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Effect of Low Sulfate Concentrations on Lactate Oxidation and Isotope Fractionation during Sulfate Reduction by Archaeoglobus fulgidus Strain Z

Danish Center of Earth System Science, University of Southern Denmark, Institute of Biology, Campusvej 55, DK-5230 Odense M, Denmark

Received 27 August 2004/ Accepted 19 January 2005

The effect of low substrate concentrations on the metabolic pathway and sulfur isotope fractionation during sulfate reduction was investigated for Archaeoglobus fulgidus strain Z. This archaeon was grown in a chemostat with sulfate concentrations between 0.3 mM and 14 mM at 80°C and with lactate as the limiting substrate. During sulfate reduction, lactate was oxidized to acetate, formate, and CO₂. This is the first time that the production of formate has been reported for A. fulgidus. The stoichiometry of the catabolic reaction was strongly dependent on the sulfate concentration. At concentrations of more than 300 µM, 1 mol of sulfate was reduced during the consumption of 1 mol of lactate, whereas only 0.6 mol of sulfate was consumed per mol of lactate oxidized at a sulfate concentration of 300 µM. Furthermore, at low sulfate concentrations acetate was the main carbon product, in contrast to the CO₂ produced at high concentrations. We suggest different pathways for lactate oxidation by A. fulgidus at high and low sulfate concentrations. At about 300 µM sulfate both the growth yield and the isotope fractionation were limited by sulfate, whereas the sulfate reduction rate was not limited by sulfate. We suggest that the cell channels more energy for sulfate uptake at sulfate concentrations below 300 to 400 µM than it does at higher concentrations. This could explain the shift in the metabolic pathway and the reduced growth yield and isotope fractionation at low sulfate levels.

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