Bio-fixation of flue gas from thermal power plants with algal biomass: Overview and research perspectives

Microalgae-mediated biofixation as an innovative technology for flue gases towards carbon neutrality: A comprehensive review

Microalgae-mediated industrial flue gas biofixation has been widely discussed as a clean alternative for greenhouse gas mitigation. Through photosynthetic processes, microalgae can fix carbon dioxide (CO2) and other compounds and can also be exploited to obtain high value-added products in a circular economy. One of the major limitations of this bioprocess is the high concentrations of CO2, sulfur oxides (SOx), and nitrogen oxides (NOx) in flue gases, according to the origin of the fuel, that can inhibit photosynthesis and reduce the process efficiency. To overcome these limitations, researchers have recently developed new technologies and enhanced process configurations, thereby increased productivity and CO2 removal rates. Overall, CO2 biofixation rates from flue gases by microalgae ranged from 72 mg L-1 d -1 to over 435 mg L-1 d-1, which were directly influenced by different factors, mainly the microalgae species and photobioreactor. Additionally, mixotrophic culture have shown potential in improving microalgae productivity. Progress in developing new reactor configurations, with pilot-scale implementations was observed, resulting in an increase in patents related to the subject and in the implementation of companies using combustion gases in microalgae culture. Advancements in microalgae-based green technologies for environmental impact mitigation have led to more efficient biotechnological processes and opened large-scale possibilities.

Carbon sequestration performance, enzyme and photosynthetic activity, and transcriptome analysis of algae-bacteria symbiotic system after antibiotic exposure

Wastewater treatment technology based on algae-bacteria successfully combines pollutant purification, CO2 reduction and clean energy production to provide new insights into climate solutions. In this study, the reciprocal mechanisms between algae and bacteria were explored through physiological and biochemical levels of algae cells and differentially expressed genes (DEGs) based on the performance of immobilized algae-bacteria symbiotic particles (ABSPs) for CO2 fixation. The results showed that ABSPs promoted the CO2 fixation capacity of microalgae. The enhanced growth capacity and photosynthetic activity of algal cells in ABSPs are key to promoting CO2 uptake, and the stimulation of photosynthetic system and the promotion of Calvin cycle were the main contributors to enhanced carbon sequestration. These findings will provide guidance for carbon reduction using immobilized ABSS as well as deciphering the algae-bacteria reciprocal mechanism.

The magic of algae-based biochar: advantages, preparation, and applications

Compared with other biomass sources, the use of algae as a raw material to prepare biochar (BC) has important advantages including safety, high yield and economy. The protein content of algae cells is as high as 3.2 mg DCW/L, and the graphitic-N and N-O functional groups generated by the pyrolysis of proteins could effectively activate free radicals. Combined with the generated pore structure, the electron transfer/exchange capability was enhanced, which is conducive to improving its catalytic performance. Algae as a natural N source, the manuscript analyzed the surface properties and physicochemical properties of algae-based BC, and investigated its degradation effect on organic/inorganic pollutants in wastewater. Subsequently, the effect of nitrogen-doped BC on the adsorption/catalysis capacity was discussed. Finally, the directed preparation of algae-based BC applied in different scenarios was summarized. Algae-based BC has the property of N doping, which broadens its application efficiency in the environmental field. Overall, this manuscript reviews how to achieve efficient utilization of algae-based BC in wastewater.

Design a novel internally illuminated mirror photobioreactor to improve microalgae production through homogeneous light distribution

In this study, a novel internally illuminated mirror photobioreactor (IIM-PBR) was designed to improve microalgae biomass production through providing a homogenous light distribution in cultivation medium. The performance of the IIM-PBR was compared with internally illuminated control photobioreactor (IIC-PBR) and externally illuminated control photobioreactor (EIC-PBR) in terms of cell growth, wastewater treatment and bioproducts generation. Compared with the IIC-PBR and EIC-PBR, the IIM-PBR increased microalgae growth rate up to 60 % and 30%, respectively. Municipal wastewater treatment revealed that the IIM-PBR could significantly improve nutrients removal as the final removal efficiencies of 90%, 95% and 90% were obtained for nitrate, phosphate and COD, respectively. Moreover, the IIM-PBR increased the total bioproducts production by 89% and 46% compared to in the IIC-PBR and EIC-PBR, respectively. Based on the energy consumption calculation, the mirror's light-reflective properties of the IIM-PBR resulted in a significant reduction of total energy consumption (∼10 times).

A critical review on phycoremediation of pollutants from wastewater-a novel algae-based secondary treatment with the opportunities of production of value-added products

Though the biological treatment employing bacterial strains has wide application in effluent treatment plant, it has got several limitations. Researches hence while looking for alternative biological organisms that can be used for secondary treatment came up with the idea of using microalgae. Since then, a large number of microalgal/cyanobacterial strains have been identified that can efficiently remove pollutants from wastewater. Some researchers also found out that the algal biomass not only acts as a carbon sink by taking up carbon dioxide from the atmosphere and giving oxygen but also is a renewable source of several value-added products that can be extracted from it for the commercial use. In this work, the cleaning effect of different species of microalgae/cyanobacteria on wastewater from varied sources along with the value-added products obtained from the algal biomass as observed by researchers during the past few years are reviewed. While a number of review works in the field of phycoremediation technology was reported in literature, a comprehensive study on phycoremediation of wastewater from different industries and household individually is limited. In the present review work, the efficiency of diverse microalgal/cyanobacterial strains in treatment of wide range of industrial effluents along with municipal wastewater having multi-pollutants has been critically reviewed.

Functional metabolism pathways of significantly regulated genes in Nannochloropsis oceanica with various nitrogen/phosphorus nutrients for CO2 fixation

To determine the optimal CO₂ concentration for microalgal biomass cultivated with industrial flue gas and improve carbon fixation capacity and biomass production. Functional metabolism pathways of significantly regulated genes in Nannochloropsis oceanica (N. oceanica) with various nitrogen/phosphorus (N/P) nutrients for CO₂ fixation were comprehensively clarified. At 100 % N/P nutrients, the optimum CO₂ concentration was 70 % and the maximum biomass production of microalgae was 1.57 g/L. The optimum CO₂ concentration was 50 % for N or P deficiency and 30 % for both N and P deficiency. The optimal combination of CO₂ concentration and N/P nutrients caused significant up regulation of proteins related to photosynthesis and cellular respiration in the microalgae, enhancing photosynthetic electron transfer efficiency and carbon metabolism. Microalgal cells with P deficiency and optimal CO₂ concentration expressed many phosphate transporter proteins to enhance P metabolism and N metabolism to maintain a high carbon fixation capacity. However, inappropriate combination of N/P nutrients and CO₂ concentrations caused more errors in DNA replication and protein synthesis, generating more lysosomes and phagosomes. This inhibited carbon fixation and biomass production in the microalgae with increased cell apoptosis.

Potential of biogenic and non-biogenic waste materials as flocculant for algal biomass harvesting: Mechanism, parameters, challenges and future prospects

In this review article, waste materials (biogenic/non-biogenic) are focused as the flocculants for harvesting of algal biomass. Chemical flocculants are widely utilized for the effective harvesting of algal biomass at a commercial scale while the high cost is a major drawback. The waste materials-based flocculants (WMBF) are started to utilize as one of the cost-effective performance for dual benefits of waste minimization and reuse for sustainable recovery of biomass. The novelty of the article is articulated with the objective that presents an insight of WMBF, classification of WMBF, preparation methods of WMBF, mechanisms of flocculation, factors affecting flocculation-mechanism, challenges and future recommendations that are required for harvesting of algae. The WMBF are shown similar flocculation mechanisms and flocculation efficiencies as chemical flocculants. Thus, the utilization of waste material for the flocculation process of algal cells minimizes the waste load into the environment and transforms the waste materials into valuable resources.

Simultaneous carbon dioxide sequestration and nitrate removal by Chlorella vulgaris and Pseudomonas sp. consortium

Carbon dioxide and nitrogen oxides are the main components of fossil flue gas causing the most serious environmental problems. Developing a sustainable and green method to treat carbon dioxide and nitrogen oxides of flue gas is still challenging. Here, a co-cultured microalgae/bacteria system, Chlorella vulgaris and Pseudomonas sp., was developed for simultaneous sequestration of CO₂ and removal of nitrogen oxides from flue gas, as well as producing valuable microalgae biomass. The co-cultured Chlorella vulgaris and Pseudomonas sp. showed the highest CO₂ fixation and NO₃^--N removal rate of 0.482 g L^-1d^-1 and 129.6 mg L^-1d^-1, the total chlorophyll accumulation rate of 65.6 mg L^-1 at the initial volume ratio of Chlorella vulgaris and Pseudomonas sp. as 1:10. The NO₃^--N removal rate can be increased to 183.5 mg L^-1d^-1 by continuous addition of 0.6 g L^-1d^-1 of glucose, which was 37% higher than that of co-culture system without the addition of glucose. Photosynthetic activity and carbonic anhydrase activity of Chlorella vulgaris were significantly increased when co-cultured with Pseudomonas sp. Excitation-emission matrix (EEM) fluorescence spectroscopy indicated that the humic acid-like substances released from Pseudomonas sp. could increase the growth of microalgae. This work provides an attractive way to simultaneously treatment of CO₂ and NO_X from flue gas to produce valuable microalgal biomass.

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.

Recent Developments on the Performance of Algal Bioreactors for CO2 Removal: Focusing on the Light Intensity and Photoperiods

This work presents recent developments of algal bioreactors used for CO₂ removal and the factors affecting the reactor performance. The main focus of the study is on light intensity and photoperiods. The role of algae in CO₂ removal, types of algal species used in bioreactors and conventional types of bioreactors including tubular bioreactor, vertical airlift reactor, bubble column reactor, flat panel or plate reactor, stirred tank reactor and specific type bioreactors such as hollow fibre membrane and disk photobioreactors etc. are discussed in details with respect to utilization of light. The effects of light intensity, light incident, photoinhibition, light provision arrangements and photoperiod on the performance of algal bioreactors for CO₂ removal are also discussed. Efficient operation of algal photobioreactors cannot be achieved without the improvement in the utilization of incident light intensity and photoperiods. The readers may find this article has a much broader significance as algae is not only limited to removal or sequestration of CO₂ but also it is used in a number of commercial applications including in energy (biofuel), nutritional and food sectors.

Industrial CO2 Capture by Algae: A Review and Recent Advances

The problem of global warming and the emission of greenhouse gases is already directly affecting the world’s energy. In the future, the impact of CO2 emissions on the world economy will constantly grow. In this paper, we review the available literature sources on the benefits of using algae cultivation for CO2 capture to decrease CO2 emission. CO2 emission accounts for about 77% of all greenhouse gases, and the calculation of greenhouse gas emissions is 56% of all CO2 imports. As a result of the study of various types of algae, it was concluded that Chlorella sp. is the best at capturing CO2. Various methods of cultivating microalgae were also considered and it was found that vertical tubular bioreactors are emerging. Moreover, for energy purposes, thermochemical methods for processing algae that absorb CO2 from flue gases were considered. Of all five types of thermochemical processes for producing synthesis gas, the most preferred method is the method of supercritical gasification of algae. In addition, attention is paid to the drying and flocculation of biofuels. Several different experiments were also reviewed on the use of flue gases through the cultivation of algae biomass. Based on this literature review, it can be concluded that microalgae are a third generation biofuel. With the absorption of greenhouse gases, the growth of microalgae cultures is accelerated. When a large mass of microalgae appears, it can be used for energy purposes. In the results, we present a plan for further studies of microalgae cultivation, a thermodynamic analysis of gasification and pyrolysis, and a comparison of the results with other biofuels and other algae cultures.

Microalgae as a solution of third world energy crisis for biofuels production from wastewater toward carbon neutrality: An updated review

The boost of the greenhouse gases (GHGs, largely carbon dioxide - CO₂) emissions owing to anthropogenic activity is one of the biggest global threats. Bio-CO₂ emission reduction has received more and more attention as an environmentally sustainable approach. Microalgae are very popular in this regard because of excellent speed of growth, low costs of production, and resistance to extreme environments. Besides, most microalgae can undergo photosynthesis, where the CO₂ and solar energy can be converted into sugar, and subsequently become biomass, providing a renewable and promising biofuel strategy with a few outstanding benefits. This review focuses on presenting CO₂ sequestration by microalgae towards wastewater treatment and biodiesel production. First, the CO₂ fixation mechanism by microalgae viz., sequestration and assimilation of CO₂ in green microalgae as well as cyanobacteria were introduced. Besides, factors affecting CO₂ sequestration in microalgae, containing microalgae species and cultivation conditions, such as light condition, photobioreactor, configuration, pH, CO₂ concentration, temperature, and medium composition, were then comprehensively discussed. Special attention was given to the production of biodiesel as third-generation biofuel from various wastewater (CO₂ biofixation), including processing steps of biodiesel production by microalgae, biodiesel production from wastewater, and improved methods. Furthermore, current life cycle assessment (LCA) and techno-economic analysis (TEA) used in biodiesel production were discussed. Finally, the research challenges and specific prospects were considered. Taken together, this review provides useful and updated information to facilitate the development of microalgal "green chemistry" and "environmental sustainability".

Utilization of Chlorella pyrenoidosa for Remediation of Common Effluent Treatment Plant Wastewater in Coupling with Co-relational Study: An Experimental Approach

Earlier investigations on biological methods of wastewater treatment have revealed that algal based wastewater treatment could be a green, cost effective and efficient approach for the removal of heavy metals. So, this study aimed to assess the potential of microalga Chlorella pyrenoidosa for remediation of heavy metals (Cr, Cu, Pb, Zn, Cd, Mn, and Ni) from varying concentration (25%, 50%, 75 and 100%) of wastewater collected from Common Effluent Treatment Plant. Heavy metals such as Cr, Cu, Pb, Zn, Cd, Mn, and Ni have been removed significantly from the wastewater, with percentage removal ranging from 73%, 60%, 75%, 66%, 87%, 83%, and 74% with 50% test solution, 57%, 59%, 70%, 56%, 72%, 66%, and 62% with 75% test solution, and 47%, 55%, 56%, 71%, 61%, 77%, and 72% with 100% test solution respectively. Studies on biochemical assay (protein, carbohydrate, and pigment) of Chlorella pyrenoidosa were also an important part of the present investigation to understand the interaction of heavy metals with algal biochemical compounds using Pearson correlation co-efficient. Biomass grown in CETP wastewater can be used for synthesis of various fruitful value-added end products like bio-diesel, pharmaceutical products, cosmetic products, bio-adsorbent etc.

Cultivation of two Chlorella species in Open sewage contaminated channel wastewater for biomass and biochemical profiles: Comparative lab-scale approach

Open sewage contaminated channel wastewater (OSCCW) has high pollutant loads, responsible for eutrophication, when mixed with various channels of urban communities. But, these pollutants can be converted and recovered into useful end products with the help of algal species. In this study, two species of Chlorella (C. vulgaris and C. pyrenoidosa) were selected and investigated for the production of algal biomass and nutrient removal efficiencies with 50% concentration of OSCCW, in a comparative way at lab-scale. Chlorella sp. cultivated in OSCCW have removed nitrate (76.9-78.8%) and phosphate (67.6-79.7%) whereas COD (72.4-76.2%) and BOD (62.3-72.4%) respectively. Correlation analysis was investigated between physico-chemical parameters and biochemical profile of both species to analyze the positive and negative correlation between two variables. The bio-chemical profile and biomass productivity of both species of Chlorella were observed well on the basis of productivity of biomass (60.1, 56.5 mg/l/d), carbohydrate (15.71, 8.82 mg/l/d), protein (11.21, 15.82 mg/l/d), lipid (20.8, 17.5 mg/l/d) and chlorophyll (0.78, 0.67 mg/l/d) in OSCCW. The maximum lipid content (34.6%) was obtained with C. pyrenoidosa as compared to C. vulgaris. Findings also support that OSCCW is well-off with nutrient resources, which can be suitable alternative for algal biomass production and remediated wastewater can be used for animal and fish farming type activities.

Reuniting the Biogeochemistry of Algae for a Low-Carbon Circular Bioeconomy

Given their advantages of high photosynthetic efficiency and non-competition with land-based crops, algae, that are carbon-hungry and sunlight-driven microbial factories, are a promising solution to resolve energy crisis, food security, and pollution problems. The ability to recycle nutrient and CO₂ fixation from waste sources makes algae a valuable feedstock for biofuels, food and feeds, biochemicals, and biomaterials. Innovative technologies such as the bicarbonate-based integrated carbon capture and algae production system (BICCAPS), integrated algal bioenergy carbon capture and storage (BECCS), as well as ocean macroalgal afforestation (OMA), can be used to realize a low-carbon algal bioeconomy. We review how algae can be applied in the framework of integrated low-carbon circular bioeconomy models, focusing on sustainable biofuels, low-carbon feedstocks, carbon capture, and advances in algal biotechnology.

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.

Key internal factors leading to the variability in CO2 fixation efficiency of different sulfur-oxidizing bacteria during autotrophic cultivation

Variability in the apparent CO₂ fixation yield of four aerobic sulfur-oxidizing bacteria (Halothiobacillus neapolitanus DSM 15147, Thiobacillus thioparus DSM 505, Thiomonas intermedia DSM 18155, and Starkeya novella DSM 506) in autotrophic culturing was studied, and mutual effects of key intrinsic factors on CO₂ fixation were explored. DSM 15147 and DSM 505 exhibited much higher CO₂ fixation yields than DSM 18155 and DSM 506. The differences in CO₂ fixation yield were determined not only by cbb gene transcription, but also by cell synthesis rate, which was determined by rRNA gene copy number; the rRNA gene copy number had a more significant effect than cbb gene transcription on the apparent CO₂ fixation yield. Moreover, accumulation of EDOC was observed in all four strains during chemoautotrophic cultivation, and the proportion of EDOC accounting for total fixed organic carbon (TOC; EDOC/TOC ratio) was much higher in DSM 18155 and DSM 506 than in DSM 15147 and DSM 505. The accumulation of EDOC led to a significant decrease in the cbb gene transcription efficiency during cultivation, and a further feedback inhibitory effect on CO₂ fixation.

Abiotic–Biological Hybrid Systems for CO2 Conversion to Value-Added Chemicals and Fuels

Abiotic–biological hybrid systems that combine the advantages of abiotic catalysis and biotransformation for the conversion of carbon dioxide (CO2) to value-added chemicals and fuels have emerged as an appealing way to address the global energy and environmental crisis caused by increased CO2 emission. We illustrate the recent progress in this field. Here, we first review the natural CO2 fixation pathways for an in-depth understanding of the biological CO2 transformation strategy and why a sustainable feed of reducing power is important. Second, we review the recent progress in the construction of abiotic–biological hybrid systems for CO2 transformation from two aspects: (i) microbial electrosynthesis systems that utilize electricity to support whole-cell biological CO2 conversion to products of interest and (ii) photosynthetic semiconductor biohybrid systems that integrate semiconductor nanomaterials with CO2-fixing microorganisms to harness solar energy for biological CO2 transformation. Lastly, we discuss potential approaches for further improvement of abiotic–biological hybrid systems.

Production of algal biomass for its biochemical profile using slaughterhouse wastewater for treatment under axenic conditions

Slaughterhouse produce large amount of wastewater, containing high pollutant load in terms of protein, fats and meat pieces, might lead to source of non-point contamination. Various concentrations (25%, 50%, 75%, and 100%) of slaughterhouse wastewater were used to increase the algal biomass production, pollutants removal and biochemical profile analysis under controlled conditions of C. pyrenoidosa. Results showed that the maximum biomass yield 430 mg L-1 was achieved at 50% concentration of wastewater to other concentration of wastewater. Direct relation was observed in between pollution load and nutrient load of SHWW with biochemical profile of C. pyrenoidosa. The COD/BOD ratio (1.9) was found to be significant on the scale of degradability by algal biomass. Sufficient nutrient removal efficiencies (23-42%, 18-48%) and pollutant load efficiencies (17-31%, 7-29%) were observed. Findings showed that slaughterhouse wastewater is rich in nutrients, which can be utilized for algal biomass production and wastewater remediation for future endeavors.

Capsules of Reactive Ionic Liquids for Selective Capture of Carbon Dioxide at Low Concentrations

The task-specific ionic liquid (IL), 1-ethyl-3-methylimidazolium 2-cyanopyrolide ([EMIM][2-CNpyr]), was encapsulated with polyurea (PU) and graphene oxide (GO) sheets via a one-pot Pickering emulsion, and these capsules were used to scrub CO₂ (0-5000 ppm) from moist air. Up to 60 wt % of IL was achieved in the synthesized capsules, and we demonstrated comparable gravimetric CO₂ capacities to zeolites and enhanced absorption rates compared to those of bulk IL due to the increased gas/liquid surface-to-volume area. The reactive IL capsules show recyclability upon mild temperature increase compared to zeolites that are the conventional absorber materials for CO₂ scrubbing. The measured breakthrough curves in a fixed bed under 100% relative humidity establish the utility of reactive IL capsules as moisture-stable scrubber materials to separate CO₂ from air, outperforming zeolites owing to their higher selectivity. It is shown that thermal stability, CO₂ absorption capacity, and rate of uptake by IL capsules can be further modulated by incorporating low-viscosity and nonreactive ILs to the capsule core. This study demonstrates an alternative and facile approach for CO₂ scrubbing, where separation from gas mixtures with extremely low partial pressures of CO₂ is required.

Enhanced biomass productivity of Arthrospira platensis using zeolitic imidazolate framework-8 as carbon dioxide adsorbents

In order to improve CO₂ diffusion in algae solution and conversion into dissolved HCO₃^-, zeolitic imidazolate framework-8 (ZIF-8) with zinc cores as unsaturated metal sites was first used as CO₂ adsorbents. Flue gas CO₂ from coal-chemical industry can be adsorbed and can be made available throughout cultivation to promote biomass productivity of Arthrospira platensis. The ZIF-8 adsorbent with particle size of 719 nm performed the largest pore area of 351.8 m²/g, which promoted CO₂ conversion into HCO₃^- by 72.9% compared to control condition without ZIF-8. The increased HCO₃^- concentration enhanced thylakoid membrane proportion in cell cross-sectional area by 1.3 times to 78.3%, which resulted in enhancement of photosynthesis in A. platensis cells. Relative electron transport rate increased by 9.4% accordingly, which was attributed to the improvement of chlorophyll a concentration by 110%. The biomass productivity using ZIF-8 adsorbent with particle size of 719 nm markedly increased by 64.0%.

Biological removal of nitrogen oxides by microalgae, a promising strategy from nitrogen oxides to protein production

Nitrogen oxides (NO_x) are the components of fossil flue gases that give rise to serious environmental and health hazards. Among the available techniques for NO_x removal, microalgae-based biological removal of NO_x (BioDeNO_x) is a promising and competent technology with eco-friendly path of low energy and low-cost solution for the pollution. In this review article, current biological technologies including bacteria-based and microalgae-related BioDeNO_x are discussed. Comparing to direct BioDeNO_x approach, indirect BioDeNO_x by microalgae is more promising since it is more stable, reliable and efficient. By transforming inorganic nitrogen nutrients to organic nitrogen, microalgae can potentially play an important role in converting NO_x into high-value added products. The microalgae-based BioDeNO_x process displays an attractive prospect for flue gas treatment to reduce environmental NO_x pollution and potentially supply protein products, establishing an efficient circular-economy strategy.

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.