Kinetics of microbially mediated reactions: dissimilatory sulfate reduction in saltmarsh sediments (Sapelo Island, Georgia, USA)

A sediment disk reactor was tested in once flow-through mode to retrieve kinetic parameters for the Monod rate law that describes sulfate reduction. The experimental method was compared with a previously described procedure by the authors where a sediment plug-flow reactor was operated in a recirculation mode. In recirculation mode, accumulation of metabolic byproducts in certain cases may result in negative feedback, thus preventing accurate determination of kinetic information. The method described in this article provides an alternative to the recirculation sediment plug-flow-through reactor technique for retrieving kinetic parameters of microbially mediated reactions in aquatic sediments. For sulfate reduction in a saltmarsh site, a maximum estimate of the half-saturation concentration, Ks, of 204 26 lM and a maximum reaction rate, Rm, of 2846 129 nmol cm3 ðwet sedimentÞ d 1 was determined. The Ks value obtained was consistent with the one estimated previously ðKs ¼ 240 20 lMÞ from a different site within the same saltmarsh mud flat using a recirculating reactor. From the Rm value and reduction rates determined using 35SO4 2 incubation experiments, we infer that sulfate reduction is limited in the field. Substrate availability is not the main contributor for the limitation, however. Competition from other microbes, such as iron reducers affects the activity of sulfate reducers in the suboxic to anoxic zones, whereas aerobes compete in the oxic zone. High sulfide concentration in the pore water may also have acted as a toxin to the sulfate reducers in the field.