Response surface methodology (RSM) was used to optimize the production of volatile fatty acids (VFAs) and hydrogen from mixed anaerobic cultures of Saccharina japonica with respect to two independent variables: methanogenic inhibitor concentration and temperature. The effects of four methanogenic inhibitors on acidogenic processes were tested, and qualitative microbial analyses were carried out. Escherichia, Acinetobacter, and Clostridium were the most predominant genera in samples treated with chloroform (CHCl3), iodoform (CHI3), 2-bromoethanesulfonate (BES), or beta-cyclodextrin (beta-CD), respectively. RSM showed that the production of VFAs reached a peak of 12.5 g/L at 38.6 A degrees C in the presence of 7.4 g/L beta-CD; these were the conditions under which hydrogen production was also nearly maximal. The quantitative polymerase chain reaction (qPCR) showed that shifts in the bacterial community population correlated with the concentrations of beta-CD indicating that this compound effectively inhibited methanogens.