The production of fatty acid methyl esters (FAMEs, biodiesel) was optimized from wet Nannochloropsis gaditana microalgal biomass (81.8 wt% water and 28.1 wt% saponifiable lipids, SLs, from the dry biomass). FAME production was carried out by direct acid-catalyzed methylation of the SLs from the microalgal biomass and FAME extraction with hexane. A three-variable, three-level Box-Behnken design (BBD) was applied to optimize the sulphuric acid (catalyst) concentration, the methanol/SL ratio and the hexane/SL ratio. The best FAME yield was obtained with a methanol/SL ratio of 254 mL/g, a sulphuric acid concentration of 3.7% (v/v) and a hexane/SL ratio of 107.7 mL/g at 100 degrees C and for 105 min. Under these conditions, 100% of the SLs were transformed into FAMEs. Finally, the FAME purity was increased from 78.7 to 86.8 wt% using an adsorption treatment with bentonite. (C) 2018 Elsevier Ltd. All rights reserved.