**Outdoor Large-Scale Cultivation of the Acidophilic Microalga** *Coccomyxa onubensis* **in a Vertical Close Photobioreactor for Lutein Production**

**Juan-Luis Fuentes 1, Zaida Montero 1, María Cuaresma <sup>1</sup> , Mari-Carmen Ruiz-Domínguez <sup>2</sup> , Benito Mogedas 1, Inés Garbayo Nores 1,\* , Manuel González del Valle <sup>3</sup> and Carlos Vílchez <sup>1</sup>**


Received: 6 February 2020; Accepted: 3 March 2020; Published: 10 March 2020

**Abstract:** The large-scale biomass production is an essential step in the biotechnological applications of microalgae. *Coccomyxa onubensis* is an acidophilic microalga isolated from the highly acidic waters of Río Tinto (province of Huelva, Spain) and has been shown to accumulate a high concentration of lutein (9.7 mg g−1dw), a valuable antioxidant, when grown at laboratory-scale. A productivity of 0.14 g L−<sup>1</sup> d−<sup>1</sup> was obtained by growing the microalga under outdoor conditions in an 800 L tubular photobioreactor. The results show a stable biomass production for at least one month and with a lutein content of 10 mg g<sup>−</sup>1dw, at pH values in the range 2.5–3.0 and temperature in the range 10–25 ◦C. Culture density, temperature, and CO2 availability in highly acidic medium are rate-limiting conditions for the microalgal growth. These aspects are discussed in this paper in order to improve the outdoor culture conditions for competitive applications of *C. onubensis*.

**Keywords:** carotenoids; extremophiles; microalgal biotechnology
