In this work, Chlorella vulgaris ESP6 was used to assimilate the soluble metabolites in the hydrogen fermentation broth of Clostridium butyricum CGS5 to obtain valuable microalgae biomass. The results show that C vulgaris ESP6 could grow on the 4-fold diluted dark fermentation broth. Acetate was efficiently utilized during the growth of C vulgaris ESP6, whereas the microalgae growth was inhibited by lactate, butyrate, and HCO3- when their concentrations were higher than 0.5, 0.1, and 2.72 g/L, respectively. C. vulgaris ESP6 could completely consume butyrate (the most abundant dark fermentation metabolite) when it was grown on Tris-Acetate-Phosphate medium under a food to microorganism (F/M) ratio of 1.11. (C) 2012 Elsevier Ltd. All rights reserved.