridinin production. This study initially screened the optimal light intensity of 55 μmol/m2/s and a mixotrophic culture condition supplemented with 5 g/L glycerol for A. carterae cultivation. Subsequently, a combined cultivation strategy achieved the highest cell density of 9.76 × 10⁶ cells/mL and a peridinin yield of 32.44 mg/L. Scale-up cultivation in column photobioreactors demonstrated the robust adaptability of A. carterae, as evidenced by the photosynthetic and respiratory rate measurements. Furthermore, comprehensive in vitro antioxidant assessments revealed the potent antioxidant activity of peridinin, which exhibited comparable efficacy to fucoxanthin and superior performance relative to β-carotene. Through systematic optimization, this study has achieved the highest cell density and peridinin yield in A. carterae reported thus far.

Protein and lysine improvement harnessed by a signal chain of red light-emitting diode light in Chlorella pyrenoidosa

Microalgae are emerging as a novel single-cell protein source that can substitute traditional plant protein feeds. In this investigation, lysine and protein accumulation in Chlorella pyrenoidosa were significantly enhanced under red light-emitting diode light, addressing challenge of limiting amino acid in plant proteins. The study employed targeted metabolomics, HPLC, and qRT-PCR to validate the light-induced pathway triggering lysine biosynthesis. Specifically, the pathway involves Ca2+-CaM as an intermediary in signal transduction, which directly inhibits PEPC activity. This inhibition directs a significant carbon flux towards central carbon metabolism, resulting in increased pyruvate levels-a critical precursor for lysine biosynthesis via the diaminopimelate pathway. Ultimately, the content of protein and lysine under red light increased by 36.02 % and 99.56 %, respectively, compared to those under white light. These findings provide a novel orientation for the precise regulation of lysine accumulation in microalgae, and moreover lay a solid theoretical foundation for producing microalgal proteins.

An assessment of the autotrophic/heterotrophic synergism in microalgae under mixotrophic mode and its contribution in high-rate phosphate recovery from wastewater

Dual carbon metabolisms and the synergism contribute to improving nutrient recovery under mixotrophy. However, how synergism influences nutrient recovery has yet to be understood, which is revealed in the current study. Due to dual carbon metabolisms and synergism,the PO4--P recovery rate under mixotrophy reached 0.34 mg L-1 h-1. Due to the internal cycling of respiratory CO2, the mutualistic index (MI) in terms of synergism helped Scenedesmus accumulate 27.49 % more biomass under mixotrophy than sum of the two controls. In contrast, MI contributed 0.26 g L-1 d-1 to the total modeled mixotrophic productivity of 1.15 g L-1 d-1. To total modeled PO4--P recovery, mixotrophic-auto, and mixotrophic-hetero shares were 42 % and 58 %. The synergism under mixotrophy contributed 20 % in total PO4--P recovery. The PO4--P recovery rate under mixotrophywas comparable to other biological P removal methods. These findings emphasize the potential of synergism in improving productivityand promoting resource recovery for sustainable wastewater treatment.

A Response Surface Methodology Study for Chlorella vulgaris Mixotrophic Culture Optimization

Glycerol is a carbon source that produces good biomass under mixotrophic conditions. Enhancing the composition of culture media in algae biomass production improves growth rates, biomass yield, nutrient utilization efficiency, and overall cost-effectiveness. Among the key nutrients in the medium, nitrogen plays a pivotal role. Urea can be effectively used as a nitrogen source and is considered a low-cost form of nitrogen compared to other sources. Urea metabolism releases some CO2 in photosynthesis, and magnesium plays a major role in urea uptake. Magnesium is another key nutrient that is key in photosynthesis and other metabolic reactions. To maximize glycerol consumption in the mixotrophic system and to obtain high biomass and lipid productions, the variations in MgSO4·7H2O and urea concentrations were evaluated in the growth medium of the microalgae. A response surface methodology (RSM) using a central composite design (CCD) was designed to maximize glycerol consumption at the initial cellular growth rates (up to four days). The magnesium and urea supply varied from 0.3 to 1.7 g L-1. Response surface methodology was utilized to analyze the results, and the highest glycerol consumption rate, 770.2 mg L-1 d-1, was observed when C. vulgaris was grown at 1.7 g L-1 urea, 1.0 g L-1 MgSO4·7H2O. Using the optimal urea and magnesium concentrations with acetate, glucose, and glycerol as carbon sources, the same lipid content (10% average) was achieved on day 4 of mixotrophic C. vulgaris culture. Overall, the results show that mixotrophic growth of C. vulgaris using urea with an optimum magnesium concentration yields large amounts of fatty acids and that the carbon source greatly influences the profile of the fatty acids.

Insight into the potential mechanism of bicarbonate assimilation promoted by mixotrophic in CO2 absorption and microalgae conversion system

CO2 absorption-microalgae conversion (CAMC) system is a promising carbon capture and utilization technology. However, the use of HCO3- as a carbon source often led to a slower growth rate of microalgae, which also limited the application of CAMC system. In this study, the assimilation efficiency of HCO3- in CAMC system was improved through mixotrophic, and the potential mechanism was investigated. The HCO3- assimilation efficiency and biomass under mixotrophic were 34.79% and 31.76% higher than that of control. Mixotrophic increased chlorophyll and phycocyanin content, which were beneficial to capture more light energy. The content of ATP and NADPH reached 566.86 μmol/gprot and 672.86 nmol/mgprot, which increased by 31.83% and 27.67% compared to autotrophic. The activity of carbonic anhydrase and Rubisco increased by 18.52% and 22.08%, respectively. Transcriptome showed that genes related to photosynthetic and respiratory electron transport were up-regulated. The synergy of photophosphorylation and oxidative phosphorylation greatly improved energy metabolism efficiency, thus accelerating the assimilation of HCO3-. These results revealed a potential mechanism of promoting the HCO3- assimilation under mixotrophic, it also provided a guidance for using CAMC system to serve carbon neutrality.

The influence of spermidine on the build-up of fucoxanthin in Isochrysis sp. Acclimated to varying light intensities

Spermidine is a type of important growth regulator, which involved in the biosynthesis of photosynthetic pigments, and has the function of promoting cell proliferation. In this study, Isochrysis sp. was selected as the research object to explore the effects of spermidine supplementation on the growth of algal cells and fucoxanthin synthesis under different light intensities. The results showed that the cell density (5.40 × 106 cells/mL) of algae were the highest at 11 days under the light intensity of 200 μmol·m-2·s-1 and spermidine content of 150 μM. The contents of diadinoxanthin (1.09 mg/g) and fucoxanthin (6.11 mg/g) were the highest when spermidine was added under low light intensity, and the growth of algal cells and fucoxanthin metabolism were the most significant. In the carotenoid synthesis pathway, PDS (phytoene desaturase) was up-regulated by 1.96 times and VDE (violaxanthin de-epoxidase) was down-regulated by 0.95 times, which may promote fucoxanthin accumulation.

Characterization of exogenous lactate addition on the growth, photosynthetic performance, and biochemical composition of four bait microalgae strains

AIMS

To quickly obtain the biomass of bait microalgae with high value-added products, researchers have examined the influence of biochemical and environmental factors on the growth rates and biochemical composition of microalgae. Previous studies have shown that lactate plays an important role in metabolic regulation in Phaeodactylum tricornutum. In this study, we investigated the effect of exogenous lactate on the growth rates, photosynthetic efficiency, and biochemical composition of four commonly used bait microalgae in aquaculture.

METHODS AND RESULTS

The optical density of the algal cultures at specific time points, YII, Fv/Fm, and the total lipid, protein, soluble sugar, insoluble sugar, chlorophyll a, and carotenoid content of P. tricornutum, Isochrysis galbana (I. galbana), Chaetoceros muelleri, and Cylindrotheca fusiformis were determined. In I. galbana, the growth rate was enhanced with the addition of lactate, even though higher concentrations of lactate were associated with a decrease in YII and Fv/Fm. In general, the total lipid content of these microalgal strains increased gradually in a concentration-dependent manner over the range of lactate concentrations. In addition, higher concentrations of lactate also induced significant changes in the total soluble and insoluble sugar levels in all microalgal strains. However, chlorophyll a and carotenoid contents increased at lower but decreased at higher concentrations of lactate in all microalgal strains. The total protein content was significantly elevated at all concentrations of lactate in P. tricornutum, whereas there were no significant differences in that of C. fusiformis.

CONCLUSIONS

Lactate effective influences in the growth, metabolism, and synthesis of important biochemical components in the four microalgal strains under investigation.

Advancements of microalgal upstream technologies: Bioengineering and application aspects in the paradigm of circular bioeconomy

Sustainable energy transition has brought the attention towards microalgae utilization as potential feedstock due to its tremendous capabilities over its predecessors for generating more energy with reduced carbon footprint. However, the commercialization of microalgae feedstock remains debatable due to the various factors and considerations taken into scaling-up the conventional microalgal upstream processes. This review provides a state-of-the-art assessment over the recent developments of available and existing microalgal upstream cultivation systems catered for maximum biomass production. The key growth parameters and main cultivation modes necessary for optimized microalgal growth conditions along with the fundamental aspects were also reviewed and evaluated comprehensively. In addition, the advancements and strategies towards potential scale-up of the microalgal cultivation technologies were highlighted to provide insights for further development into the upstream processes aimed at sustainable circular bioeconomy.

Biotechnologies for bulk production of microalgal biomass: from mass cultivation to dried biomass acquisition

Microalgal biomass represents a sustainable bioresource for various applications, such as food, nutraceuticals, pharmaceuticals, feed, and other bio-based products. For decades, its mass production has attracted widespread attention and interest. The process of microalgal biomass production involves several techniques, mainly cultivation, harvesting, drying, and pollution control. These techniques are often designed and optimized to meet optimal growth conditions for microalgae and to produce high-quality biomass at acceptable cost. Importantly, mass production techniques are important for producing a commercial product in sufficient amounts. However, it should not be overlooked that microalgal biotechnology still faces challenges, in particular the high cost of production, the lack of knowledge about biological contaminants and the challenge of loss of active ingredients during biomass production. These issues involve the research and development of low-cost, standardized, industrial-scale production equipment and the optimization of production processes, as well as the urgent need to increase the research on biological contaminants and microalgal active ingredients. This review systematically examines the global development of microalgal biotechnology for biomass production, with emphasis on the techniques of cultivation, harvesting, drying and control of biological contaminants, and discusses the challenges and strategies to further improve quality and reduce costs. Moreover, the current status of biomass production of some biotechnologically important species has been summarized, and the importance of improving microalgae-related standards for their commercial applications is noted.