Improving high carbon dioxide tolerance and carbon dioxide fixation capability of Chlorella sp. by adaptive laboratory evolution

Proteomics reveals toxin tolerance and polysaccharide accumulation in Chlorococcum humicola under high CO2 concentration

Algae have great application prospects in excess sludge reclamation and recovery of high-value biomass. Chlorococcum humicola was cultivated in this research, using sludge extract (mixed with SE medium) with additions of 10%, 20%, and 30% CO2 (v/v). Results showed that under 20% CO2, the dry weight and polysaccharide yield reached 1.389 ± 0.070 g/L and 313.49 ± 10.77 mg/L, respectively. 10% and 20% CO2 promoted the production of cellular antioxidant molecules to resist the toxic stress and the toxicity of 20% CO2 group decreased from 62.16 ± 3.11% to 33.02 ± 3.76%. 10% and 20% CO2 accelerated the electron transfer, enhanced carbon assimilation, and promoted the photosynthetic efficiency, while 30% CO2 led to photosystem damage and disorder of antioxidant system. Proteomic analysis showed that 20% CO2 mainly affected energy metabolism and the oxidative stress level on the early stage (10 d), while affected photosynthesis and organic substance metabolism on the stable stage (30 d). The up-regulation of PSII photosynthetic protein subunit 8 (PsbA, PsbO), A0A383W1S5 and A0A383VRI4 promoted the efficiency of PSII and chlorophyll synthesis, and the up-regulation of A0A383WH74 and A0A2Z4THB7 led to the accumulation of polysaccharides. The up-regulation of A0A383VDH1, A0A383VX37 and A0A383VA86 promoted respiration. Collectively, this work discloses the regulatory mechanism of high-concentration CO2 on Chlorococcum humicola to overcome toxicity and accumulate polysaccharides.

Transcriptome analysis of the effects of different carbon dioxide concentrations on paramylon accumulation in Euglena gracilis Z

Appropriate concentration of carbon dioxide (CO2) will promote algae growth and metabolism. Building upon this finding, the present study investigated the impact of different CO2 concentrations (5% and 20%) on the carbon sequestration capacity of E. gracilis through aeration culturing, employing a combination of physiological analyses and transcriptome analysis. The results demonstrated that under 5% CO2 concentration, the cell density of E. gracilis was 1.79 times higher than that achieved in an air culture condition, and the paramylon content of E. gracilis was found to be 6.18 times higher than that of the air group. Based on transcriptome analysis, the carbon metabolism of E. gracilis was discussed. Significant up-regulation expression of genes associated with carbon synthesis was validated by an increase in paramylon content. This study revealed that under 5% CO2 conditions, E. gracilis exhibited elevated growth rate and enhanced photosynthetic carbon assimilation efficiency.

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.

Intestinal Immune Cell Populations, Barrier Function, and Microbiomes in Broilers Fed a Diet Supplemented with Chlorella vulgaris

This study aimed to evaluate the effects of dietary Chlorella vulgaris (CV) on the distribution of immune cells, intestinal morphology, intestinal barrier function, antioxidant markers, and the cecal microbiome in 10-day-old broiler chickens. A total of 120 day-old Ross 308 male broiler chicks were assigned to two dietary treatments using a randomized complete block design, with body weight as the blocking factor. Birds fed a diet containing CV showed an increase in CD4⁺ T cells (p < 0.05) compared to those fed the control diet. The relative mRNA expression of intestinal epithelial barrier function-related markers (occludin and avian β-defensin 5) was elevated (p < 0.05) in the CV-supplemented group compared to the control group. The alpha diversity indices (Chao1 and observed features) of the cecal microbiome in 10-day-old birds increased (p < 0.05), indicating higher richness within the cecal bacterial community. In the microbiome analysis, enriched genera abundance of Clostridium ASF356 and Coriobacteriaceae CHKCI002 was observed in birds fed the diet containing CV compared to those fed the control diet. Taken together, dietary CV supplementation might alter intestinal barrier function, immunity, and microbiomes in 10-day-old broiler chickens.

Unlocking the potential of microalgae bio-factories for carbon dioxide mitigation: A comprehensive exploration of recent advances, key challenges, and energy-economic insights

Microalgae are promising alternatives to mitigate atmospheric CO₂ owing to their fast growth rates, resilience in the face of adversity and ability to produce a wide range of products, including food, feed supplements, chemicals, and biofuels. However, to fully harness the potential of microalgae-based carbon capture technology, further advancements are required to overcome the associated challenges and limitations, particularly with regards to enhancing CO₂ solubility in the culture medium. This review provides an in-depth analysis of the biological carbon concentrating mechanism and highlights the current approaches, including species selection, optimization of hydrodynamics, and abiotic components, aimed at improving the efficacy of CO₂ solubility and biofixation. Moreover, cutting-edge strategies such as gene mutation, bubble dynamics and nanotechnology are systematically outlined to elevate the CO₂ biofixation capacity of microalgal cells. The review also evaluates the energy and economic feasibility of using microalgae for CO₂ bio-mitigation, including challenges and prospects for future development.

The Influence of Elevated CO2 Concentrations on the Growth of Various Microalgae Strains

The influence of elevated CO₂ concentrations on the growth and viability of various microalgae strains was studied. Arthrospira platensis, Chlorella ellipsoidea, Chlorella vulgaris, Gloeotila pulchra, and Elliptochloris subsphaerica were tested. The cultivation of microalgae was carried out at constant CO₂ concentrations (0.04, 3, 6, or 9%-sequentially from lower to higher concentrations), under constant (24 h·day^-1) illumination with an intensity of 74.3 µmol quanta·m^-2·s^-1, and a constant temperature of 23.5 ± 0.5 °C. The optical density of the microalgae biomass, pH, and the chemical composition of the culture medium were measured. Microscopy (including the cytochemical microscopic method) was conducted to monitor the state of the microalgae. The highest biomass growth rate (0.37 g·L^-1·day^-1), among all experiments, was achieved for Chlorella vulgaris at CO₂ = 3% and for Chlorella ellipsoidea at CO₂ = 6 and 9%. The lowest growth rate (0.12 g·L^-1·day^-1) was achieved for Arthrospira platensis at CO₂ = 3 and 9%. The microscopy results showed the absence or a minimum number of dead cells of the strains under selected conditions. The ability to maintain the viability of cultures up to significant concentrations of CO₂ = 9% was due to adaptation (gradual increase in CO₂ concentrations in the experiments).

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.

Comparative Evaluation of Chlorella vulgaris and Anabaena variabilis for Phycoremediation of Polluted River Water: Spotlighting Heavy Metals Detoxification

This study investigated the phycoremediation abilities of Chlorella vulgaris (microalga) and Anabaena variabilis (cyanobacterium) for the detoxification of polluted river water. Lab-scale phycoremediation experiments were conducted for 20 days at 30 °C using the microalgal and cyanobacterial strains and water samples collected from the Dhaleswari river in Bangladesh. The physicochemical properties such as electrical conductivity (EC), total dissolved solids (TDS), biological oxygen demand (BOD), hardness ions, and heavy metals of the collected water samples indicated that the river water is highly polluted. The results of the phycoremediation experiments demonstrated that both microalgal and cyanobacterial species significantly reduced the pollutant load and heavy metal concentrations of the river water. The pH of the river water was significantly raised from 6.97 to 8.07 and 8.28 by C. vulgaris and A. variabilis, respectively. A. variabilis demonstrated higher efficacy than C. vulgaris in reducing the EC, TDS, and BOD of the polluted river water and was more effective at reducing the pollutant load of SO₄^2- and Zn. In regard to hardness ions and heavy metal detoxification, C. vulgaris performed better at removing Ca²⁺, Mg²⁺, Cr, and Mn. These findings indicate that both microalgae and cyanobacteria have great potential to remove various pollutants, especially heavy metals, from the polluted river water as part of a low-cost, easily controllable, environmentally friendly remediation strategy. Nevertheless, the composition of polluted water should be assessed prior to the designing of microalgae- or cyanobacteria-based remediation technology, since the pollutant removal efficiency is found to be species dependent.

CO2 gradient domestication improved high-concentration CO2 tolerance and photoautotrophic growth of Euglena gracilis

In order to improve CO₂ biofixation efficiency of microalgae cultivated with coal-chemical flue gas, CO₂ gradient domestication was employed to improve high-concentration CO₂ tolerance and photoautotrophic growth of acid-tolerant Euglena gracilis. The dried biomass yield of photoautotrophic growth of E.gracilis increased from 1.09 g/L (wild-type strain) by 21 % to 1.32 g/L with CO₂ gradient domestication to 15 % CO₂. The RuBisCO activity and biomass production of E.gracilis strain domesticated to 99 % CO₂ were 2.63 and 3.4 times higher, respectively, than those of wild-type strain. The chlorophyll a and b contents were 2.52 and 1.79 times higher, respectively, than those of wild-type strain. Superoxide dismutase and catalase activities of 99 % CO₂-domesticated strain increased to 1.24 and 6 times, which reduced peroxide damage under high carbon stress and resulted in lower apoptotic and necrotic rates of domesticated strain. Thus, this work provides valuable guidance for CO₂ fixation and adaptive evolution of E. gracilis in industrial flue gas.

Adaptive laboratory evolution for increased temperature tolerance of the diatom Nitzschia inconspicua

Outdoor microalgal cultivation for the production of valuable biofuels and bioproducts typically requires high insolation and strains with high thermal (>37°C) tolerance. While some strains are naturally thermotolerant, other strains of interest require improved performance at elevated temperatures to enhance industrial viability. In this study, adaptive laboratory evolution (ALE) was performed for over 300 days using consecutive 0.5°C temperature increases in a constant temperature incubator to attain greater thermal tolerance in the industrially relevant diatom Nitzschia inconspicua str. Hildebrandi. The adapted strain was able to grow at a constant temperature of 37.5°C; whereas this constant temperature was lethal to the parental control, which had an upper-temperature boundary of 35.5°C before adaptive evolution. Several high-temperature clonal isolates were obtained from the evolved population following ALE, and increased temperature tolerance was observed in the clonal, parent, and non-clonal adapted cultures. This ALE method demonstrates the development of enhanced industrial algal strains without the production of genetically modified organisms (GMOs).

A two-prong mutagenesis and adaptive evolution strategy to enhance the temperature tolerance and productivity of Nannochloropsis oculata

Incorporation of microalgae in biorefineries intended to help society reach carbon neutrality is hindered by algal growth inhibition at high temperatures, necessitating the use of costly and carbon-intensive cooling systems. In the present study, a two-prong strategy of random mutagenesis and adaptive laboratory evolution to generate robust thermotolerant strains of Nannochloropsis oculata, was used. The best mutants demonstrated increased productivity at 35 °C, which was 10 °C higher than the optimal temperature of the wild type. In a 2-L photobioreactor at 35 °C, biomass and lipid productivity were 1.43-fold and 2.24-fold higher, respectively, than wild type at 25 °C. Higher pigment and carbohydrate content contributed to the mutants' rapid growth and enhanced photosynthetic efficiency. Metabolomics and lipidomics showed rewiring of the central carbon metabolism and membrane lipid synthesis in thermotolerant strains to ensure cellular homeostasis without compromising productivity. Tagatose and phosphatidylethanolamine upregulation were identified as future genetic targets for further enhancing lipid production.

Enhancing microalgal lipid accumulation for biofuel production

Microalgae have high lipid accumulation capacity, high growth rate and high photosynthetic efficiency which are considered as one of the most promising alternative sustainable feedstocks for producing lipid-based biofuels. However, commercialization feasibility of microalgal biofuel production is still conditioned to the high production cost. Enhancement of lipid accumulation in microalgae play a significant role in boosting the economics of biofuel production based on microalgal lipid. The major challenge of enhancing microalgal lipid accumulation lies in overcoming the trade-off between microalgal cell growth and lipid accumulation. Substantial approaches including genetic modifications of microalgal strains by metabolic engineering and process regulations of microalgae cultivation by integrating multiple optimization strategies widely applied in industrial microbiology have been investigated. In the present review, we critically discuss recent trends in the application of multiple molecular strategies to construct high performance microalgal strains by metabolic engineering and synergistic strategies of process optimization and stress operation to enhance microalgal lipid accumulation for biofuel production. Additionally, this review aims to emphasize the opportunities and challenges regarding scaled application of the strategic integration and its viability to make microalgal biofuel production a commercial reality in the near future.

Insights into activated sludge/Chlorella consortia under dark condition compared with light condition

Bacteria-algae consortia in the light bring the benefit of O₂ production and CO₂ reduction for wastewater treatment, while the bottleneck for application is how it behaves in the dark. In this study, inoculum ratio and sludge retention time (SRT) affected nutrient removal rather than chemical oxygen demand (COD) removal. Dark conditions (with a sludge/Chlorella inoculum ratio of 1:2 at a SRT of 15 d) achieved comparable performance to those of light conditions, due to bacteria contribution and mechanical aeration. Compared with light conditions, the ratio of Chla/Chlb decreased and Caro/(Chla + Chlb) increased to response oxidative stress. In the dark, algae were associated with Nitrosomonas and Dechloromonas. Flavobacterium disassociated with Chlorella in the dark but associated with Chlorella in the light. Moreover, nitritation genes (amo and Hao) and denitrifying gene (narH) were up-regulated, while P metabolism genes (PPX and PPK) were down-regulated. It is proposed to enrich Nitrosomonas in the night and denitrify polyphosphate accumulating organisms (DPAO) in the daytime to establish short-cut nitrification and denitrifying phosphorus removal in practical applications.

Exploring kinetics, removal mechanism and possible transformation products of tigecycline by Chlorella pyrenoidosa

The accumulation of antibiotics in wastewater leads to broad antibiotic resistance, threating human health. Microalgae have been receiving attention due to their ability to remove antibiotics from wastewater. Tigecycline (TGC) is a broad-spectrum glycylcycline antibiotic. It has not been investigated for removal by microalgae. The removal kinetics of TGC by Chlorella pyrenoidosa were evaluated under different initial dry cell densities, TGC concentrations, temperatures and light intensity conditions. Approximately 90% of TGC could be removed when the TGC concentration was 10 mg∙L^-1 and the initial dry cell density was more than 0.2 g∙L^-1. A low value of TGC per g dry cell weight ratio led to a high removal efficiency of TGC. The initial dry cell density of microalgae was also critical for the removal of TGC. A high initial dry cell density is better than a low initial dry cell density to remove TGC when the ratio of the TGC concentration to dry cell weight are the same at the beginning of the cultivation. The removal mechanisms were investigated. Photolysis was a slow process that did not lead to removal at the beginning. Adsorption, hydrolysis, photolysis and biodegradation by microalgae were the main contributors to the removal of TGC. TGC was easily hydrolyzed under high -temperature conditions. Three transformation products of TGC by microalgae were identified. The stability of TGC was evaluated in water and salt solutions of citric acid, K₂HPO₄·3H₂O and ferric ammonium citrate. TGC was stable in ultrapure water and citric acid solution. TGC was hydrolyzed in K₂HPO₄·3H₂O and ferric ammonium citrate solutions.

Distinctive correlations between cell concentration and cell size to microalgae biomass under increasing carbon dioxide

Carbon capture and storage (CCS) via microalgae cultivations is getting renewed interest as climate change mitigation effort, owing to its excellent photosynthetic and CO₂ fixation capability. Microalgae growth is monitored based on their biomass, cell concentrations and cell sizes. The key parametric relationships on microalgae growth under CO₂ are absent in previous studies and this inadequacy hampers the design and scale-up of microalgae-based CCS. In this study, three representative microalgae species, Chlorella, Nostoc and Chlamydomonas, were investigated for establishing key correlations of cell concentrations and sizes towards their biomass fluctuations under CO₂ influences of 0% to 20% volume ratios (v/v). This revealed that Chlorella and Chlamydomonas cell concentrations significantly contributed towards increasing biomass concentration under CO₂ elevations. Chlorella and Nostoc cell sizes were enhanced at 20% (v/v). These findings provided new perspectives on growth responses under increasing CO₂ treatment, opening new avenues on CCS schemes engineering designs and biochemical production.

Effects of Different pH Control Strategies on Microalgae Cultivation and Nutrient Removal from Anaerobic Digestion Effluent

This study investigated nutrient removal from anaerobic digestion effluent by cultivating mixed-culture microalgae enriched from anaerobic sludge under different pH conditions: R_UC (uncontrolled), R_7–8 (maintained at 7–8), and R_<8 (maintained below 8). Significant amounts of NH₄⁺-N were lost by volatilization in R_UC cultures due to increased pH values (≤8.6) during the early period of cultivation. The pH control strategies significantly affected the biological NH₄⁺-N removal (highest in R_7–8), microalgal growth (highest in R_7–8), biomass settleability (highest in R_<8), and microalgal growth relative to bacteria (highest in R_<8) in the cultures. Parachlorella completely dominated the microalgal communities in the inoculum and all of the cultures, and grew well at highly acidic pH (<3) induced by culture acidification with microalgal growth. Microalgae-associated bacterial community structure developed very differently among the cultures. The findings call for more attention to the influence and control of pH changes during cultivation in microalgal treatment of anaerobic digestion effluent.

Screening and application of Chlorella strains on biosequestration of the power plant exhaust gas evolutions of biomass growth and accumulation of toxic agents

To use microalgae for the biosequestration of carbon dioxide (CO₂) emitted from the coal-fired power plants, the screening of high CO₂ tolerant microalgae and their accumulation of toxic agents have attracted significant research attention. This study evaluated 10 Chlorella strains for high CO₂ tolerance using combined growth rates and growth periods subjected to logistic parameters. We selected LAMB 31 with high r (0.89 ± 0.10 day^-1), high k (6.51 ± 0.19), and medium Tp (5.17 ± 0.15 day) as a candidate for CO₂ biosequestration. Correspondingly, six genes involving carbon fixation and metabolism processes were upregulated in LAMB 31 under high CO₂ conditions, verifying its high CO₂ tolerant ability. LAMB 31 cultures exposed to exhaust gas of power plant under different flow rates grew well, but the high flow rate (0.6 L/h) showed inhibition effects compared with low flow rates (0.2 and 0.3 L/h) at the end of the culturing period. The toxic agents in the exhaust gas including sulfur, arsenic, and mercury accumulated in LAMB 31 biomass but were deemed safe for use in the production of both human food and animal feed based on the National Food Safety Standard in China. This study showed a complete process involving high CO₂ tolerant microalgae screening, high CO₂ tolerant verification, and in situ application in a power plant. Data results provide valuable information as the basis for future research studies in microalgae application on CO₂ mitigation at emission sources.

Application of Microalgal Stress Responses in Industrial Microalgal Production Systems

Adaptive laboratory evolution (ALE) has been widely utilized as a tool for developing new biological and phenotypic functions to explore strain improvement for microalgal production. Specifically, ALE has been utilized to evolve strains to better adapt to defined conditions. It has become a new solution to improve the performance of strains in microalgae biotechnology. This review mainly summarizes the key results from recent microalgal ALE studies in industrial production. ALE designed for improving cell growth rate, product yield, environmental tolerance and wastewater treatment is discussed to exploit microalgae in various applications. Further development of ALE is proposed, to provide theoretical support for producing the high value-added products from microalgal production.

Cultivation and Biorefinery of Microalgae (Chlorella sp.) for Producing Biofuels and Other Byproducts: A Review

Microalgae-based carbon dioxide (CO2) biofixation and biorefinery are the most efficient methods of biological CO2 reduction and reutilization. The diversification and high-value byproducts of microalgal biomass, known as microalgae-based biorefinery, are considered the most promising platforms for the sustainable development of energy and the environment, in addition to the improvement and integration of microalgal cultivation, scale-up, harvest, and extraction technologies. In this review, the factors influencing CO2 biofixation by microalgae, including microalgal strains, flue gas, wastewater, light, pH, temperature, and microalgae cultivation systems are summarized. Moreover, the biorefinery of Chlorella biomass for producing biofuels and its byproducts, such as fine chemicals, feed additives, and high-value products, are also discussed. The technical and economic assessments (TEAs) and life cycle assessments (LCAs) are introduced to evaluate the sustainability of microalgae CO2 fixation technology. This review provides detailed insights on the adjusted factors of microalgal cultivation to establish sustainable biological CO2 fixation technology, and the diversified applications of microalgal biomass in biorefinery. The economic and environmental sustainability, and the limitations and needs of microalgal CO2 fixation, are discussed. Finally, future research directions are provided for CO2 reduction by microalgae.

Bio-removal of PtCl62- complex by Galdieria sulphuraria

Bio-removal of negative charged platinum complex is of great challenge owing to electrostatic repulsions between PtCl₆^2- and general extracellular polymeric substance (EPS) of microorganism. Galdieria sulphuraria (GS) are thermophilic and acidophilic microalga with specific metabolism, which subsequently lead to their unique cellular compositions such as EPS and phycocyanin, possibly providing a strategy to deal with negative charged metal complex. Accordingly, G. sulphuraria are employed to remove negative charged PtCl₆^2- complex with initial concentrations ranging from 0, 10, 20, 30, to 45 ppm. The growth rates of G. sulphuraria with microalgae named as GS-0, GS-10, GS-20, GS-30, and GS-45, respectively, and simultaneously bio-removal efficiencies of PtCl₆^2- are investigated. G. sulphuraria are independent to PtCl₆^2- within 0-30 ppm, while they are inhibited within 45 ppm of PtCl₆^2-. The PtCl₆^2- removal efficiencies of GS-10, GS-20, and GS-30 increase from 94.58%, 95.52%, to 95.92%, while decrease to 71.81% of GS-45. About 92.39%, 93.77%, 94.29%, and 75.21% of PtCl₆^2- adsorbed are accumulated within GS-10, GS-20, GS-30, GS-45, with few in EPS. The PtCl₆^2- complexes accumulated in EPS and algae cells are possibly decomposed to PtCl₄ according to the increasing zeta potentials of EPS and algae cells. The results indicate that PtCl₆^2- is efficiently removed by G. sulphuraria, achieving bio-removal of negative charged PtCl₆^2- complex from wastewater.

Adaptive Laboratory Evolution of Microalgae: A Review of the Regulation of Growth, Stress Resistance, Metabolic Processes, and Biodegradation of Pollutants

Adaptive laboratory evolution (ALE) experiments are a serviceable method for the industrial utilization of the microalgae, which can improve the phenotype, performance, and stability of microalgae to obtain strains containing beneficial mutations. In this article, we reviewed the research into the microalgae ALE test and assessed the improvement of microalgae growth, tolerance, metabolism, and substrate utilization by ALE. In addition, the principles of ALE and the key factors of experimental design, as well as the issues and drawbacks of the microalgae ALE method were discussed. In general, improving the efficiency of ALE and verifying the stability of ALE resulting strains are the primary problems that need to be solved in future research, making it a promising method for the application of microalgae biotechnology.

Research progress in bioremediation of petroleum pollution

With the enhancement of environmental protection awareness, research on the bioremediation of petroleum hydrocarbon environmental pollution has intensified. Bioremediation has received more attention due to its high efficiency, environmentally friendly by-products, and low cost compared with the commonly used physical and chemical restoration methods. In recent years, bacterium engineered by systems biology strategies have achieved biodegrading of many types of petroleum pollutants. Those successful cases show that systems biology has great potential in strengthening petroleum pollutant degradation bacterium and accelerating bioremediation. Systems biology represented by metabolic engineering, enzyme engineering, omics technology, etc., developed rapidly in the twentieth century. Optimizing the metabolic network of petroleum hydrocarbon degrading bacterium could achieve more concise and precise bioremediation by metabolic engineering strategies; biocatalysts with more stable and excellent catalytic activity could accelerate the process of biodegradation by enzyme engineering; omics technology not only could provide more optional components for constructions of engineered bacterium, but also could obtain the structure and composition of the microbial community in polluted environments. Comprehensive microbial community information lays a certain theoretical foundation for the construction of artificial mixed microbial communities for bioremediation of petroleum pollution. This article reviews the application of systems biology in the enforce of petroleum hydrocarbon degradation bacteria and the construction of a hybrid-microbial degradation system. Then the challenges encountered in the process and the application prospects of bioremediation are discussed. Finally, we provide certain guidance for the bioremediation of petroleum hydrocarbon-polluted environment.

CsubMADS1, a lag phase transcription factor, controls development of polar eukaryotic microalga Coccomyxa subellipsoidea C-169

MADS-box transcription factors (TFs) have not been functionally delineated in microalgae. In this study, the role of CsubMADS1 from microalga Coccomyxa subellipsoidea C-169 has been explored. Unlike Type II MADS-box proteins of seed plants with MADS, Intervening, K-box, and C domains, CsubMADS1 only has MADS and Intervening domains. It forms a group with MADS TFs from algae in the phylogenetic tree within the Type II MIKC^C clade. CsubMADS1 is expressed strongly in the lag phase of growth. The CsubMADS1 monomer does not have a specific localization in the nucleus, and it forms homodimers to localize exclusively in the nucleus. The monomer has two nuclear localization signals (NLSs): an N-terminal NLS and an internal NLS. The internal NLS is functional, and the homodimer requires two NLSs for specific nuclear localization. Overexpression (OX) of CsubMADS1 slows down the growth of the culture and leads to the creation of giant polyploid multinucleate cells, resembling autospore mother cells. This implies that the release of autospores from autospore mother cells may be delayed. Thus, in wild-type (WT) cells, CsubMADS1 may play a crucial role in slowing down growth during the lag phase. Due to starvation in 2-month-old colonies on solid media, the WT colonies produce mucilage, whereas OX colonies produce significantly less mucilage. Thus, CsubMADS1 also negatively regulates stress-induced mucilage production and probably plays a role in stress tolerance during the lag phase. Taken together, our results reveal that CsubMADS1 is a key TF involved in the development and stress tolerance of this polar microalga.

Recent Advances in Carbon Dioxide Conversion: A Circular Bioeconomy Perspective

Managing the concentration of atmospheric CO2 requires a multifaceted engineering strategy, which remains a highly challenging task. Reducing atmospheric CO2 (CO2R) by converting it to value-added chemicals in a carbon neutral footprint manner must be the ultimate goal. The latest progress in CO2R through either abiotic (artificial catalysts) or biotic (natural enzymes) processes is reviewed herein. Abiotic CO2R can be conducted in the aqueous phase that usually leads to the formation of a mixture of CO, formic acid, and hydrogen. By contrast, a wide spectrum of hydrocarbon species is often observed by abiotic CO2R in the gaseous phase. On the other hand, biotic CO2R is often conducted in the aqueous phase and a wide spectrum of value-added chemicals are obtained. Key to the success of the abiotic process is understanding the surface chemistry of catalysts, which significantly governs the reactivity and selectivity of CO2R. However, in biotic CO2R, operation conditions and reactor design are crucial to reaching a neutral carbon footprint. Future research needs to look toward neutral or even negative carbon footprint CO2R processes. Having a deep insight into the scientific and technological aspect of both abiotic and biotic CO2R would advance in designing efficient catalysts and microalgae farming systems. Integrating the abiotic and biotic CO2R such as microbial fuel cells further diversifies the spectrum of CO2R.

Chlorella vulgaris cultivation in simulated wastewater for the biomass production, nutrients removal and CO2 fixation simultaneously

Chlorella vulgaris (C. vulgaris) was promising microalgae to simultaneously achieve biomass production, carbon dioxide (CO₂) fixation, nutrients removal and proteins production especially under different conditions of CO₂ gas and wastewaters. Results presented that maximal specific growth rate of C. vulgaris was 0.21-0.35 d^-1 and 0.33-0.43 d^-1 at 0.038% and 10% CO₂ respectively, and corresponding maximal CO₂ fixation rate was attended with 4.51-14.26 and 56.26-85.72 mg CO₂·L^-1·d^-1. C. vulgaris showed good wastewater removal efficiency of nitrogen and phosphorus at 10% CO₂ with 96.12%-99.61% removal rates. Nitrogen fixation amount achieved 41.86 mg L^-1 when the initial NH₄Cl concentration was set at 60 mg L^-1 at 10% CO₂. Improved total protein (25.01-365.49 mg) and amino acids (24.56-196.44 mg) contents of C. vulgaris biomass was observed with the increasing of added CO₂ and ammonium concentrations. Moreover, the developed kinetic function of C. vulgaris growth depends on both phosphorus quota and nitrogen quota with correlation coefficient (R²) ranged from 0.68 to 0.97. Computed maximal consumed nutrients concentrations (ΔC_max) based on Logistic function are positively related to initial NH₄⁺-N concentrations, which indicated that adding ammonium could stimulate the utilization of both phosphorus and nitrogen.

Effects of three antibiotics on growth and antioxidant response of Chlorella pyrenoidosa and Anabaena cylindrica

Antibiotics are essential for treatments of bacterial infection and play important roles in the fields of aquaculture and animal husbandry. Antibiotics are accumulated in water and soil due to the excessive consumption and incomplete treatment of antibiotic wastewater. The accumulation of antibiotics in ecological systems leads to global environmental risks. The toxic effects of spiramycin (SPI), tigecycline (TGC), and amoxicillin (AMX) on Chlorella pyrenoidesa and Anabaena cylindrica were evaluated based on growth inhibition experiments, and determinations of ROS production and antioxidant enzyme activities (catalase, superoxide dismutase, and malondialdehyde). Half maximal effective concentrations (EC50) of TGC, SPI, and AMX for A. cylindrica were 62.52 μg/L, 38.40 μg/L, and 7.66 mg/L, respectively. Those were 6.20 mg/L, 4.58 mg/L, and > 2 g/L for C. pyrenoidesa, respectively. It was shown that A. cylindrica was much more sensitive to these antibiotics than C. pyrenoidesa. In addition, EC50 values of SPI and TGC were lower than that of AMX. It was indicated that SPI and TGC had higher toxic than AMX to C. pyrenoidesa and A. cylindrica. The current study is helpful to evaluating possible ecological risks of TGC, SPI, and AMX by green microalgae and cyanobacteria.

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.

Growth Performance and Antioxidative Response of Chlorella pyrenoidesa, Dunaliella salina, and Anabaena cylindrica to Four Kinds of Ionic Liquids

Ionic liquids are widely used for lipid and pigment extractions from microalgae. It is possible that ionic liquids are discharged into environments. The evaluation of growth performance and antioxidative response of ionic liquids to microalgae is helpful to explore the stress regulation mechanism and investigate possible environmental risk. Ionic liquids induce production of reactive oxygen species (ROS) to microalgae. These oxidative stresses are possible from cations, anions, and salinity. In this study, the growth inhibitions of [BMIM]Br, [BMIM]Cl, [EMIM]Cl, and [EMIM]EtOSO₃ to Anabaena cylindrica, Chlorella pyrenoidesa, and Dunaliella salina were evaluated. It was interesting that Br^- and two kinds of cations, [BMIM] and [EMIM], had significant effects on growth inhibitions of these microalgae. IC50 values of these ionic liquids for A. cylindrica, C. pyrenoidesa, and D. salina were also estimated based on the results of growth inhibitions. It was proved that [EMIM]Cl is relatively harmless to C. pyrenoidesa and D. salina, and [EMIM]EtOSO₃ is relatively or practically harmless to C. pyrenoidesa. [BMIM]Br and [BMIM]Cl are practically harmless to A. cylindrica and C. pyrenoidesa, and relatively harmless to D. salina. More than 0.8 g/L [EMIM]EtOSO₃ led to bleaching of both A. cylindrica and D. salina at 48 h which was shown that the anion, EtOSO₃^-, had higher inhibition to A. cylindrica and D. salina than Cl^-. In addition, high concentration of ionic liquids led to reductions of chlorophyll content in these three kinds of microalgae, increase of ROS levels and malondialdehyde contents for most of the cases. High concentration of ionic liquids also increased the activities of superoxide dismutase in three kinds of microalgae. There were positive correlations between ROS levels or MDA content, and inhibitions ratios of these ionic liquids to microalgae except [EMIM]Cl to A. cylindrica. These antioxidant enzymes were beneficial for reducing the ROS induced by ionic liquids.

Exploring the Role of a Cytokinin-Activating Enzyme LONELY GUY in Unicellular Microalga Chlorella variabilis

LONELY GUY has been previously characterized in flowering plants to be involved in the direct activation of cytokinins. In this study, the function of the only LONELY GUY gene (CvarLOG1) from unicellular green microalga Chlorella variabilis NC64A has been investigated. CvarLOG1 expressed mainly in the lag and log phases of growth and was confirmed to be a cytokinin-activating enzyme. Overexpression of CvarLOG1 in Chlorella led to extended life in culture by almost 10-20 days, creating a "stay-green" phenotype. In the transformed alga, the cell cycle was lengthened due to delayed entry into the G2/M phase contrary to the known role of cytokinins in stimulating G2/M transition possibly due to excessive levels of this hormone. However, due to the sustained growth and delayed senescence, there was an increase in cell number by 11% and in biomass by 46% at the stationary phase, indicating a potential application for the biofuel industry. The total carbohydrate and lipid yield increased by approximately 30 and 20%, respectively. RNA-Seq-based transcriptomic analysis revealed that the genes associated with light and dark reactions of photosynthesis were upregulated, which may be the reason for the increased biomass. These data show that LOG plays an essential role during the cell cycle and in the functioning of the chloroplast and that the pathway leading to direct activation of cytokinins via LOG is functional in algae.

Enhancing PUFA-rich polar lipids in Tisochrysis lutea using adaptive laboratory evolution (ALE) with oscillating thermal stress

Adaptive laboratory evolution is a powerful tool for microorganism improvement likely to produce enhanced microalgae better tailored to their industrial uses. In this work, 12 wild-type strains of Tisochrysis lutea were co-cultivated under increasing thermal stress for 6 months. Indeed, temperature was oscillating daily between a high and a low temperature, with increasing amplitude along the experiment. The goal was to enhance the polyunsaturated fatty acid content of the polar lipids. Samples were taken throughout the evolution experiment and cultivated in standardized conditions to analyze the evolution of the lipid profile. Genomic analysis of the final population shows that two strains survived. The lipid content doubled, impacting all lipid classes. The fatty acid analyses show a decrease in SFAs correlated with an increase in monounsaturated fatty acids (MUFAs), while changes in polyunsaturated fatty acid (PUFAs) vary between both photobioreactors. Hence, the proportion of C18-MUFAs (18:1 n-9) and most C18-PUFAs (18:2 n-6, 18:3 n-3, and 18:4 n-3) increased, suggesting their potential role in adjusting membrane fluidity to temperature shifts. Of particular interest, DHA in polar lipids tripled in the final population while the growth rate was not affected. KEY POINTS: • Adaptive laboratory evolution on a mix of 12 T. lutea strains led to survival of 2 • Thermal stress impacted cell size, total lipid cell content, and all lipid classes • DHA cell content partitioned to polar lipids tripled throughout the experiment.

Design and optimization of bioreactor to boost carbon dioxide assimilation in RuBisCo-equipped Escherichia coli

Global warming is a surging issue that has provoked the demand of green process to mitigate carbon dioxide. In this context, RuBisCo-equipped Escherichia coli has first developed and evaluated the CO₂-assimiliable capability based on the mass balance in three devices: Flask-based in CO₂ incubator (FIC), two-layered device (TLD) and CO₂ bubbling device (CBD) systematically. With the forced diffusion of 5% CO₂ in CBD, which confers an efficient attack of CO₂ to RuBisCo, the CO₂ assimilation increased from -5.03 to -2.63 g-CO₂/g-DCW. Furthermore, boosted CO₂ assimilation ability was observed by co-expression of GroELS chaperone with 71% reduction on CO₂ release. By DNA sequencing and tandem MS/MS analysis, the toxicity of RuBisCo and PRK was identified to interfere the sugar metabolism and energy producing, while the cell morphology was changed and observed in RuBisCo-equipped E. coli. Our study provides a new perspective of higher CO₂ assimilation for sustainable to eco-friendly green bioprocess.

Adaptive Evolution Improves Algal Strains for Environmental Remediation

Microalgae have potential for environmental remediation, but they must better tolerate stress. Adaptive laboratory evolution (ALE) is effective to construct evolved strains, but its efficiency is low. Highly efficient ALE relies on selecting suitable environmental stress, original strain selection, and optimizing initial cell density and stress strategy.

Harnessing the Power of Mutagenesis and Adaptive Laboratory Evolution for High Lipid Production by Oleaginous Microalgae and Yeasts

Oleaginous microalgae and yeasts represent promising candidates for large-scale production of lipids, which can be utilized for production of drop-in biofuels, nutraceuticals, pigments, and cosmetics. However, low lipid productivity and costly downstream processing continue to hamper the commercial deployment of oleaginous microorganisms. Strain improvement can play an essential role in the development of such industrial microorganisms by increasing lipid production and hence reducing production costs. The main means of strain improvement are random mutagenesis, adaptive laboratory evolution (ALE), and rational genetic engineering. Among these, random mutagenesis and ALE are straight forward, low-cost, and do not require thorough knowledge of the microorganism’s genetic composition. This paper reviews available mutagenesis and ALE techniques and screening methods to effectively select for oleaginous microalgae and yeasts with enhanced lipid yield and understand the alterations caused to metabolic pathways, which could subsequently serve as the basis for further targeted genetic engineering.

The regulating mechanisms of CO2 fixation and carbon allocations of two Chlorella sp. strains in response to high CO2 levels

The extreme high CO₂ in industrial exhaust gas cannot be tolerated by microalgae is the key challenge for the application of microalgae in CO₂ bio-sequestration. To provide better insights for this challenge, we chose one high CO₂ tolerant (Chlorella sp. LAMB 31) and non-tolerant (Chlorella sp. LAMB 122) Chlorella sp. to examine their different CO₂ fixation and carbon allocation responses to 40% CO₂. The results indicated LAMB 31 had a 24-h "lag phase" of biomass increase, during which the transition from PSII-PSI and the increase of lipid synthesis happened to acclimate high CO₂ conditions, followed by the increase of pigments synthesis, carbon fixation rates and polysaccharide productions. However, no acclimating mechanism was observed in LAMB 122, whose biomass, photosynthesis and material synthesis were all gradually collapsed under 40% CO₂. Finally, four parameters including Chl a, polysaccharides, carbon fixation rates and MDA were selected to be good physiological biomarkers for high CO₂ tolerant strains screenings in the future.

Exergy analysis for docosahexaenoic acid production by fermentation and strain improvement by adaptive laboratory evolution for Schizochytrium sp

Exergy analysis is powerful tool for process optimization and mechanism analysis. In this study, exergy analysis was performed for docosahexaenoic acid (DHA) fermentation process. More than 86% of input exergy was contributed by glucose. The exergy of biomass was about 64.66% of the total output exergy when the phosphate concentration was 4 g L^-1. The exergy efficiencies of DHA (η_DHA) for the starting strains and the evolved strains under high oxygen concentration, low temperature, and two-factor conditions were also investigated. The η_DHA in the collected experimental data was not more than 20.9%. It was proved that there was a positive correlation between η_DHA and the biomass yield. It was indicated that adaptive laboratory evolution (ALE) improved biomass yield which had the most important effect on enhancing η_DHA and DHA yield (or DHA productivity). It is necessary to improve η_DHA through process optimization and ALE in future.

Digital quantification and selection of high-lipid-producing microalgae through a lateral dielectrophoresis-based microfluidic platform

Microalgae are promising alternatives to petroleum as renewable biofuel sources, however not sufficiently economically competitive yet. Here, a label-free lateral dielectrophoresis-based microfluidic sorting platform that can digitally quantify and separate microalgae into six outlets based on the degree of their intracellular lipid content is presented. In this microfluidic system, the degree of cellular lateral displacement is inversely proportional to the intracellular lipid level, which was successfully demonstrated using Chlamydomonas reinhardtii cells. Using this functionality, a quick digital quantification of sub-populations that contain different intracellular lipid level in a given population was achieved. In addition, the degree of lateral displacement of microalgae could be readily controlled by simply changing the applied DEP voltage, where the level of gating in the intracellular lipid-based sorting decision could be easily adjusted. This allowed for selecting only a very small percentage of a given population that showed the highest degree of intracellular lipid content. In addition, this approach was utilized through an iterative selection process on natural and chemically mutated microalgal populations, successfully resulting in enrichment of high-lipid-accumulating microalgae. In summary, the developed platform can be exploited to quickly quantify microalgae lipid distribution in a given population in real-time and label-free, as well as to enrich a cell population with high-lipid-producing cells, or to select high-lipid-accumulating microalgal variants from a microalgal library.

Comprehensive approach to improving life-cycle CO2 reduction efficiency of microalgal biorefineries: A review

Along with the increase in global awareness of rising CO₂ levels, microalgae have attracted considerable interest as a promising CO₂ reduction platforms since they exhibit outstanding biomass productivity and are capable of producing numerous valuable products. At this moment, however, two major barriers, relatively low photosynthetic CO₂ fixation efficiency and necessity of carbon-intensive microalgal process, obstruct them to be practically utilized. This review suggests effective approaches to improve life-cycle CO₂ reduction of microalgal biorefinery. In order to enhance photosynthetic CO₂ fixation, strategies to augment carbon content and to increase biomass productivity should be considered. For reducing CO₂ emissions associated with the process operations, introduction of efficient process elements, designing of energy-saving process routes, reuse of waste resources and utilization of process integration can be noteworthy options. These comprehensive strategies will provide guidance for microalgal biorefineries to become a practical CO₂ reduction technology in near future.

Storage of starch and lipids in microalgae: Biosynthesis and manipulation by nutrients

Microalgae accumulate starch and lipid as storage metabolites under nutrient depletion, which can be used as sustainable feedstock for biorefinery. Omics analysis coupled with enzymatic and genetic verifications uncovered a partial picture of pathways and important enzymes or regulators related to starch and lipid biosynthesis as well as the carbon partitioning between them under nutrient depletion conditions. Depletion of macronutrients (N, P, and S) resulted in considerable enhancement of starch and/or lipid content in microalgae, but the accompanying declined photosynthesis hampered the achievements of high concentrations. This review summarized the current knowledge on the pathways and the committed steps as well as their carbon allocation involved in starch and lipid biosynthesis, and focused on the manipulation of different nutrients and the alleviation of oxidative stress for enhanced storage metabolites production. The biological and engineering approaches to cope with the conflict between biomass production and storage metabolites accumulation are proposed.

Modification and improvement of microalgae strains for strengthening CO2 fixation from coal-fired flue gas in power plants

Biological CO₂ capture using microalgae is a promising new method for reducing CO₂ emission of coal-fired flue gas. The strain of microalgae used in this process plays a vital role in determining the rate of CO₂ fixation and characteristics of biomass production. High requirements are put forward for algae strains due to high CO₂ concentration and diverse pollutants in flue gas. CO₂ can directly diffuse into the cytoplasm of cells by extra- and intracellular CO₂ osmotic pressure under high CO₂ concentrations. The flue gas pollutants, such as SO_x, NO_x and fly ashes, have negative effects on the growth of microalgae. This work reviewed the state-of-the-art advances on microalgae strains used for CO₂ fixation, focusing on the modification and improvement of strains that are used for coal-fired flue gas. Methods such as genetic engineering, random mutagenesis, and adaptive evolution have the potential to facilitate photosynthesis, improve growth rate and reduce CO₂ emission.

Effect of light quality on growth rate, carbohydrate accumulation, fatty acid profile and lutein biosynthesis of Chlorella sp. AE10

Microalgae are feedstocks for multiple product development based on algal biorefinery concept. The effects of light quality (white, red and blue light emitting diodes) and macro-element starvations on Chlorella sp. AE10 were investigated under 20% CO₂ and 850 µmol m^-2 d^-1. Nitrogen and phosphorus starvations had negative effects on its growth rate. The biomass productivities were decreased from day 1 and the highest one was 1.90 g L^-1 d^-1 under white light conditions. Phosphorus starvation promoted carbohydrate accumulation under three LED light sources conditions and the highest carbohydrate content was 75.9% using red light. Blue light increased lutein content to 9.58 mg g^-1. The content of saturated fatty acids was significantly increased from 37.51% under blue light and full culture medium conditions to 77.44% under blue light and nitrogen starvation conditions. Chlorella sp. AE10 was a good candidate for carbohydrate and lutein productions.

Kinetic model for effects of simulated flue gas onto growth profiles of Chlorella sp. AE10 and Chlorella sp. Cv

Microalgae are potential candidate for biofuel production as alternative one for fossil fuels. CO₂ in flue gas is available carbon source to support microalgae growth. In this study, the effects of different concentrations of the simulated flue gas onto algal growth and photosynthetic activity were evaluated for both Chlorella sp. AE10 and Chlorella sp. Cv. The growth profiles were correlated by a simple kinetic model. It was indicated that the simulated flue gas led to low pH and the photosynthetic activity was partially destroyed. Chlorella sp. Cv can tolerate full simulated flue gas, 10% CO₂  + 200 ppm NO_x  + 100 ppm SO_x . The pH in medium maintained at 6 and the photosynthetic activity was more than 0.6 at the first 6 days. If the concentration of NO_x was more 100 ppm and that of SO_x was more than 50 ppm, the pH was declined to 4 at day 2 or 3 for Chlorella sp. AE10. At the same time, the related photosynthetic activities of Chlorella sp. AE10 were less than 0.4, which was not suitable for algal growth. It was shown that Chlorella sp. Cv could be used for CO₂ fixation from the simulated flue gas.