Optimizing heterotrophic production of Chlorella sorokiniana SU-9 proteins potentially used as a sustainable protein substitute in aquafeed

Enhanced carbon capture and utilization in transgenic Chlorella sorokiniana harboring pyridoxal kinase under dynamic carbon dioxide levels

Microalgae are crucial in carbon capture, utilization, and storage due to the efficient CO2 assimilation through photosynthesis and potential for high-value biochemical production. However, limited research has explored genetic strain to enhance carbon capture under dynamic CO2 conditions. This research aimed to optimize carbon capture in Chlorella sorokiniana by introducing pyridoxal kinase (pdxY) and cultivation in fluctuating CO2 concentrations. The sequential optimization successfully led to 34% increase in growth with improved carbon capture efficiency to 88.5%. Transgenic strains 2023PY and BSLPY demonstrated superior performance under high (2%) and low (0.04%) CO2, respectively. Addition of Tris base to the medium stabilized pH at favorable level, which is crucial for optimum growth. Scale-up cultivation in 2-L photobioreactor achieved net-zero carbon emissions across all strains. These findings highlight the potential of genetic engineering and process optimization in advancing microalgal carbon capture, along with the production of protein, starch, and lipid for sustainable applications.

Production of single-cell protein from vinegar residue by Rhodotorula glutinis and techno-economic analysis

Single-cell protein (SCP) is attracting attention due to its high value. This study aimed to produce SCP by Rhodotorula glutinis from vinegar residue (VR). The fermentation conditions were optimized by using glucose and then VR was utilized to produce SCP. The titer of SCP was 32.14 g/L, and the protein content and lipid content were 26.67 % and 7.94 %. The yield of SCP was 0.32 g/g VR. Economic analysis of producing SCP from VR showed that processing 100,000 tons of VR annually can produce 32,140 tons of SCP at a production cost of 1,308.38 USD/ton. Sensitivity analysis revealed that the costs of cellulase and yeast were the main factors influencing the production cost of SCP, accounting for 33.6 % and 51.1 % of the raw material cost. Substituting urea for yeast powder could reduce the production cost of SCP to 761.65 USD/ton. This study provided important insights into the industrial production of SCP from VR.

Discovery of mannose as an alternative non-nutrient-deficient regulator of lipid accumulation in microalgae

INTRODUCTION

Microalgae are considered promising bioenergy producers, but their commercial potential is limited by low lipid yields. Nutrient deprivation, particularly nitrogen starvation, is a primary strategy to enhance lipid synthesis efficiency in microalgae. However, controlling this process flexibly, effectively, and accurately remains challenging. Moreover, nutrient deficiency triggers expression changes of numerous genes, complicating the identification of key lipid biosynthesis regulators.

OBJECTIVES

For the first time, we investigated mannose as a novel non-nutrient-deficient regulator of lipid accumulation in microalgae and explored its potential underlying mechanisms.

METHODS

We examined how mannose induction affectslipid accumulation in Chlorella sorokiniana W1 under various culture conditions and compared its effects with nitrogen-starvation. Transcriptome analysis and genome-scale metabolic modeling were used to elucidate the regulatory mechanisms underlying mannose-induced lipid synthesis. Additionally, potential transcription factors were identified using weighted gene co-expression network analysis.

RESULTS

Mannose drives rapid and sustained lipid accumulation in C. sorokiniana under various cultivation conditions, independent of nutrient deficiencies. Under autotrophic conditions, mannose increased lipid content of microalgae by 80.1 %. Notably, mannose was not consumed during cultivation, supporting its role as an inducer. Transcriptomic analysis revealed that mannose increased carbon flux by upregulating genes associated with the Calvin cycle, glycolysis, the TCA cycle, and starch degradation. It also redirected carbon towards lipid accumulation by upregulating lipid synthesis pathways and downregulating lipid degradation pathways. Additionally, two SBP1 transcription factors specifically responsive to mannose were identified and may regulate carbon metabolism in microalgae.

CONCLUSION

Our study introduces mannose as a novel non-nutrient-deficiency regulatory factor for lipid accumulation in C. sorokiniana W1, and explores its metabolic and regulatory mechanisms under various nutrient conditions. The research demonstrates that mannose induction has significant potential for improving microalgal lipid production in practical applications.

Membrane photobioreactor for biogas capture and conversion - Enhanced microbial interaction in biofilm

The urgency to mitigate greenhouse gas emissions has driven interest in sustainable biogas utilization. This study investigates a 1 L enclosed membrane photobioreactor (MPBR) using a microalgae-methanotroph coculture for biogas capture. Operating with a hydraulic and solid retention time of 7 days and a biogas loading rate of 2.7 L /day, the introduction of gas membrane module increased CO2-C and CH4-C uptake rates by 12 % and 50 %, respectively. Biofilm formation on the membrane surface enhanced system performance, with imaging analyses revealing methanotroph predominantly located near the membrane surface and photosynthetic microorganisms distributed throughout. Metagenomic analysis showed shifts in key metabolic pathways, including increased abundance of soluble methane monooxygenase genes and enhanced vitamin B synthesis in the biofilm. These findings highlight the spatial organization and metabolic interactions in methanotroph-microalgae coculture system, providing insights into the role of membrane-induced biofilms in improving MPBR performance for sustainable biogas utilization.

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.

Recent advances and challenges in single cell protein (SCP) technologies for food and feed production

The global population is increasing, with a predicted demand for 1250 million tonnes of animal-derived protein by 2050, which will be difficult to meet. Single-cell protein (SCP) offers a sustainable solution. This review covers SCP production mechanisms, microbial and substrate choices, and advancements in metabolic engineering and CRISPR-Cas. It emphasizes second-generation substrates and fermentation for a circular economy. Despite challenges like high nucleic acid content, SCP promises to solve the global nutrition problem.

Comprehensive Analysis of Biomass from Chlorella sorokiniana Cultivated with Industrial Flue Gas as the Carbon Source

Chlorella sorokiniana, isolated from a pond adjacent to a cement plant, was cultured using flue gas collected directly from kiln emissions using 20 L and 25000 L photobioreactors. Lipids, proteins, and polysaccharides were analyzed to understand their overall composition for potential applications. The lipid content ranged from 17.97% to 21.54% of the dry biomass, with carotenoid concentrations between 8.4 and 9.2 mg/g. Lutein accounted for 55% of the total carotenoids. LC/MS analysis led to the identification of 71 intact triacylglycerols, 8 lysophosphatidylcholines, 10 phosphatidylcholines, 9 monogalactosyldiacylglycerols, 12 digalactosyldiacylglycerols, and 1 sulfoquinovosyl diacylglycerol. Palmitic acid, oleic acid, linoleic acid, and α-linolenic acid were the main fatty acids. Polyunsaturated fatty acid covers ≥ 56% of total fatty acids. Protein isolates and polysaccharides were also extracted. Protein purity was determined to be ≥75% by amino acid analysis, with all essential amino acids present. Monomer analysis of polysaccharides suggested that they are composed of mainly D-(+)-mannose, D-(+)-galactose, and D-(+)-glucose. The results demonstrate that there is no adverse effect on the metabolite profile of C. sorokiniana biomass cultured using flue gas as the primary carbon source, revealing the possibility of utilizing such algal biomass in industrial applications such as animal feed, sources of cosmeceuticals, and as biofuel.

Simultaneous production of algal biomass and lipid by heterotrophic cultivation of linoleic acid-rich oleaginous microalga Chlorella sorokiniana using high acetate dosage

The low-cost carbon source, acetate, was utilized to feed a linoleic acid-rich Chlorella sorokiniana for microalgal biomass and lipid accumulation. Remarkably high tolerance capability to high acetate dosage up to 30 g/L was observed, with heterotrophy being the preferred trophic mode for algal growth and lipogenesis when supplemented 20 g/L acetate. Transcriptome analysis revealed a marked activation of pathways involved in acetate bioconversion and lipogenesis upon exposure to high-level of acetate. However, the enhancement of photorespiration inhibited photosynthesis, which ultimately led to a decrease in biomass and lipid under mixotrophy. Heterotrophic acetate-feeding generated more superior amino acid profiling of algal biomass and a predominant linoleic acid content (50 %). Heterotrophic repeat fed-batch strategy in 5 L fermenter significantly increased the growth performance and lipid titer, with the highest levels achieved being 23.4 g/L and 7.0 g/L, respectively. This work provides a viable approach for bio-products production through acetate-based heterotrophic algal cultivation.

Unlocking microalgal host-exploring dark-growing microalgae transformation for sustainable high-value phytochemical production

Microalgae have emerged as a promising, next-generation sustainable resource with versatile applications, particularly as expression platforms and green cell factories. They possess the ability to overcome the limitations of terrestrial plants, such non-arable land, water scarcity, time-intensive growth, and seasonal changes. However, the heterologous expression of interested genes in microalgae under heterotrophic cultivation (dark mode) remains a niche area within the field of engineering technologies. In this study, the green microalga, Chlorella sorokiniana AARL G015 was chosen as a potential candidate due to its remarkable capacity for rapid growth in complete darkness, its ability to utilize diverse carbon sources, and its potential for wastewater treatment in a circular bioeconomy model. The aims of this study were to advance microalgal genetic engineering via dark cultivation, thereby positioning the strain as promising dark-host for expressing heterologous genes to produce high-value phytochemicals and ingredients for food and feed. To facilitate comprehensive screening based on resistance, eleven common antibiotics were tested under heterotrophic condition. As the most effective selectable markers for this strain, G418, hygromycin, and streptomycin exhibited growth inhibition rates of 98%, 93%, and 92%, respectively, ensuring robust long-term transgenic growth. Successful transformation was achieved through microalgal cell cocultivation with Agrobacterium under complete darkness verified through the expression of green fluorescence protein and β-glucuronidase. In summary, this study pioneers an alternative dark-host microalgal platform, using, Chlorella, under dark mode, presenting an easy protocol for heterologous gene transformation for microalgal host, devoid of the need for expensive equipment and light for industrial production. Furthermore, the developed genetic transformation methodology presents a sustainable way for production of high-value nutrients, dietary supplements, nutraceuticals, proteins and pharmaceuticals using heterotrophic microalgae as an innovative host system.

Wastewater reuse in agriculture: Prospects and challenges

Sustainable water recycling and wastewater reuse are urgent nowadays considering water scarcity and increased water consumption through human activities. In 2015, United Nations Sustainable Development Goal 6 (UN SDG6) highlighted the necessity of recycling wastewater to guarantee water availability for individuals. Currently, wastewater irrigation (WWI) of crops and agricultural land appears essential. The present work overviews the quality of treated wastewater in terms of soil microbial activities, and discusses challenges and benefits of WWI in line with wastewater reuse in agriculture and aquaculture irrigation. Combined conventional-advanced wastewater treatment processes are specifically deliberated, considering the harmful impacts on human health arising from WWI originating from reuse of contaminated water (salts, organic pollutants, toxic metals, and microbial pathogens i.e., viruses and bacteria). The comprehensive literature survey revealed that, in addition to the increased levels of pathogen and microbial threats to human wellbeing, poorly-treated wastewater results in plant and soil contamination with toxic organic/inorganic chemicals, and microbial pathogens. The impact of long-term emerging pollutants like plastic nanoparticles should also be established in further studies, with the development of standardized analytical techniques for such hazardous chemicals. Likewise, the reliable, long-term and extensive judgment on heavy metals threat to human beings's health should be explored in future investigations.

Optimization of Heterotrophic Culture Conditions for the Microalgae Euglena gracilis to Produce Proteins

Euglena gracilis is one of the few permitted edible microalgae. Considering consumer acceptance, E. gracilis grown heterotrophically with yellow appearances have wider food industrial applications such as producing meat analogs than green cells. However, there is much room to improve the protein content of heterotrophic culture cells. In this study, the effects of nitrogen sources, temperature, initial pH, and C/N ratios on the protein production of E. gracilis were evaluated under heterotrophic cultivation. These results indicated that ammonium sulfate was the optimal nitrogen source for protein production. The protein content of E. gracilis cultured by ammonium sulfate increased by 113% and 44.7% compared with that cultured by yeast extract and monosodium glutamate, respectively. The manipulation of the low C/N ratio further improved E. gracilis protein content to 66.10% (w/w), which was 1.6-fold of that in the C/N = 25 group. Additionally, amino acid analysis revealed that the nitrogen-to-protein conversion factor (NTP) could be affected by nitrogen sources. A superior essential amino acid index (EAAI) of 1.62 and a balanced amino acid profile further confirmed the high nutritional value of E. gracilis protein fed by ammonium sulfate. This study highlighted the vast potency of heterotrophic cultured E. gracilis as an alternative dietary protein source.