Intracellular phosphorous compounds and the reversibility of dissimilatory sulfate reduction: what do we learn from oxygen isotopes?

Dissimilatory sulfate reduction (DSR) leads to an overprint of the oxygen isotope composition of sulfate by the oxygen isotope composition of water. This overprint is assumed to occur via cell-internally formed sulfuroxy intermediates in the sulfate reduction pathway. Unlike sulfate, the sulfuroxy intermediates can readily exchange oxygen isotopes with water. Subsequent to the oxygen isotope exchange, these intermediates, e.g. sulfite, are re-oxidized by reversible enzymatic reactions to sulfate, incorporating the oxygen used for the re-oxidation of the sulfur intermediates. Consequently, the rate and expression of DSR-mediated oxygen isotope exchange between sulfate and water depends not only on the oxygen isotope exchange between sulfuroxy intermediates and water, but also on cell-internal forward and backward reactions. The latter are the very same processes that control the extent of sulfur isotope fractionation expressed by DSR. In the stepwise reduction of sulfate to sulfide, intracellular phosphorous compounds are pivotal for the conversion of intracellular sulfate to sulfite. Because of thermodynamics, the concentration of thereby produced intracellular phosphorous compounds affects the reversibility of this reduction step and thereby impacts the oxygen isotope composition of sulfate. Consequently, there should be a link between cell-internal management of phosphorous pools and the expression of sulfur and oxygen isotope effects. The measurement of multiple sulfur isotope fractionation has successfully been applied to obtain information on the reversibility of individual enzymatically catalyzed steps in DSR. Similarly, also the oxygen isotope signature of sulfate reveals information on the reversibility of DSR. High reversibility (i.e. large isotope effects) is generally assumed to be tied to low energy availability. This raises the question if and how cell-internal management of phosphorous pools could be tied to survival strategies under energy limitation.