Optimization of outdoor cultivation of the marine microalga Nannochloropsis gaditana in flat-panel reactors using industrial exhaust flue gases Alberto Rodríguez-López 1 & Francisco Javier Fernández-Acero 2 & Roberto Andrés-Vallejo 3 & Palmira Guarnizo-García 1 & María Dolores Macías-Sánchez 4 & Miguel Gutiérrez-Díaz 1 & Silvia Burgos-Rodríguez 5 Received: 23 April 2019 /Revised and accepted: 7 November 2019 # Springer Nature B.V. 2019 Abstract The market for microalgae is substantial and currently expanding. To meet this demand, it is necessary to demonstrate the feasibility of large-scale biomass production over long periods. This study aims to demonstrate the feasibility and sustainability of a semi-industrial process for the outdoor cultivation of the microalga Nannochloropsis gaditana in flat-panel reactors using flue gases on demand from a coal-fired power plant. To this end, four secondary objectives were defined: (i) To generate an accurate estimate of the annual average biomass production using this process; (ii) To determine how the production rate and the biochemical composition of the biomass vary with the weather conditions over the course of a year; (iii) To reduce operational costs by developing a new culture medium based on natural seawater enriched with agricultural fertilizers; and (iv) To increase the productivity yield by optimizing the air flow rate in outdoor flat-panel reactors. This research is relevant because it fills an information gap in the available literature regarding experimentation with outdoor microalgae culture, on a semi-industrial scale, using flue gases, for a period of 2 full years. Results showed that biomass production and growth rate range from 0.040 ± 0.022 g (dry weight) L -1 d -1 and 0.088 ± 0.044 d -1 to 0.078 ± 0.033 g dwt L -1 d -1 and 0.179 ± 0.079 d -1 , respectively, depending on the environmental conditions. Protein and lipid content range from 34.4 ± 8.163 to 43.5 ± 13.250% dry weight and from 12.5 ± 2.243 to 14.4 ± 0.936% dwt, respectively. The culture was maintained during a complete cycle of two years, with the stated conditions, and demonstrated the technical feasibility and sustainability of the process. Keywords Nannochloropsis gaditana . Semi-industrial scale . Biochemical composition . Flue gases . Culture medium Introduction Many potential uses have been developed for microalgae, including biofertilizers, biopolymers, bioplastics, paints, dyes, colorants, animal feed (Borowitzka 1997; Ledda et al. 2015), and as a source of proteins with potential uses in biomedical applications (Fernandez-Acero et al. 2019) and even energy generation (Macías-Sánchez et al. 2015). Thus the demand for microalgae biomass has increased significantly in recent years. To satisfy this increasing de- mand for biomass, it has become essential to maximize and optimize the production of these types of microorganism (Benemann 1992) and to develop techniques and new strat- egies, with the object of increasing production rates while reducing operational costs. Vuppaladadiyam et al. (2018) presented a review giving a summary of the different costs of the biomass produced depending on the production strat- egy and type of photobioreactor employed to grow microalgae, and Pérez-López et al. (2017) identified the * Alberto Rodríguez-López rdz.lopez.alberto@gmail.com 1 Neoalgae Micro Seaweed Products, Semi-industrial Microalgae Plant, UPT-litoral, Ctra. Faro Mesa Roldán s/n 04140, Carboneras, Almería, Spain 2 Microbiology Laboratory, IVAGRO Marine and Environmental Sciences Faculty, University of Cádiz, Pol. Río San Pedro s/n, Puerto Real, Cádiz, Spain 3 Research Project Manager of Innovation Department (ENDESA Generación, S.A.), Calle Ribera del Loira 60, 28042 Madrid, Spain 4 Department of Chemical Engineering and Food Technology, Science Faculty, University of Cádiz, Avda. República Saharaui s/n, 11510 Puerto Real, Cádiz, Spain 5 Open Innovation at Innovation and Product Lab, Enel Iberia Srl, C/ Ribera del Loira 60, 28042 Madrid, Spain Journal of Applied Phycology https://doi.org/10.1007/s10811-019-01990-8