Microalgae can be used to upgrade biogas to biomethane and subsequently be digested for biogas production. However, the low C:N ratio of species such as Arthrospira platensis may cause ammonia inhibition and low process stability during anaerobic digestion. This study investigates co-fermentation of A. platensis with carbon-rich co-substrates (barley straw, beet silage and brown seaweed) at a C:N ratio of 25 to enhance biomass conversion. No synergistic effects on biomethane potential could be proven in batch fermentation tests. However continuous digestion trials showed significantly improved process stability. Mono-digestion of A. platensis was stable only at an organic loading of 1.0 g VS L-1 d(-1). The optimum process co-digested A. platensis with seaweed and achieved stable operation at an organic loading of 4.0 g VS L-1 d(-1). Co-digestion of microalgae and seaweed can be effectively applied to integrated coastal biomethane systems. (C) 2016 Elsevier Ltd. All rights reserved.