Table T14. Procedure for sulfate reduction measurements.

Table T14. Procedure for sulfate reduction measurements.


1) In the cold room, whole-round cores were cleanly and anoxically cut and transferred into an N₂-gassed (filtered) sampling bucket. After removing 1 cm of surface sediment, four 5-cm³ subsamples were taken with sterile syringes with the Luer end removed.
2) The syringes were closed gas-tight with sterile butyl rubber and placed in a N₂-filled aluminum bag for transport.
3) After transfer to the radioisotope van, the syringe samples were kept at in situ temperature in a thermostated incubator to equilibrate before further processing.
4) To start the experiment, 10 µL of a ³⁵SO₄^2- solution was injected by microsyringe into three of the four parallel syringes through the butyl rubber stopper and along a 1- to 2-cm line in the middle of the sediment. The radiotracer stock solution for injections was made up as ³⁵SO₄^2- in sterile saline solution (containing sulfate concentrations appropriate for the site and depth) by diluting a commercial stock solution (Perkin-Elmer/NEN, 185 MBq).
5) Three types of blank measurements were performed:
a. From each vial of radiolabeled solution, a 10-µL aliquot was fixed in Zn acetate solution.
b. At selected depths, a "time-zero" control was performed on a syringe subsample. The radiolabeled sulfate was injected into the sediment and allowed to incubate for ~30 min (<1% of the total incubation time), after which time it was fixed in Zn acetate solution.
c. A third "killed" control consisted of adding the sediment to the Zn acetate solution. After 30 min, 10 µL of ³⁵SO₄^2- was then added to the Zn acetate-sediment slurry.
All controls are distilled and counted as for live samples.
6) The radiolabeled syringe samples were placed in N₂-filled gas bags. Two squares of an oxygen scrubber tray (Anaerocult A; Merck) were added to the bag and wetted. The bags were sealed and placed in the appropriate incubator.
7) At the conclusion of the incubation period, ranging from 7 to 42 days, the bacterial activity was stopped by mixing the sediment into pre-tared 50-mL Corning screw-top vials containing 14 mL of 20% (w/v) zinc acetate solution. The zinc solution kills the bacteria and preserves any free sulfides by precipitating them as ZnS.
8) These fixed samples were transported frozen to the shore-based institute in Bremen, Germany, for further processing. For safe handling and transport, all vials were packed break-proof in original styrofoam, double-bagged, and packed in a International Air Transport Association approved carton (shipped as UN2910-Radioactive material, excepted package—limited quantity of material).
9) The processing (as modified from Fossing and Jørgensen, 1989) involves the separation of the injected ³⁵SO₄^2- and the reduced ³⁵S-sulfide (in total reduced inorganic sulfides) in chromous acid solution by cold distillation of the released H₂³⁵S into Zn-acetate traps. By measurement of the ³⁵SO₄^2- and Zn³⁵S radioactivities on a liquid scintillation counter, the fraction of sulfate reduced during the experiment can be determined. This is converted to total amount of sulfate reduced per unit time (nmol SO₄^2-/cm³/day) based on the interstitial water concentration of sulfate, the porosity, and the incubation time (cf. Jørgensen, 1978).