Lawrence Berkeley National Laboratory

D. vulgaris, a sulfate reducing bacterium, is known to grow well on lactate or pyruvate using sulfate as the electron acceptor anaerobically (5 to 7 hours generation time). D. vulgaris can also ferment pyruvate without any electron acceptor. Although all added pyruvate (60 mM) is consumed, growth of D. vulgaris by pyruvate fermentation is slow (generation time over 100 hours) and the final protein yield is low, about 1/3 that of pyruvate or lactate respiration. When pyruvate is oxidized to acetate, three products are resulted, electrons, protons, and CO2. Hydrogen production through the many hydrogenases of D. vulgaris can serve to rid the cell of electrons in addition to the production of formate, lactate, ethanol, or succinate. To understand the metabolic pathway of pyruvate fermentation, the possible inhibition of metabolic end products on pyruvate fermentative growth was investigated. D. vulgaris pyruvate fermentation was very sensitive to added H2 gas in a sealed growth tube. It also was sensitive to added CO2 gas. Furthermore the fermentation was slowed by adding NaHCO3, Na2CO3, formate, lactate, or ethanol. With an inert gaseous headspace, growth by pyruvate fermentation was proportional to headspace volumes of the growth tube. Fermentation end products analyzed by HPLC revealed that when the fermentation was inhibited by added H2 or CO2 gas, pyruvate oxidation was slowed proportionally. Twenty six deletion mutants of D. vulgaris were chosen for fermentation growth studies. Some mutants (12 of them) had no effect on pyruvate fermentation. Others (13 of them) either showed no growth [JW381 (nhaD-1) and JW9021 (qmoABC)] or slower growth on pyruvate fermentation than the wild type. One deletion mutant, JW385 (cycA), showed better growth and larger final protein yields than the wild type when fermenting pyruvate.