Anaerobic digestion of microalgae has been enhanced by several pretreatments; however the reported net energy ratio was negative. In order to cope with this issue, this investigation focused on thermal pretreatment (120 degrees C for 40 min) at increasing biomass loads of Chlorella vulgaris and Scenedesmus sp. During that thermal pretreatment, carbohydrates solubilisation prevailed over proteins for both strains. Regardless the biomass load pretreated, anaerobic biodegradability of C. vulgaris was enhanced by 50% and therefore, pretreatments of high biomass loads was suggested to counterbalance the energy input. On the other hand, thermally pretreated Scenedesmus sp. biomass supported an enhancement of 21-27%. The specific cell wall composition was suggested as a potential reason for the differences registered on their anaerobic biodegradabilities. (C) 2014 Elsevier Ltd. All rights reserved.