Effects of Mo(VI) on phototrophic hydrogen production by Rhodobacter sphaeroides

Effects of Mo(6+) concentration on phototrophic hydrogen production of Rhodobacter sphaeroides were investigated using lactate as the sole carbon source. Results showed that an increase of Mo(6+) from nil to 1000 microg l(-1) led to increases in hydrogen yield, maximum production rate, conversion efficiency, biomass yield and lactate removal. At 100 microg-Mo l(-1), the maximum rate was 12.0 ml h(-1) with a conversion efficiency of 36.1%, the cell yields were 1.11 g-cell g(-1) -lactate and 2.4 g-cell g(-1)-TOC removed. Further increase of Mo(6+) improved hydrogen production only marginally. Degradation of lactate by R. sphaeroides produced not just hydrogen but also acetate, butyrate, i-valerate, i-caproate, hexanoate and some unidentified organic intermediates, but did not produce propionate and alcohols. Nitrogenase activity, as measured by the acetylene reduction method, had no clear correlation with either Mo(6+) concentration or hydrogen yield.