The increase of inoculum density of Rhodobacter sphaeroides B-3059 above 2.0 mg bacteriochlorophyll/1 in 15 mM K-phosphate buffer inhibited hydrogen photoproduction but favored PHB synthesis. This inhibition was substrate-specific and was observed on media containing acetate. The inhibition resulted from sharp increase in pH above 10 due evidently to fast acetate consumption. The lower the buffer concentration, the lower the inhibitory inoculum density was. The dependence of H-2 photoproduction from acetate on buffer concentration at low and high inoculum density was different for different strains of purple bacteria. However, conditions were found to provide H-2 production by each of 9 tested strains. Acetate-dependent hydrogen photoproduction in 10 mM K-phosphate buffer was unreliable even with low inoculum density. Ammonium traces and high acetate concentration also facilitated medium alkalization and, consequently, inhibited H-2 production. The increase of buffer concentration above 20 mM helped to prevent pH rise independent of the triggering factor. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.