*4.1. Microorganism and cultivation conditions*

*Coccomyxa acidophila,* the algal material used in this work, was isolated from the acidic water of the Tinto River's mining area, in Huelva (Spain). 

Initially, an axenic culture of the microalga was obtained by streaking it on basal agar medium at 

pH 2.5. After that, isolated colonies were transferred from the solid medium to a liquid medium modified by Silverman and Lundgren [29]. *Coccomyxa acidophila* mother cultures were maintained by periodic transfers in sterile medium adjusted to pH 2.5 with concentrated H2SO4. Unless otherwise indicated, standard cultivation conditions were batch cultures grown at 25 ºC into 1 L-Roux flasks, bubbled with air containing 5% (v/v) CO2 and continuously illuminated with fluorescent lamps (Philips TLD, 30 W, 150 μE·mƺ2·sƺ1 at the surface of the flasks). In those cases where CO2 was not supplied to the cultures, it was necessary to put a carbon dioxide trap with KOH 5 M buffer for removing it from the air mix. Every day, pH was controlled and adjusted at 2.5 ± 0.1 by adding diluted HCl or NaOH. 

The irradiance was measured with a quantum/photometer Licor (mod. LI 250A). 
