Ulva biomass was evaluated as a co-substrate for anaerobic digestion of spent coffee grounds at varying organic loads (0.7-1.6 g chemical oxygen demand (COD)/L d) and substrate compositions. Co-digestion with Ulva (25%, COD basis) proved beneficial for SCG biomethanation in both terms of process performance and stability. The beneficial effect is much more pronounced at higher organic and hydraulic loads, with the highest COD removal and methane yield being 51.8% and 0.19 L/g COD fed, respectively. The reactor microbial community structure changed dynamically during the experiment, and a dominance shift from hydrogenotrophic to aceticlastic methanogens occurred with increase in organic loading rate. Network analysis provides a comprehensive view of the microbial interactions involved in the system and confirms a direct positive correlation between Ulva input and methane productivity. A group of populations, including Methanobacterium-and Methanoculleus-related methanogens, was identified as a possible indicator for monitoring the biomethanation performance. (C) 2017 Elsevier Ltd. All rights reserved.