Half-saturation constants, sulfate

Following the calculations in Section 18.5, we take a rate constant k+ for sulfate reduction of 10-9 mol mg-1 s-1, a half-saturation constant for acetate of 70 p, molal, and a growth yield of 4300 mg mol-1 from a study of the kinetics of Desulfobacter postgatei by Ingvorsen el al. (1984). We set a value for KA, the half-saturation constant for sulfate, of 200 p molal, as suggested by Ingvorsen el al. (1984) and Pallud and Van Cappellen (2006). 

Much confusion exists over the effects of sulfate concentration and carbon availability on rates of sulfate reduction (cf. 1, 4, 5, 72, 85, 106). Sulfate reducers in lake sediments exhibit low half-saturation constants for sulfate (10-70 xmol/L 4, 72, 78, 85) as well as for acetate and hydrogen (4, 13, 87). The low half-saturation constants allow them to outcompete methano-gens for these substrates until sulfate is largely consumed within pore waters (4, 90). Low concentrations of sulfate in lakes confine the zone of sulfate reduction to within a few centimeters of the sediment surface (e.g., 4, 90, 98). The comparability of rates of sulfate reduction in freshwater and marine... 

Methane oxidation kinetics was assessed as follows. In these experiments, cells were inoculated into medium containing different initial amounts of copper sulfate and grown to an optical density at 600 nm of 0.5-0.7. Aliquots were then placed in closed vials at different initial dissolved methane concentrations. These aliquots were incubated under optimal conditions, and head-space samples were taken at four different time points (1-4 h) for determination of methane concentrations by gas chromatography. From these data, initial methane consumption rates determined for different methane concentrations were used to obtain the Michaelis-Menten parameters Ks (half-saturation constant) and Vmax (rate at substrate saturation). Under these conditions sMMO was not expressed, as described previously (9). 

The true physiological role of these reductases in phytoplankton is not known and it is unclear whether electron transport out of the cell occurs in nature. Although the oxidation and reduction of the extracellular solutes may just be an adventitious reaction of these enzymes with no significance to the microorganism, under certain conditions, such as the presence of favorable redox couples, such electron export may occur. Because of the high half-saturation constants measured for metal reduction by Jones et al. (1987), it seems unlikely that these or similar trace metal complexes are the major electron acceptors in nature. We cannot rule out, however, the possibility of reduction of other (major) solutes such as sulfate or intermediate redox sulfur species. 

Place 200 cc. of concentrated ammonia (25 percent) and 50 cc. of water in a flask and conduct in hydrogen sulfide to saturation (closed flask). Divide the solution into two equal portions. Saturate the one with sulfur (60 g.) at 40°, filter, and add to the other half. Prepare a solution of 20 g. of crystallized copper sulfate in 200 cc. of water and add slowly and with constant shaking to the first solution until a permanent precipitate results (CuS). Filter as rapidly as possible through a large folded filter and bring the filtrate into an Erlenmeyer flask which is of a size to be nearly filled by the solution. By allowing the solution to stand, especially if cooled by ice, reddish crystals separate which should be filtered off the next day, washed with a little water and with alcohol, and dried over lime in a desiccator. To the mother liquor more copper sulfate can be added, and the process repeated. 

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