Utilization of the cyanobacteria Anabaena sp. CH1 in biological carbon dioxide mitigation processes

Microalgae-based flue gas CO2 sequestration for cleaner environment and biofuel feedstock production: a review

Anthropogenic CO2 emissions are the prime cause of global warming and climate change, promoting researchers to develop suitable technologies to reduce carbon footprints. Among various CO2 sequestration technologies, microalgal-based methods are found to be promising due to their easier operation, environmental benefits, and simpler equipment requirements. Microalgae-based carbon capture and storage (CCS) technology is essential for addressing challenges related to the use of industrial-emitted flue gases. This review focuses on the literature concerning the microalgal application for CO2 sequestration. It highlights the primary physiochemical parameters that affect microalgal-based CO2 biofixation, including light exposure, microalgal strain, temperature, inoculum size, pH levels, mass transfer, CO2 concentration, flow rate, cultivation system, and mixing mechanisms. Moreover, the inhibition effect of different flue gas components including NOx, SOx, and Hg on growth kinetics is discussed to enhance the capacity of microalgal-based CO2 biofixation, along with deliberated challenges and prospects for future development. Overall, the review indicated microalgal-based flue gas CO2 fixation rates range from 80 mg L-1 day-1 to over 578 mg L-1 day-1, primarily influenced by physiochemical parameters and flue gas composition. This article summarizes the mechanisms and stages of microalgal-based CO2 sequestration and provides a comprehensive review based on international interest in this green technology.

Realization process of microalgal biorefinery: The optional approach toward carbon net-zero emission

Increasing carbon dioxide (CO2) emission has already become a dire threat to the human race and Earth's ecology. Microalgae are recommended to be engineered as CO2 fixers in biorefinery, which play crucial roles in responding climate change and accelerating the transition to a sustainable future. This review sorted through each segment of microalgal biorefinery to explore the potential for its practical implementation and commercialization, offering valuable insights into research trends and identifies challenges that needed to be addressed in the development process. Firstly, the known mechanisms of microalgal photosynthetic CO2 fixation and the approaches for strain improvement were summarized. The significance of process regulation for strengthening fixation efficiency and augmenting competitiveness was emphasized, with a specific focus on CO2 and light optimization strategies. Thereafter, the massive potential of microalgal refineries for various bioresource production was discussed in detail, and the integration with contaminant reclamation was mentioned for economic and ecological benefits. Subsequently, economic and environmental impacts of microalgal biorefinery were evaluated via life cycle assessment (LCA) and techno-economic analysis (TEA) to lit up commercial feasibility. Finally, the current obstacles and future perspectives were discussed objectively to offer an impartial reference for future researchers and investors.

Environmental pollution mitigation through utilization of carbon dioxide by microalgae

Anthropogenic emissions of CO₂ have reached a critical level and the global surface temperature is expected to rise by 1.5 °C between 2030 and 2050. To ameliorate the current global warming scenario, the research community has been struggling to find more economical and innovative solutions for carbon sequestration. Among such techniques, the use of microalgal species such as Chlorella sp., Dunaliella tertiolecta, Spirulina platensis, Desmodesmus sp., and Nannochloropsis sp., among others have shown high carbon tolerance capacity (10-100%) for establishing carbon capture, utilization and storage systems. To make microalgal-based carbon capture more economical, the microalgal biomass (∼2 g/L) can be converted biofuels, pharmaceuticals and nutraceuticals through biorefinery approach with product yield in the range of 60-99.5%. Further, CRISPR-Cas9 has enabled the knockout of specific genes in microalgal species that can be used to generate low pH tolerant strains with high lipid production. Inspite of the emerging developments in pollution control by microalgae, only limited investigations are available on its economic aspects which indicate a production cost of ∼$ 0.5-15/kg microalgal biomass. This review intends to summarize the advancements in different carbon sequestration techniques while highlighting their mechanisms and major research areas that need attention for economical microalgae-based carbon sequestration.

Modeling of carbon dioxide fixation by microalgae using hybrid artificial intelligence (AI) and fuzzy logic (FL) methods and optimization by genetic algorithm (GA)

In this study, we are reporting a novel prediction model for forecasting the carbon dioxide (CO₂) fixation of microalgae which is based on the hybrid approach of adaptive neuro-fuzzy inference system (ANFIS) and genetic algorithm (GA). The CO₂ fixation rate of various algal strains was collected and the cultivation conditions of the microalgae such as temperature, pH, CO₂ %, and amount of nitrogen and phosphorous (mg/L) were taken as the input variables, while the CO₂ fixation rate was taken as the output variable. The optimization of ANFIS parameters and the formation of the optimized fuzzy model structure were performed by genetic algorithm (GA) using MATLAB in order to achieve optimum prediction capability and industrial applicability. The best-fitting model was figured out using statistical analysis parameters such as root mean square error (RMSE), coefficient of regression (R²), and average absolute relative deviation (AARD). According to the analysis, GA-ANFIS model depicted a greater prediction capability over ANFIS model. The RMSE, R², and AARD for GA-ANFIS were observed to be 0.000431, 0.97865, and 0.044354 in the training phase and 0.00056, 0.98457, and 0.032156 in the testing phase, respectively, for the GA-ANFIS Model. As a result, it can be concluded that the proposed GA-ANFIS model is an efficient technique having a very high potential to accurately predict the CO₂ fixation rate.

Algal biomass to biohydrogen: Pretreatment, influencing factors, and conversion strategies

Hydrogen has gained attention as an alternative source of energy because of its non-polluting nature as on combustion it produces only water. Biological methods are eco-friendly and have benefits in waste management and hydrogen production simultaneously. The use of algal biomass as feedstock in dark fermentation is advantageous because of its low lignin content, high growth rate, and carbon-fixation ability. The major bottlenecks in biohydrogen production are its low productivity and high production costs. To overcome these issues, many advances in the area of biomass pretreatment to increase sugar release, understanding of algal biomass composition, and development of fermentation strategies for the complete recovery of nutrients are ongoing. Recently, mixed substrate fermentation, multistep fermentation, and the use of nanocatalysts to improve hydrogen production have increased. This review article evaluates the current progress in algal biomass pretreatment, key factors, and possible solutions for increasing hydrogen production.

Microalgal Biomass as Feedstock for Bacterial Production of PHA: Advances and Future Prospects

The search for biodegradable plastics has become the focus in combating the global plastic pollution crisis. Polyhydroxyalkanoates (PHAs) are renewable substitutes to petroleum-based plastics with the ability to completely mineralize in soil, compost, and marine environments. The preferred choice of PHA synthesis is from bacteria or archaea. However, microbial production of PHAs faces a major drawback due to high production costs attributed to the high price of organic substrates as compared to synthetic plastics. As such, microalgal biomass presents a low-cost solution as feedstock for PHA synthesis. Photoautotrophic microalgae are ubiquitous in our ecosystem and thrive from utilizing easily accessible light, carbon dioxide and inorganic nutrients. Biomass production from microalgae offers advantages that include high yields, effective carbon dioxide capture, efficient treatment of effluents and the usage of infertile land. Nevertheless, the success of large-scale PHA synthesis using microalgal biomass faces constraints that encompass the entire flow of the microalgal biomass production, i.e., from molecular aspects of the microalgae to cultivation conditions to harvesting and drying microalgal biomass along with the conversion of the biomass into PHA. This review discusses approaches such as optimization of growth conditions, improvement of the microalgal biomass manufacturing technologies as well as the genetic engineering of both microalgae and PHA-producing bacteria with the purpose of refining PHA production from microalgal biomass.

Role of Microalgae in Global CO2 Sequestration: Physiological Mechanism, Recent Development, Challenges, and Future Prospective

The rising concentration of global atmospheric carbon dioxide (CO2) has severely affected our planet’s homeostasis. Efforts are being made worldwide to curb carbon dioxide emissions, but there is still no strategy or technology available to date that is widely accepted. Two basic strategies are employed for reducing CO2 emissions, viz. (i) a decrease in fossil fuel use, and increased use of renewable energy sources; and (ii) carbon sequestration by various biological, chemical, or physical methods. This review has explored microalgae’s role in carbon sequestration, the physiological apparatus, with special emphasis on the carbon concentration mechanism (CCM). A CCM is a specialized mechanism of microalgae. In this process, a sub-cellular organelle known as pyrenoid, containing a high concentration of Ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), helps in the fixation of CO2. One type of carbon concentration mechanism in Chlamydomonas reinhardtii and the association of pyrenoid tubules with thylakoids membrane is represented through a typical graphical model. Various environmental factors influencing carbon sequestration in microalgae and associated techno-economic challenges are analyzed critically.

Marine Microalgae Contribution to Sustainable Development

The burning of fossil fuels is an unsustainable activity, which is leading to an increase in greenhouse gases (GHGs) emissions and related global warming. Among sustainable energy sources, microalgae represent a promising alternative to fossil fuel and contribute to the achievement of important Sustainable Development Goals (SDGs). In particular, the potential contribution of marine microalgae to sustainable development is large as, among other benefits, they represent a carbon negative energy source and may be applied in many coastal areas around the world. Despite this, significant economic and technological improvements are needed in order to make microalgae biofuels viable on a large scale. This review aims to explore how and to what extent third-generation biofuels (marine microalgae, but also the latest advances in freshwater microalgae) can benefit the realization of these SDGs. From this study we concluded that the production of large-scale marine microalgae biofuels is not yet feasible from the economic perspective at a large scale. However, the cultivation of microalgae in seawater holds great potential for increasing the small to medium viability of this biofuel source. The possibilities for improvement along with the contributions to sustainable development lay the groundwork for continuing to study and apply the potential of sustainable production of microalgae bioenergy.

Advances of Carbon Capture and Storage in Coal-Based Power Generating Units in an Indian Context

India is a nation with a diverse economy that requires tremendous resources to completely meet the desires of its compatriots in various sectors. In terms of energy resources and requirements, coal-based power plants can fulfill the bulk of these electricity needs. India is very reliant on coal, which is used in power plants as a primary energy source. However, the usage of coal energy at a higher level continuously pollutes the atmosphere. The Indian power market alone accounts for half of the country’s CO2 emissions, which implies that significant action is needed to contain environmental pollution. Carbon Capture and Storage (CCS) is a bridging technique and feasible alternative for the carbon fired plant processing of CO2. However, the application of CCS in coal-fired power stations is still uncommon in the nation. At the UNFCCC Paris Summit, India committed to reduce its carbon emission intensity by approximately 30–33% by 2030. In this work, several CCS systems, possible CO2 origins, and emission levels in India are discussed. Various advanced methods for CO2 capture and separation are also highlighted. Furthermore, the current work discusses CCS situations and the applications of CCS in India along with its manifold challenges.

Bioproduct Potential of Outdoor Cultures of Tolypothrix sp.: Effect of Carbon Dioxide and Metal-Rich Wastewater

Rising CO2 levels, associated climatic instability, freshwater scarcity and diminishing arable land exacerbate the challenge to maintain food security for the fast growing human population. Although coal-fired power plants generate large amounts of CO2 emissions and wastewater, containing environmentally unsafe concentrations of metals, they ensure energy security. Nitrogen (N2)-fixation by cyanobacteria eliminate nitrogen fertilization costs, making them promising candidates for remediation of waste CO2 and metals from macronutrient-poor ash dam water and the biomass is suitable for phycocyanin and biofertilizer product development. Here, the effects of CO2 and metal mixtures on growth, bioproduct and metal removal potential were investigated for the self-flocculating, N2-fixing freshwater cyanobacterium Tolypothrix sp. Tolypothrix sp. was grown outdoors in simulated ash dam wastewater (SADW) in 500 L vertical bag suspension cultures and as biofilms in modified algal-turf scrubbers. The cultivation systems were aerated with air containing either 15% CO2 (v/v) or not. CO2-fertilization resulted in ∼1.25- and 1.45-fold higher biomass productivities and ∼40 and 27% increased phycocyanin and phycoerythrin contents for biofilm and suspension cultures, respectively. CO2 had no effect on removal of Al, As, Cu, Fe, Sr, and Zn, while Mo removal increased by 37% in both systems. In contrast, Ni removal was reduced in biofilm systems, while Se removal increased by 73% in suspension cultures. Based on biomass yields and biochemical data obtained, net present value (NPV) and sensitivities analyses used four bioproduct scenarios: (1) phycocyanin sole product, (2) biofertilizer sole product, (3) 50% phycocyanin and 50% biofertilizer, and (4) 100% phycocyanin and 100% biofertilizer (residual biomass) for power station co-located and not co-located 10 ha facilities over a 20-year period. Economic feasibility for the production of food-grade phycocyanin either as a sole product or with co-production of biofertilizer was demonstrated for CO2-enriched vertical and raceway suspension cultures raised without nitrogen-fertilization and co-location with power stations significantly increased profit margins.

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.

Analysis of the light intensity dependence of the growth of Synechocystis and of the light distribution in a photobioreactor energized by 635 nm light

Synechocystis gathered momentum in modelling studies and biotechnological applications owing to multiple factors like fast growth, ability to fix carbon dioxide into valuable products, and the relative ease of genetic manipulation. Synechocystis physiology and metabolism, and consequently, the productivity of Synechocystis-based photobioreactors (PBRs), are heavily light modulated. Here, we set up a turbidostat-controlled lab-scale cultivation system in order to study the influence of varying orange–red light intensities on Synechocystis growth characteristics and photosynthetic activity. Synechocystis growth and photosynthetic activity were found to raise as supplied light intensity increased up to 500 μmol photons m⁻² s⁻¹ and to enter the photoinhibition state only at 800 μmol photons m⁻² s⁻¹. Interestingly, reverting the light to a non-photo-inhibiting intensity unveiled Synechocystis to be able to promptly recover. Furthermore, our characterization displayed a clear correlation between variations in growth rate and cell size, extending a phenomenon previously observed in other cyanobacteria. Further, we applied a modelling approach to simulate the effects produced by varying the incident light intensity on its local distribution within the PBR vessel. Our model simulations suggested that the photosynthetic activity of Synechocystis could be enhanced by finely regulating the intensity of the light incident on the PBR in order to prevent cells from experiencing light-induced stress and induce their exploitation of areas of different local light intensity formed in the vessel. In the latter case, the heterogeneous distribution of the local light intensity would allow Synechocystis for an optimized usage of light.

Impact of Flue Gas Compounds on Microalgae and Mechanisms for Carbon Assimilation and Utilization

To shift the world to a more sustainable future, it is necessary to phase out the use of fossil fuels and focus on the development of low-carbon alternatives. However, this transition has been slow, so there is still a large dependence on fossil-derived power, and therefore, carbon dioxide is released continuously. Owing to the potential for assimilating and utilizing carbon dioxide to generate carbon-neutral products, such as biodiesel, the application of microalgae technology to capture CO₂ from flue gases has gained significant attention over the past decade. Microalgae offer a more sustainable source of biomass, which can be converted into energy, over conventional fuel crops because they grow more quickly and do not adversely affect the food supply. This review focuses on the technical feasibility of combined carbon fixation and microalgae cultivation for carbon reuse. A range of different carbon metabolisms and the impact of flue gas compounds on microalgae are appraised. Fixation of flue gas carbon dioxide is dependent on the selected microalgae strain and on flue gas compounds/concentrations. Additionally, current pilot-scale demonstrations of microalgae technology for carbon dioxide capture are assessed and its future prospects are discussed. Practical implementation of this technology at an industrial scale still requires significant research, which necessitates multidisciplinary research and development to demonstrate its viability for carbon dioxide capture from flue gases at the commercial level.

CO2 capture from air by Chlorella vulgaris microalgae in an airlift photobioreactor

In this work, hydrodynamics and CO₂ biofixation study was conducted in an airlift bioreactor at the temperature of 30±2°C. The main objective of this work was to investigate the effect of high gas superficial velocity on CO₂ biofixation using Chlorella vulgaris microalgae and its growth. The study showed that Chlorella vulgaris in high input gas superficial velocity also had the ability to grow and remove the CO₂ by less than 80% efficiency.

Reduction and biofixation of carbon dioxide in palm oil mill effluent using developed microbial granules containing photosynthetic pigments

The developed microbial granules containing photosynthetic pigments had successfully achieved approximately 18-21% of carbon dioxide (CO₂) removal in POME for one complete SBR cycle. Also, the granules had reached CO₂ removal at 15-29% within 24h and removal of 25% after 5 days. Both results were inconsistent possibly due to the slow mass transfer rate of CO₂ from gas to liquid as well as the simultaneous effect of CO₂ production and respiration among the microbes. Furthermore, results showed the removal of CO₂ from air increases proportionally with the CO₂ removed in liquid. The CO₂ biofixation of granules attained was approximately 0.23g/L/day for a week. Using the regression model, the removal of CO₂ between liquid and gas, CO₂ biofixation rate were highly correlated with the treatment time. A statistically significant relationship was obtained between CO₂ concentration in liquid, biomass productivity and treatment time for the CO₂ biofixation rate of the granules.

Biorefineries of carbon dioxide: From carbon capture and storage (CCS) to bioenergies production

Greenhouse gas emissions have several adverse environmental effects, like pollution and climate change. Currently applied carbon capture and storage (CCS) methods are not cost effective and have not been proven safe for long term sequestration. Another attractive approach is CO2 valorization, whereby CO2 can be captured in the form of biomass via photosynthesis and is subsequently converted into various form of bioenergy. This article summarizes the current carbon sequestration and utilization technologies, while emphasizing the value of bioconversion of CO2. In particular, CO2 sequestration by terrestrial plants, microalgae and other microorganisms are discussed. Prospects and challenges for CO2 conversion are addressed. The aim of this review is to provide comprehensive knowledge and updated information on the current advances in biological CO2 sequestration and valorization, which are essential if this approach is to achieve environmental sustainability and economic feasibility.

Algal remediation of CO₂ and nutrient discharges: A review

The recent literature pertaining to the application of algal photobioreactors (PBRs) to both carbon dioxide mitigation and nutrient abatement is reviewed and the reported data analysed. The review appraises the influence of key system parameters on performance with reference to (a) the absorption and biological fixation of CO2 from gaseous effluent streams, and (b) the removal of nutrients from wastewaters. Key parameters appraised individually with reference to CO2 removal comprise algal speciation, light intensity, mass transfer, gas and hydraulic residence time, pollutant (CO2 and nutrient) loading, biochemical and chemical stoichiometry (including pH), and temperature. Nutrient removal has been assessed with reference to hydraulic residence time and reactor configuration, along with C:nutrient ratios and other factors affecting carbon fixation, and outcomes compared with those reported for classical biological nutrient removal (BNR). Outcomes of the review indicate there has been a disproportionate increase in algal PBR research outputs over the past 5-8 years, with a significant number of studies based on small, bench-scale systems. The quantitative impacts of light intensity and loading on CO2 uptake are highly dependent on the algal species, and also affected by solution chemical conditions such as temperature and pH. Calculations based on available data for biomass growth rates indicate that a reactor CO2 residence time of around 4 h is required for significant CO2 removal. Nutrient removal data indicate residence times of 2-5 days are required for significant nutrient removal, compared with <12 h for a BNR plant. Moreover, the shallow depth of the simplest PBR configuration (the high rate algal pond, HRAP) means that its footprint is at least two orders of magnitude greater than a classical BNR plant. It is concluded that the combined carbon capture/nutrient removal process relies on optimisation of a number of process parameters acting synergistically, principally microalgal strain, C:N:P load and balance, CO2 and liquid residence time, light intensity and quality, temperature, and reactor configuration. This imposes a significant challenge to the overall process control which has yet to be fully addressed.

Biosequestration of atmospheric CO2 and flue gas-containing CO2 by microalgae

The unceasing rise of greenhouse gas emission has led to global warming and climate change. Global concern on this phenomenon has put forward the microalgal-based CO2 sequestration aiming to sequester carbon back to the biosphere, ultimately reducing greenhouse effects. Microalgae have recently gained enormous attention worldwide, to be the valuable feedstock for renewable energy production, due to their high growth rates, high lipid productivities and the ability to sequester carbon. The photosynthetic process of microalgae uses atmospheric CO2 and CO2 from flue gases, to synthesize nutrients for their growth. In this review article, we will primarily discuss the efficiency of CO2 biosequestration by microalgae species, factors influencing microalgal biomass productions, microalgal cultivation systems, the potential and limitations of using flue gas for microalgal cultivation as well as the bio-refinery approach of microalgal biomass.

Assessment of the CO2 fixation capacity of Anabaena sp. ATCC 33047 outdoor cultures in vertical flat-panel reactors

The extent of biological CO2 fixation was evaluated for outdoor cultures of the cyanobacterium Anabaena sp. ATCC 33047. Culture conditions were optimized indoors in bubble-column photochemostats operating in continuous mode, subjected to irradiance cycles mimicking the light regime outdoors. Highest values achieved for CO2 fixation rate and biomass productivity were 1 and 0.6 g L(-1) day(-1), respectively. The comparison among different reactors operating simultaneously - open pond, horizontal tubular reactor and vertical flat-panel - allowed to assess their relative efficiency for the outdoor development of Anabaena cultures. Despite the higher volumetric CO2 fixation capacity (and biomass productivity) exhibited by the tubular photobioreactor, yield of the flat-panel reactor was 50% higher than that of the tubular option on a per area basis, reaching values over 35 g CO2 fixed m(-2) d(-1). The flat-panel reactor actually represents a most suitable system for CO2 capture coupled to the generation of valuable biomass by Anabaena cultures.

CO2 biofixation and carbonic anhydrase activity in Scenedesmus obliquus SA1 cultivated in large scale open system

The present study deals with the large scale open system cultivation of the novel microalga: Scenedesmus obliquus SA1 (KC733762) previously isolated in our laboratory. SA1 strain was cultivated in open system at varying CO2 levels ranging from 0.03% to 35% (v/v) and subsequently the carbonic anhydrase activity (CA) and the biochemical properties were monitored. Maximum biomass concentration (1.39 ± 0.023 g L(-1)), CO2 fixation rate (97.65 ± 1.03 mg L(-1)d(-1)) and total CA activity (166.86 ± 3.30 E.U./mg chla) were obtained at 35% CO2. CA inhibitors: acetazolamide and ethoxyzolamide inhibited the external and internal enzyme activity in SA1. High CO2 levels were favorable for the accumulation of lipids and chlorophyll. The present results suggested that SA1 possessed high CO2 tolerance and high carbohydrate, lipid and chlorophyll content when cultivated in open system thus being suitable for CO2 mitigation in outdoor ponds and subsequent generation of value added products.

Development of a process for efficient use of CO2 from flue gases in the production of photosynthetic microorganisms

A new methodology to use efficiently flue gases as CO(2) source in the production of photosynthetic microorganisms is proposed. The CO(2) is absorbed in an aqueous phase that is then regenerated by microalgae. Carbonated solutions could absorb up to 80% of the CO(2) from diluted gas reaching total inorganic carbon (TIC) concentrations up to 2.0 g/L. The pH of the solution was maintained at 8.0-10.0 by the bicarbonate/carbonate buffer, so it is compatible with biological regeneration. The absorption process was modeled and the kinetic parameters were determined. Anabaena sp. demonstrated to tolerate pH (8.0-10.0) and TIC (up to 2.0 g/L) conditions imposed by the absorption step. Experiments of regeneration of the liquid phase demonstrated the feasibility of the overall process, converting CO(2) into organic matter. The developed process avoids heating to regenerate the liquid whereas maximizing the efficiency of CO(2) use, which is relevant to achieve the commercial production of biofuels from microalgae.