Sulfate Transport in the Sulfate Reducing Bacterium; Desulfovibrio Gigas

The characteristics and kinetics of sulfate uptake and reduction by Desulfovibrio gigas were studied using radio-labelled sulfate, in order to assess the nature of terminal electron acceptor transport with respect to current membrane transport theory. Although sulfate is not accumulated into an intracellular pool, it is transported across the cell membrane prior to reduction as evidenced by studies on the localization of sulfate binding and reduction by cellular fractions. Sulfate is rapidly bound by whole cells to a stead, -state level which is a function of extracellular sulfate concentration and the availability of an electron donor. Hydrogen sulfide evolution begins immediately upon cellular exposure to sulfate and is linear with time at a rate which is also a function of sulfate concentration and the availability of an electron donor. Pyruvate stimulates both uptake and reduction activities in whole cells to maximal levels, while lactate and molecular hydrogen do not stimulate initial activities above endogenous levels. The same is true for reduction of sulfate by cell free extracts. It is concluded that the phosphoroclastic cleavage of pyruvate couples to sulfate reduction. Uptake and reduction activities are therefore respiratory coupled, and act as a single unit in vivo. The rate of hydrogen sulfide production reflects the rate of steady state sulfate transport since the cellular level is constant at any given sulfate concentration. It was observed that the Michaelis constant for sulfate uptake was identical to that for reduction, 2.0 mM. Evidence for system specificity was derived from kinetic measurements of sulfate uptake in the presence of Group VI anions and other sulfur compounds. Proton ionophores, uncouplers of oxidative phosphorylation, and membrane potential inhibitors are effective inhibitors of sulfate uptake which indicates that sulfate metabolism is dependent upon cellular energy metabolism and an energized membrane state. Kinetic analysis of initial sulfate uptake with varying pH indicates that the kinetic mechanism of sulfate uptake is a rapid equilibrium ordered sequence with protons adding to the cell carrier first, followed by sulfate addition to a protonated carrier. It is concluded that transport of sulfate, the terminal electron acceptor in D. gigas, proceeds in a fashion consistent with the chemiosmotic theory of membrane transport.