The objective of this work was to determine the benefit of thermal pretreatment on biogas yield from microalgae-fed anaerobic digester mesocosms. Replicate Nanochloropsis oculata cultures were heated for 4 h at 30, 60, and 90 degrees C, as well as at a constant temperature of 90 degrees C for 1, 3.5, and 12 h. Net biogas production increased from 0.28 L biogas/g volatile solids added (VSa) for the control to 0.39 L biogas/g VSa (p < 0.01) when heated at 90 degrees C, but there was no improvement at 30 or 60 degrees C. Increased biogas production correlated with increased soluble chemical oxygen demand (COD). Net biogas production increased as a function of heating time, from 0.32 L biogas/g VSa for the control, to 0.41, 0.43, and 0.44 L biogas/g VSa (p < 0.05 for all combinations vs. control) when preheated at 90 degrees C for 1, 3.5, and 12 h, respectively. However, despite enhanced biogas production the energy balance is negative for thermal pretreatment. (C) 2013 Elsevier Ltd. All rights reserved.