CO2 Sequestration using Microalga Scenedesmus Obliquus SA1 Isolated from Bio-diversity Hotspot Region of Assam

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Flue gases emitted from coal-fired thermoelectric plants is responsible for up to 7% (v/v) of global CO2 emissions, about 10-15% (v/v) of the flue gases emitted from the power plants being in the form of CO2. Increased CO2 concentration in the atmosphere is responsible for global warming and climate change. The thesis focuses on the isolation and characterization of high CO2 and temperature tolerant microalga capable of sequestering CO2 from flue gas and subsequent cultivation of the microalga in bench scale open system and lab scale photobioreactor for enhanced CO2 sequestration. Microalga strain SA1 was isolated from a freshwater body of Assam and identified as Scenedesmus obliquus (KC733762). At 13.8 ± 1.5% inlet CO2 concentration and 25 °C, maximum biomass of 4.975 ± 0.003 g L−1 and maximum CO2 fixation rate of 252.883 ± 0.361 mg L−1 d−1 were obtained in the lab scale closed system studies. Also, at elevated temperature (40 °C) and 13.8 ± 1.5% CO2 supply maximum biomass value of 0.883 ± 0.001 g L−1 and maximum specific growth rate of 0.54 ± 0.020 d−1 were obtained. . The carbohydrate, protein, lipid, and chlorophyll content of the CO2 treated SA1 obtained in the lab scale closed system studies were 30.87 ± 0.64%, 9.48 ± 1.65%, 33.04 ± 0.46 and 6.03 ± 0.19% respectively. The inlet CO2 concentration of 13.8 ± 1.5% was reduced to 0.5% during logarithmic growth phase of SA1. Since the power plant flue gas contains high concentration of CO2 (around 12-15%) and is released from the power plant at high temperature (around 40-50 ºC after the desulfurization process), tolerance of high CO2 concentration and high temperature of 40 ºC temperature by S. obliquus SA1 makes it a potential strain for CO2 sequestration from flue gases. SA1 strain was subsequently cultivated in bench scale open system at varying CO2 levels ranging from 0.03-35% (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 Carbonic anhydrase (CA) activity (166.86 ± 3.30 E.U. /mg chla) were obtained at 35% CO2 at a culture depth of 0.17 m. The culture depth was varied at 15% CO2 concentration from 0.0425 m to 0.17 m. Overall biomass productivity (54.33 ± 0.19 mg L−1 d−1), CO2 fixation rate (102.13 ± 0.36 mg L−1 d−1) and maximum biomass productivity (156.8 ± 4.37 mg L−1 d−1) were the highest at a culture depth of 0.085m. As evident from literature reports, CA activity is strongly induced when algae are grown in a low-CO2 environment. This fact was evident from our experimental finding, as CA activity of control culture (grown at ambient CO2 concentration) > CA activity of 15% CO2 treated culture > CA activity of 35% CO2 treated culture for most of the experimental period. CA inhibitors: acetazolamide and ethoxyzolamide inhibited the external and internal enzyme activity respectively in SA1, thereby confirming the presence of periplasmic (external) and intracellular CA in the SA1 strain. High CO2 levels were favorable for the accumulation of lipids and chlorophyll in the SA1 strain the values of which were 41.17 ± 0.77% and 8.47 ± 0.15% respectively. The increased lipid content could make the SA1 strain useful in biodiesel production. Also, chlorophyll is a useful commercial pigment and is regarded as an economically valuable co-product of the CO2 sequestration process. Finally, the operational parameters were varied to maximize the CO2 utilization efficiency by the SA1 strain. In these optimization studies, SA1 strain was cultivated in a lab scale cylindrical glass photobioreactor (open system) under 15% CO2 concentration at varied operational conditions (light intensity, CO2 sparging duration and CO2 flow rates). At light intensity of 4351 lux, CO2 sparging duration of 12 h per day and flow rate of 0.43 liter per hour, maximum biomass concentration of 3.32 ± 0.022 g L−1, maximum specific growth rate of 1.24 ± 0.028 d−1, maximum CO2 fixation rate of 1035.25 ± 52.98 mgL−1d−1 and maximum CO2 utilization efficiency of 10.23% were obtained which were higher than most of the relevant literature reports. These parameters were thus inferred to be the optimum condition for maximum CO2 utilization by the microalga in lab scale photobioreactor. SA1 has high biomass productivity, fast growth rates, an attractive biochemical profile, high CO2 fixation rates and utilization efficiency when cultivated in presence of 15% CO2 (typical flue gas concentration). It can thus prove to be a potential candidate for...
Supervisor: Kaustubha Mohanty