CO2 Bio-sequestration Research Articles

Novel paradigm of light and CO2 feeding to improve the ‎ co-production of ‎astaxanthin and polyunsaturated fatty acids in Haematococcus ‎pluvialis

This study aimed to develop an innovative paradigm of lighting and CO2 flow rate in dilution and photoinduction stages of H. pluvialis cultivation to overcome the cell number reduction and stimulate the astaxanthin and polyunsaturated fatty acids accumulation joined by carbon bio-sequestration. The appropriate paradigm of lighting to reach the maximum cell number, astaxanthin concentration, and astaxanthin content in the dilution and photoinduction stages was obtained 2-fold (300 µmol/m2/s) and 6-fold (900 µmol/m2/s) of the final light intensity in the phototrophy stage (150 µmol/m2/s), respectively. Under the inventive lighting paradigm, feeding the 225 mL/L/min CO2 recorded optimum consequences in the dilution and photoinduction stages. The cell number, astaxanthin productivity, lipid content, polyunsaturated fatty acid and CO2 bio-fixation rate at the end of the photoinduction stages by lighting/CO2 balance were 94 %, 96 %, 32 %, 61 % and 40 % higher than control, respectively. Additionally, the levels of omega-6 fatty acids and omega-3 fatty acids were 70 % and 73 % higher than the control, respectively. This substantial increase was achieved by maintaining an optimal ratio of omega-6 to omega-3 fatty acids (3.05). Furthermore, to maximize the cell growth of microalgae and enhance the accumulation of valuable metabolites through CO2 bio sequestration, it is recommended to focus on the CO2 flow rate parameter instead of the CO2 percentage. That is affected by both the CO2 percentage and the aeration rate. Our findings provide a promising cost-effective and ecofriendly approach to improve the co-production of astaxanthin and polyunsaturated fatty acids in H. pluvialis.

Agriculture models for restoring degraded land to enhance CO2 biosequestration and carbon credits in the Vindhyan region of India

The study's objective was to evaluate the status of converted degraded land into productive agricultural models by improving the physicochemical properties of the soil, soil organic matter (SOM), soil organic carbon (SOC) fractions (active and passive), and microbial biomass carbon (MBC), while also generating carbon (C) credit for additional farmers' income. Six models were analyzed, namely: (1) Arjun forest-based agroecosystems (AFBAE); (2) Lemon grass-based agroecosystems (LGBAE); (3) Legume-cereal-moong-based agroecosystems (LCMBAE); (4) Bael-black mustard-based agroecosystems (BMBAE); (5) Guava-wheat-based agroecosystems (GWBAE), and (6) Custard apple -lentil -based agroecosystems (CALBAE). These models were replicated three times in a randomized block design (RBD). Soil samples were collected from the study area at two depths (0–0.30 and 0.30–0.60 m). At a 0–0.30 m depth, the highest bulk density (ρb) of 1.50 Mg m−3 was observed in LCMBAE, while the lowest ρb of 1.43 Mg m−3 was recorded in BMBAE. The soil organic carbon (SOC) and SOC stock values exhibited a range of 4.2–7.7 g kg−1 and 19.0–33.4 Mg ha−1, respectively. In the AFBAE, the highest levels of 163.1 % MBC were found over LCMBAE. At a 0–0.30 m depth, the recalcitrant index (RI) and lability index (LI) ranged from 0.35–0.46 to 1.97–2.11, respectively. Additionally, the AFBAE exhibited the highest total biomass accumulation (39.23 Mg ha−1), carbon dioxide (CO2) biosequestration (287.9 Mg ha−1), and the total social cost of CO2 at US$ 277 ha−1. Furthermore, in the AFBAE, there was a 198.1 % increase in total C credit (US$ 161 ha−1) compared to LCMBAE (US$ 54 ha−1). However, at 0.30–0.60 m depths, GWBAE and CALBAE were statistically equivalent (p ≤ 0.05) in total C stocks. Principal component analysis (PCA) reveals that component-1 accounts for 77.4 % of the variability, while component-2 contributes 18.6 %. This article aimed to convert the degraded land into a sustainable agricultural module by increasing SOC and CO2 biosequestration and producing more C-credit, or climate currency, on underutilized land.

Microalgae cultivation: from CO2 fixation to single-cell protein production

Bio-sequestration of CO2 using microalgae to recycle CO2 into valuable products such as single-cell proteins (SCP) is one of the most promising fields nowadays. Microalgae are able to use CO2 as their carbon source and subsequently build carbohydrates, proteins, nucleic acids, and lipids. Nevertheless, all microalgae strains do not have the same CO2 tolerance and culture conditions. Moreover, pure CO2 is less soluble in water, which leads to a low carbon capture and fixation rate. Thus, to optimise SCP production in relation to CO2 mitigation, studies of the enhancement of solubilization of CO2 in water as well as the determination of the optimum process parameter for CO2 fixation and SCP production need to be done. Consequently, this study aims to review the cultivation conditions for single-cell protein production.

Application study for the high CO2 tolerant Chlorella strain by flue gas culture: Evaluation of growth performance and adaptive mechanisms

The application of microalgae for CO2 biosequestration of flue gas emissions from coal-fired power plants has attracted widespread attention. In this study, one high CO2 tolerant Chlorella strain (LAMB 31) screened by previous experiments was cultured with flue gas of a power plant, whose multiple growth performance and the adaptive mechanisms under the flue gas were investigated. The results indicated LAMB 31 grew well under the flue gas with a specific growth rate of 0.15/d and a maximum carbon fixation rate of 0.24 g/L/d. Correspondingly, LAMB 31 increased intracellular chlorophyll and fatty acid content (especially unsaturated fatty acids), while relatively low intracellular arsenic and mercury were accumulated, showing advantages of LAMB 31 for further food/feed and biodiesel production. Transcriptional results showed that enhancing carbon turnover and storage processes were the major mechanism for LAMB 31 adapting flue gas culture. Furthermore, inhibited uptake and enhanced detoxification of heavy mental were also found, verifying that LAMB 31 could effectively control the intracellular heavy mental accumulations. This study comprehensively evaluated the growth performance, elucidated possible adaptive strategies of Chlorella sp. responding to the flue gas, and provided further directions for the subsequent applications of LAMB 31 biomass.

Industrial garbage-derived biocompost enhances soil organic carbon fractions, CO2 biosequestration, potential carbon credits and sustainability index in a rice-wheat ecosystem

The objectives of this experiment were i) to study on the garbage composting to improve the soil organic carbon (SOC) pools (active and passive), ii) work out the carbon (C) budgeting, and iii) cut off C footprints (CFs) in the rice (Oryza sativa L.)-wheat (Triticum aestivum L.) farming to achieve the long-term sustainability. The main plots show four fertilizer levels (F0 = control, F1 = 112.5:45:45 kg nitrogen; phosphorus; potassium (NPK) ha−1, F2 = 150:60:60 kg NPK ha−1 and F3 = 150:60:60 kg NPK ha−1+ 5 kg iron (Fe) + 5 kg zinc (Zn) were applied, while in sub plots with the combination of three industrial garbage (I1 = carpet garbage; I2 = pressmud; I3 = bagasse) and three microbial culture (M1 = Pleurotus sajor-caju, M2 = Azotobacter chroococcum; M3 = Trichoderma viride) made into nine treatment combinations were applied. Based on the interaction, treatment F3 × I1+M3 resulted in a maximum of 25.1 and 22.4 Mg ha−1 total CO2 biosequestration by rice and wheat, respectively. However, it was cut off CFs by 29.9 and 22.2% more than F1 × I3+M1. Based on the soil C fractionation study, in the main plot treatment, F3 was active very labile C (VLC) and moderately labile C (MLC) and passive less labile C (LLC) and recalcitrant C (RC) SOC fractions contributed by 68.3 and 30.0%, respectively, of total SOC. However, in the sub plot, treatment I1+M3 found 68.2% and 29.8% active and passive SOC fractions, respectively, of total SOC. Regarding the soil microbial biomass C (SMBC) study, F3 had 37.7% higher than F0. However, in the sub plot, I1+M3 was seen to be 21.5% greater than I2+M1. Furthermore, wheat and rice had higher 1002 and 897 US$ ha−1 potential C credit in F3 × I1+M3, respectively. SOC fractions were perfectly positively correlated with SMBC. A positive (+) correlation was observed among grain yield (wheat and rice) and SOC pools in soil. However, a negative correlation was found between the C sustainability index (CSI) and greenhouse gas intensity (GHGI). The variability in wheat and rice grain yield was 46 and 74%, respectively, contributed by the SOC pools. Therefore, this study hypothesised that applying inorganic nutrients and industrial garbage converted into biocompost cut off C emissions and reduced the demand for chemical fertilizers, opening garbage disposal, and simultaneously enhancing the SOC pools.

The Possibility of Deploying CO2 from Biogas Combustion to Improve the Productivity of a Periodical Chlorella vulgaris Culture

Carbon dioxide (CO2) is the major contributor to the global emissions of greenhouse gases, which necessitates the search for its fixation and utilization methods. Engaging photosynthesizing microorganisms for its biosequestration is one of the prospective technologies applied to this end. Considering the paucity of literature works on the possibilities of deploying CO2 from biogas combustion to intensify microalgae production, this research aimed to identify the feasibility of using this type of CO2 in Chlorella vulgaris culture by evaluating biomass production yield and CO2 biosequestration effectiveness. The experiment was performed in glass PBR, in which the culture medium occupied the volume of 1.0 dm3, and the gaseous phase occupied 0.3 dm3. The reactors were continuously illuminated by fluorescent lamps. The temperature of flue gases and air fed to reactors, and culture temperature was 20 °C ± 2 °C. The use of flue gases promoted a more rapid biomass growth, reaching 77.8 ± 3.1 mgVS/dm3⋅d, and produced a higher microalgae concentration, i.e., 780 ± 58 mgVS/dm3. Nevertheless, the flue gas-fed culture turned out to be highly sensitive, which was manifested in a decreased culture medium pH and relatively quickly achieved decay phase of the C. vulgaris population. The microalgae effectively assimilated CO2, reducing its concentration from 13 ± 1% to 1 ± 0.5% in the effluent from the photobioreactor. The flue gases were found not to affect the qualitative composition of the microalgal biomass. However, strict control and monitoring of microalgae biomass production is necessary, as well as rapid responses in flue gas-fed systems. This is an important hint for potential operators of such technological systems on the large scale. Regardless of the possibility of deploying microalgae to fix and utilize CO2, a justified avenue of research is to look for cheap sources of CO2-rich gases.

Bioremediation strategies of palm oil mill effluent and landfill leachate using microalgae cultivation: An approach contributing towards environmental sustainability

Palm oil mill effluent (POME) is defined as the wastewater that contains high concentrations of organics, nutrients and oil and grease generated from the production process of palm oil. Therefore, proper discharge and management of POME is important to avoid deleterious impact on the environment. In fact, solid waste generation is a precursor for its disposal issues as most of the solid waste generated in developing nations is dumped into landfills. This has led to the threat posed by the generation of landfill leachate (LL). LL is a complex dark coloured liquid consisting of organic matter, inorganic substances, trace elements and xenobiotics. Hence, it is essential to effectively treat the landfill leachate before discharging it to avoid contamination of soil, surface & groundwater bodies. Conventional treatment methods comprises of physical, biological and chemical treatment, however, microalgal-based treatment could also be incorporated. Furthermore, with the benefits offered by microalgae in valorisation, the application of microalgae in POME and leachate treatment as well as biofuel production, is considerably viable. This paper provides an acumen of the microalgae-based treatment of POME and LL, integrated with biofuel production in a systematic and critical manner. The pollutants assimilation from wastewater and CO2 biosequestration are discussed for environmental protection. Cultivation systems for wastewater treatment with simultaneous biomass production and its valorisation, are summarised. The study aims to provide insight to industrial stakeholders on economically viable and environmentally sustainable treatment of wastewaters using microalgae, and eventually contributing to the circular bioeconomy and environmental sustainability.

Energy metabolism and intracellular pH regulation reveal different physiological acclimation mechanisms of Chlorella strains to high concentrations of CO2

The intolerance of high CO2 in the exhaust gas is the “bottleneck” limiting the wide application of microalgae for CO2 biosequestration. Around this topic, we selected high-CO2-tolerant (LAMB 33 and 31) and nontolerant (LAMB 122) Chlorella strains to study their different energy metabolisms and cytoplasmic pH regulations in response to high CO2. Under 40 % CO2, LAMB 33 and 31 both showed elevated ATP synthesis, accelerated ATP consumption and fast cytoplasmic pH regulation while exhibiting different acclimating strategies therein: chloroplast acclimations were reflected by high chlorophyll contents in 33 but photosystem transitions in 31; faster mitochondrial acclimations occurred in 33 than in 31; cellular organic carbon mainly flowed to monosaccharide synthesis for 33 but to monosaccharide and protein synthesis for 31; and cytoplasmic pH regulation was attributed to V-ATPase in 31 but not in 33. All the above metabolic processes gradually collapsed in 122, leading to growth inhibition. Our study identified different metabolic acclimation strategies among Chlorella strains to high CO2 and provided new traits for breeding microalgae for CO2 biosequestration.

Exploring indigenous freshwater chlorophytes in integrated biophotovoltaic system for simultaneous wastewater treatment, heavy metal biosorption, CO2 biofixation and biodiesel generation

The study explored the combined photosynthetic activities of two green microalgal species, Tetradesmus obliquus and Tetradesmus reginae, on an integrated biophotovoltaic (BPV) platform for simultaneous wastewater treatment, toxic metal biosorption, carbon biofixation, bioelectricity generation and biodiesel production. The experimental setup comprised of a dual-chambered BPV with copper anode surrounded by T. obliquus in BG11 media, and copper cathode with T. reginae in municipal wastewater separated by Nafion 117 membrane. The study reported a maximum power density of 0.344 Wm−2 at a cell potential of 0.415 V with external resistance of 1000 Ω and 0.3268 V maximum open-circuit voltage. The wastewater electrical conductivity and pH increased from 583 ± 22 to 2035 ± 29.31 mS/cm and 7.403 ± 0.174 to 8.263 ± 0.055 respectively, signifying increased photosynthetic and electrochemical activities. Residual nitrogen, phosphorus, chemical oxygen demand, arsenic, cadmium, chromium and lead removal efficiencies by T. reginae were 100%, 80.68%, 71.91%, 47.6%, 88.82%, 71.24% and 92.96%, respectively. T. reginae accumulated maximum biomass of 0.605 ± 0.033 g/L with a CO2 biosequestration rate of 0.166 ± 0.010 gCO2/L/day and 42.40 ± 1.166% lipid content. Methyl palmitate, methyl undecanoate and 13-octadecenoic acid with relative abundances of 37.24%, 24.80% and 12.02%, respectively were confirmed.

Optimization of the Growth and Performance of Several Cynobacteria Species in a Pilot Scale Raceway Pond for CO2 Bio-Sequestration

Due to the limited availability of fresh water and the high cost of land for plant culture, microalgae cultivation has attracted significant attention in recent years and has been shown to be the best option for CO2 bio-sequestration. Bio-sequestration of CO2 through algae bioreactors has been hailed as one of the most promising and ecologically benign methods available. This research study was taken up to alleviate certain limitations associated with the technology such as low CO2 sequestration efficiency and low biomass yields. In this study three distinct cyanobacterial strains, Chlorella sp., Synechococcus sp., and Spirulina sp., were tested in 10 litre raceway ponds for their capacity for CO2 bioconversion and high biomass production under various CO2 concentrations at different EC. The highest growth rate of all tested cyanobacterial strains was observed during the first 4 days of cultivation under CO2 5% to 10%. Additionally, all these cyanobacterial strains were explored for their bioremediation capabilities. The results showed that the Chlorella sp., Synechococcus sp., and Spirulina sp. were able to remove COD of the wastewater by 56%, 48% and 77% respectively and the BOD removal efficiency was 48%, 30% and 52% respectively. The primary results indicated that the Spirulina sp. was to be the best cynobacteria studied in terms of biomass production, CO2 bioconversion, and bioremediation capacities. Therefore, the Spirulina sp. was further scaled up in 1500 litre raceway pond for CO2 bio-sequestration and biomass production. The biomass collected was utilised to extract biomolecules such as protein, carbohydrate and lipids.

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.

Optimization of Bio-Foamed Concrete Brick Strength via Bacteria Based Self-Healing and Bio-Sequestration of CO2.

This research aimed to optimize the compressive strength of bio-foamed concrete brick (B-FCB) via a combination of the natural sequestration of CO2 and the bio-reaction of B. tequilensis enzymes. The experiments were guided by two optimization methods, namely, 2k factorial and response surface methodology (RSM). The 2k factorial analysis was carried out to screen the important factors; then, RSM analysis was performed to optimize the compressive strength of B-FCB. Four factors, namely, density (D), B. tequilensis concentration (B), temperature (T), and CO2 concentration, were selectively varied during the study. The optimum compressive strength of B-FCB was 8.22 MPa, as deduced from the following conditions: 10% CO2, 3 × 107 cell/mL of B, 27 °C of T and 1800 kg/m3 of D after 28 days. The use of B. tequilensis in B-FCB improved the compressive strength by 35.5% compared to the foamed concrete brick (FCB) after 28 days. A microstructure analysis by scanning electronic microscopy (SEM), energy dispersive X-ray (EDX) and X-ray diffraction analysis (XRD) reflected the changes in chemical element levels and calcium carbonate (CaCO3) precipitation in the B-FCB pores. This was due to the B. tequilensis surface reactions of carbonic anhydrase (CA) and urease enzyme with calcium in cement and sequestered CO2 during the curing time.

Immobilized Microalgae-Based Photobioreactor for CO2 Capture (IMC-CO2PBR): Efficiency Estimation, Technological Parameters, and Prototype Concept

Microalgae-mediated CO2 sequestration has been a subject of numerous research works and has become one of the most promising strategies to mitigate carbon dioxide emissions. However, feeding flue and exhaust gas into algae-based systems has been shown to destroy chloroplasts, as well as disrupt photosynthesis and other metabolic processes in microalgae, which directly limits CO2 uptake. CO2 biosequestration in existing photobioreactors (PBRs) is also limited by the low biomass concentration in the growth medium. Therefore, there is a real need to seek alternative solutions that would be competitive in terms of performance and cost-effectiveness. The present paper reports the results of experiments aimed to develop an innovative trickle bed reactor that uses immobilized algae to capture CO2 from flue and exhaust gas (IMC-CO2PBR). In the experiment, ambient air enriched with technical-grade CO2 to a CO2 concentration of 25% v/v was used. The microalgae immobilization technology employed in the experiment produced biomass yields approximating 100 g DM/dm3. A relationship was found between CO2 removal rates and gas volume flux: almost 40% of CO2 was removed at a feed of 25 dm3 of gas per hour, whereas in the 200 dm3/h group, the removal efficiency amounted to 5.9%. The work includes a determination of basic process parameters, presentation of a developed functional model and optimized lighting system, proposals for components to be used in the system, and recommendations for an automation and control system for a full-scale implementation.

Bioprospecting wild South African microalgae as a potential third-generation biofuel feedstock, biological carbon-capture agent and for nutraceutical applications

Microalgae are among the few biological resources studied that are found to possess vast biotechnological potential. This study isolated, identified and investigated two wild green microalgal species with substantial potential as a bioresource and climate change mitigation importance. Two isolates, Chlorella sorokiniana and Tetradesmus reginae were cultivated in selected artificial media under laboratory conditions. The isolates were analysed for nutrient consumption, biomass productivity, CO2 biosequestration rate, elemental composition and fatty acid methyl profiles/composition. The outcome showed maximum daily biomass productivity of 0.128 ± 0.003 and 0.2 ± 0.004 g L−1 for C. sorokiniana and T. reginae, respectively. CO2 biosequestration rate of T. reginae was the highest among the isolates, indicating that it can act as a biological climate change mitigation agent. Moreover, T. reginae recorded a significantly higher (p < 0.05) total lipid and carbohydrate content than C. sorokiniana. The C/N ratio for T. reginae was significantly higher than the C/N ratio for C. sorokiniana. Tetradesmus reginae also demonstrated the ability to produce a considerable quantity of omega-3 oils; hence, the species is of nutraceutical importance. Furthermore, T. reginae demonstrated maximal carbohydrate content and is therefore considered a potential feedstock for bioethanol production. Chlorella sorokiniana, on the other hand, showed a remarkable (p < 0.05) protein content making it a potential source for human food and animal feed supplement. Finally, the two isolates met both European and American quality biodiesel standards with exceptional cetane (CN) and iodine numbers (IV).

Microalgae as promising source for integrated wastewater treatment and biodiesel production

Microalgae have been studied for their potential of wastewater treatment as well as a promising source for biodiesel production. This study investigates the potential of microalgae to remove nutrients from domestic wastewater (DWW) while producing lipids-rich biomass for biodiesel production. Eight microalgae were cultivated in (DWW) to evaluate their nutrients removal capacity and biomass production. Total phosphorus (TP) of DWW reduced from 2 mg L−1 to 0.02 mg L−1 with the treatment efficiency of 99.15% and the highest performance was noted in Chlamydomonas reinhardtii (C. reinhardtii). For total nitrogen (TN), treatment efficiency climbed to 99.07%. It is reduced from 18.35 to 0.17 mg L−1 recorded in C. reinhardtii and Chlorella pyrenoidosa (C. pyrenoidosa). On the other hand, all microalgae showed a high lipids-rich biomass in wastewater compared to BG11. The highest lipid content was 36.93% noted in Chlorella sorokiniana (C. sorokiniana). Fatty acids methyl ester (FAME) profiles showed a high content of palmitic C16:0, oleic C18:1 and stearic acids C18:0 in studied microalgae strains. In summary, microalgae envisage its potential application in integrated wastewater treatment and biodiesel production. In perspective, the authors focus on the validation of this bioprocess in pilot scale. Furthermore, the use of microalgae for other applications such CO2 biosequestration and added value products. Novelty statement: The present study investigates the potential of Moroccan microalgae as candidates to wastewater remediation and high biomass production with high lipid rate for biodiesel production.

Integrated treatment of submerged membrane and adsorption using dried Aphanothece sp for removing cadmium from synthetic wastewater

Water pollution from the discharge of wastewater containing toxic heavy metals such as cadmium (Cd) to the natural water bodies is one of the biggest problem affecting humanity. Different technologies such as sorption, membrane filtration etc. have been used to remove heavy metal from the polluted water. In this work, the effects of operational parameters on the ability of the submerged membrane adsorption hybrid system (SMAHS) to remove Cd2+ from synthetic wastewater using dried Aphanothece sp A15 as a suspended biosorbent was evaluated. This biomass originates from a cyanobacterium was cultivated in a photobioreactor system enriched with 15 % of CO2. The operational parameters investigated were filtration flux, biomass dosage, frequency of replacing biosorbent, and initial metal concentration. Of the various parameters, the filtration flux and the initial concentration of Cd were the most influential parameters concerning biosorption efficiency in this study. More than 95 % removal of Cd was achieved with initial Cd concentration of 4.89 mg/L at a filtration flux of 20 L/m2.h. The highest biosorption capacity (112.89 mg/g) was obtained with a flux of 60 L/m2.h.The biosorbent became saturated after 20 h of operation without biosorbent replacement. This suggests that the dried biomass of Aphanothece sp A15 has the potential to be implemented in the industrial wastewater treatment plant using SMAHS processes integrated within the CO2 bio-sequestration unit. Therefore, cadmium removal from wastewater can be coupled with the reduction of CO2 emissions from flue gas.

The Effects of Total Dissolved Carbon Dioxide on the Growth Rate, Biochemical Composition, and Biomass Productivity of Nonaxenic Microalgal Polyculture

The biosequestration of CO2 using microalgae has emerged as a promising means of recycling CO2 into biomass via photosynthesis, which could be used to produce biofuels as an attractive approach to CO2 mitigation. We investigated the CO2 fixation capability of the native nonaxenic microalgal culture using a 2 L photobioreactor operated in batch mode. The cultivation was carried out at varying concentrations of total dissolved CO2 (Tco2) in the bulk media ranging from 200 to 1000 mg L−1, and the temperature and light intensities were kept constant. A maximum CO2 fixation rate was observed at 400 mg L−1 of Tco2. Characteristic growth parameters such as biomass productivity, specific growth rate, maximum biomass yield, and biochemical parameters such as carbohydrate, protein, and lipids were determined and discussed. We observed that the effect of CO2 concentration on growth and biochemical composition was quite significant. The maximum biomass productivity was 22.10 ± 0.70 mg L−1 day−1, and the rate of CO2 fixation was 28.85 ± 3.00 mg L−1 day−1 at 400 mg L−1 of Tco2. The maximum carbohydrate (8.17 ± 0.49% dry cell weight) and protein (30.41 ± 0.65%) contents were observed at 400 mg L−1, whereas the lipid content (56.00 ± 0.82% dry cell weight) was the maximum at 800 mg L−1 of Tco2 in the bulk medium.

Using seawater-based Na2CO3 medium for scrubbing the CO2 released from Bio-CNG plant for enhanced biomass production of Pseudanabaena limnetica

The concentration of CO2, one of the most important greenhouse gases (GHG), has reached to 409.8 ± 0.1 ppm in 2019. Although there are many carbon capture and storage (CCS) methods, they are very costly and their long term use raises concern about environmental safety. Alternatively, bio-sequestration of CO2 using microalgal cell factories has emerged as a promising way of recycling CO2 into biomass via photosynthesis. In the present study, Indigenous algal strain Pseudanabaena limnetica was cultivated in pneumatically agitated 60-L flat-panel photobioreactor system. The gas was released from Bio-CNG plant as by-product into Na2CO3-rich medium and cultivated in semicontinuous mode of operation. It was observed that when CO2 was sparged in seawater-based 0.02 M Na2CO3 solution, maximum CO2 was dissolved in the system and was used for algal cultivation. Control system produced 0.64 ± 0.035 g/L of biomass at the end of 15 days, whereas CO2 sparged Na2CO3 medium produced 0.81 ± 0.046 g/L of biomass. When CO2 from Bio-CNG station was fed, it resulted in biomass production of 1.62 ± 0.070 g/L at the end of 18 days compared to 1.46 ± 0.066 g/L of biomass produced in control system which was not fed with gas released from Bio-CNG plant as by-product. Thus, feeding CO2 directly into Na2CO3 medium and operating the system semicontinuously would be efficient for scrubbing CO2 from commercial Bio-CNG plant. This study proves that feeding CO2 gas from Bio-CNG plant into Na2CO3-rich alkaline system can be used to feed algae for enhanced biomass production.

The molecular mechanisms of Chlorella sp. responding to high CO2: A study based on comparative transcriptome analysis between strains with high- and low-CO2 tolerance

High CO2 acclimation for microalgae has attracted large research attention owing to the usefulness of microalgae in bio-sequestration of CO2 from the emission source. In this study, one high CO2 tolerant (LAMB 31) and non-tolerant (LAMB 122) Chlorella sp. strains were transferred from air to 40% CO2, during which four time points were chosen for comparative transcriptome analysis. Gene changes started in the lag phase (T1) of population growth with more genes (7889) upregulated in LAMB 31 than in LAMB 122 (1092). Further function enrichments indicated: In LAMB 31, up-regulation of genes in cyclic electron transportation, F-type ATPase and Calvin cycle were associated with the enhancement of carbon fixation abilities; upregulation of genes in phosphorylation together with V-ATPase, which contributed to cytoplasmatic pH stability; Lastly, enhancement of carbon metabolisms including TCA cycle and glycolysis accelerated the consumption of cellular organic carbon. Most of the genes in these pathways and processes showed downregulation in LAMB 122. This study disclosed the most complete transcriptional molecular mechanisms of Chlorella sp. responding to high CO2 by combining CO2 fixation, transportation, and metabolic processes. The results provided valuable genetic information for future screening and breeding of microalgae with high-CO2 tolerance for more efficient CO2 bio-sequestration.

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.