RESEARCH ARTICLE CO 2 bio-fixation and biofuel production in an airlift photobioreactor by an isolated strain of microalgae Coelastrum sp. SM under high CO 2 concentrations Shokouh Mousavi 1 & Ghasem D. Najafpour 1 & Maedeh Mohammadi 1 Received: 25 March 2018 /Accepted: 21 August 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Microalgae cultivation is a promising approach to remove ambient CO 2 via photosynthesis process. This paper investigates the impact of high CO 2 concentrations (6, 12, and 16%) on algae growth, CO 2 biofixation, lipid and carbohydrate contents, and nutrient removal of newly isolated microalgae, Coelastrum sp. SM. In addition, the ability of microalgae to produce biodiesel at optimal condition was studied. The microalgae were cultivated in wastewater using an airlift photobioreactor. Under 12% CO 2 , the maximum biomass productivity and CO 2 fixation rate were 0.267 g L -1 day -1 and 0.302 g L -1 h -1 , respectively. Total Kjeldahl nitrogen (TKN), total phosphorous (TP), nitrate, and sCOD removal efficiency were 84.01, 100, 86.811, and 73.084%, respectively. Under 12% CO 2 and at the same condition for cell growth, the highest lipid and carbohydrate contents were 3 7.91 and 58.45%, respectively. The composition of fatty acids methyl ester (FAME) of the microalga lipid was defined. Based on the obtained results and FAME profile, Coelastrum sp. SM was a suitable feedstock for biodiesel production and also, the organism had a great potential for CO 2 biofixation, which is also more suitable than any other reported strains in other related studies. Keywords Microalgae . CO 2 bio-fixation . Coelastrum sp. SM . Biodiesel . Lipid . Carbohydrate . Nutrient removal Introduction Today’s biggest concern is that the world is facing reduction of fossil fuels, which are non-renewable energy sources. There is a need to conserve non-renewable resources and keep the envi- ronment safe from the negative side effects of burning fossil fuels which are the main reasons for the growing demand for green energy, such as biodiesel (Choi et al. 2015; Kandimalla et al. 2016; Nayak et al. 2016). In addition to the depletion of non-renewable fuels, climate changes and global warming have also caused serious environmental pollution problems, for ex- ample, extreme weather events, ocean acidification, and sea level rise. Since 1950, most of the climate changes have been caused by carbon dioxide emissions due to human activities (García-Cubero et al. 2017; McMichael 2013; Nithiya et al. 2017). Currently available methods for capturing carbon diox- ide are adsorption, absorption, cryogenic separation, and mem- brane separation. However, the application of these methods is limited due to high cost, high energy utilization, and many concerns about the CO 2 storage environmental sustainability (Lee and Weng 2018; Pires et al. 2011). Photosynthetic organisms like microalgae and cyanobacteria can play an important role in reducing carbon dioxide from the atmosphere or industrial gas due to their ability to absorb CO 2 and conversion into biomass. But, the biomass production and consequently the biofixation of carbon dioxide by microalgae are only economically viable along with production of another valuable product such as biofuel (Cheng et al. 2016; García- Cubero et al. 2017). Therefore, it is necessary to find some species that have high CO 2 tolerance and simultaneously produce significant amount of lipid/carbohydrate. In addition to lipid and carbohydrate con- tent, the amount of biomass production is also very important. Strains of algae that produce a good combination of biomass productivity and lipid/carbohydrate content can be useful to Responsible editor: Philippe Garrigues * Ghasem D. Najafpour najafpour@nit.ac.ir 1 Faculty of Chemical Engineering, Babol Noshirvani University of Technology, Babol, Iran Environmental Science and Pollution Research https://doi.org/10.1007/s11356-018-3037-4