(703a) Isolation, Identification and Characterization of High CO2 Tolerant Microalgae for Efficient CO2 Sequestration From Two Different Locations of India

Isolation, Identification and Characterization of high CO₂ tolerant microalgae for efficient CO₂ sequestration from two different locations of India.

Samarpita Basu¹, AbhijitSarma Roy², K.Mohanty^{1, 3}, A.K.Ghoshal*^{1, 2, 3}

¹Centre for Environment, Indian Institute of Technology Guwahati, Assam-781039, India

²Centre for Energy, Indian Institute of Technology Guwahati, Assam-781039, India

³Department of Chemical Engineering, Indian Institute of Technology Guwahati, Assam-781039, India

*Corresponding author (E-mail:aloke@iitg.ernet.in)

    The accumulation of greenhouse gases like CO₂ in the earth’s atmosphere as a result of human activities and industrialization causes serious consequences like global warming thereby increasing the temperatures of the surface air and subsurface ocean. Apart from other causes combustion of fossil fuel for energy is one of the most important for increased CO₂in the atmosphere. Emissions of CO₂ are likely to increase significantly in the near future due to increased energy demand from developing countries.Public concerns over severe environmental consequences that might result due to globally rising temperatures have brought forward the necessities to find out ways to reduce the emission of CO₂ to the atmosphere. Some of the strategies employed to reduce CO₂ emissions include the implementation of renewable energy sources as well as capturing of CO₂  to limit its release into the atmosphere. The current methods of capturing CO₂ include the use of physicochemical absorbents and mineral carbonation of CO_2.However, these methods have several drawbacks like high space requirements, high cost and potential CO₂ leakage over time. Therefore, a lot of attention is now being given to the biological fixation of CO₂. Photosynthetic microorganisms like microalgae and cyanobacteria use inorganic carbon for growth and hence can convert CO₂ from a point source into biomass. Among the photosynthetic microorganisms, microalgal capture of CO₂ is thought to be a feasible technology with energy efficient and environment friendly natureMostly microalgae are unicellular photosynthetic microorganisms that can fix CO₂ effectively from gaseous effluents (e.g., industrial flue gas) and have much higher photosynthetic efficiency and biomass productivity as compared to other energy crops  Moreover, the microalgae biomass produced after CO₂ fixation can be put to several uses such as feed stock for a variety of biofuels, medications, cosmetics and nutritious foods, representing additional benefits from the microalgal CO₂ reduction process. The present study aimed to isolate high CO₂ and temperature tolerant microalgae capable of sequestering CO₂ from flue gas. Water samples were collected from two different locations, one from a freshwater body (26°11'15"N 91°45'4"E) of Guwahati in Assam, located in north eastern region of India and another from a freshwater body (22°55′22″N88°22′46″E) located in the vicinity of a thermal power plant in West Bengal, located in eastern region of India. Isolation of microalgae strains were carried out from the samples using BG 11(ATCC Medium 616) as the culture medium .14 hours of light and 10 hours of dark regime was employed during growth using daylight-type 20 W fluorescent tubes and light intensity maintained was 5496 lux. In totality, three microalgal strains designated as SA1 (from Assam), SA2 and SA3 (from West Bengal) were isolated. After isolation of pure culture, CO₂ fixation studies were initiated using these strains using 13.8 ± 1.5 % CO₂ as the sole carbon source for microalgal cultivation. 28s rRNA gene sequencing of SA1 followed by homology matching with NCBI GenBank database confirmed SA1 to be the green microalga, Scenedesmus obliquus. The 28s rRNA gene sequence was then submitted to NCBI Genbank and accession number (KC733762) was obtained. Growth kinetics and CO₂ fixation rates of the isolates were determined at varying culture conditions (nitrate and phosphate concentrations, photoperiods  and temperatures). Studies were carried out using these strains at elevated cultivation temperatures of upto 40°C since flue gas has a temperature of around 40°C after the desulfurization process. Characterizations of the strains were done in scanning electron microscope as well as FTIR. FTIR gave an idea of the biochemical composition of the microalgal strains. Carbohydrate, protein, lipid and pigment content of the microalgal strains were determined to elucidate the potential of these strains for the production of value-added substances like biofuels which would offer additional benefits to the CO₂ sequestration process. The various growth kinetic parameters like maximum biomass value, maximum specific growth rate, overall biomass productivity and CO₂ fixation rates were estimated for the strain SA1.The microalgae strains SA2and SA3 isolated from West Bengal are currently being studied and DNA isolation and sequencing are being carried out. The gross calorific values of the strains were estimated to elucidate the lipid content of the microalgae strains and also to validate the use of the algal biomass for biofuel generation after the carbon sequestration process. SA1 showed enhanced CO₂ fixation rates and high biomass productivities which were higher than most of the related studies. At 13.8 ± 1.5%CO₂ and 25 °C, maximum biomass (4.975 ± 0.003 gL⁻¹) and maximum CO₂ fixation rate (252.883 ± 0.361mgL⁻¹d⁻¹) were obtained. At elevated temperature  (40°C) and 13.8 ± 1.5% CO_2, maximum biomass(0.883 ± 0.001 g L⁻¹) was obtained. The carbohydrate, protein, lipid, and chlorophyll content of the SA1 after CO₂ sequestration were 30.87 ± 0.64%, 9.48 ± 1.65%, 33.04 ± 0.46 and 6.03 ± 0.19% respectively, which were higher than previous reports. The Gross calorific value (GCV) was 23.02 MJ/Kg for SA1 after CO₂ sequestration which was significantly higher than most of the previous reports.

Keywords: Global warming, microalgae, CO₂ fixation, flue gas, Scenedesmus obliquus