Algae may be fermented to produce hydrogen. However micro-algae (such as Arthrospira platensis) are rich in proteins and have a low carbon/nitrogen (C/N) ratio, which is not ideal for hydrogen fermentation. Co-fermentation with macro-algae (such as Laminaria digitata), which are rich in carbohydrates with a high (C/N) ratio, improves the performance of hydrogen production. Algal biomass, pre-treated with 2.5% dilute H2SO4 at 135 degrees C for 15 min, effected a total yield of carbohydrate monomers (CMs) of 0.268 g/g volatile solids (VS). The CMs were dominating by glucose and mannitol and most (ca. 95%) were consumed by anaerobic fermentative micro-organisms during subsequent fermentation. An optimal specific hydrogen yield (SHY) of 85.0 mL/g VS was obtained at an algal C/N ratio of 26.2 and an algal concentration of 20 g VS/L. The overall energy conversion efficiency increased from 31.3% to 54.5% with decreasing algal concentration from 40 to 5 VS g/L. (C) 2016 Elsevier Ltd. All rights reserved.