Sediment samples were taken during May 2017 on research vessel (R/V) Meteor (M137) at four stations (74, 128, 243, and 752 m) along the 12º S depth transect traversing the Peruvian OMZ. Samples were retrieved using a TV-guided multicorer (MUC) equipped with seven core liners as described previously. Core liners were 60 cm long with an inner diameter of 10 cm. Filaments of giant sulfur oxidizing bacteria were observable by eye at the sediment surface (dense mat) and inside sediment at the 128 and 243 m stations. Filaments were detected mostly inside sediment at the 74 m station and were not observed at the 752 m station. Retrieved cores were immediately transferred to cold rooms (12ºC) for further processing. At each station, two small push cores (length 20 cm, inner diameter 2.6 cm) were subsampled from one MUC core. One of the replicate sub-cores (hereafter 'spiked core') was amended with unlabeled sulfide: 16 µl saturated sulfide solution (2.42 M stock solution: 250 g Na2S • 9 H2O in 420 ml ultrapure water) was injected into the sediment push core through pre-drilled holes placed at 1 cm depth increments following the principle of the whole-round core injection method. Based on an average sediment water content of ~80%, the added sulfide resulted in a final sulfide concentration of 10 mM in the porewater after its dilution into the sediment (5 cm3 sediment per injection point). The second sub-core remained untreated (hereafter 'unspiked core'). After an equilibration for 1-2 hrs at 12ºC in the dark, 10 μL of carrier-free 35S-sulfate radiotracer (dissolved in water, 1.86 MBq, specific activity 37 TBq mmol-1) was injected into both sub-cores at 1 cm depth increments according to the whole-core injection method. In the spiked core, radiotracer was injected through the same ports used for sulfide injection. Both spiked and unspiked cores were incubated with radiotracer for 6-8 hrs at 12ºC in the dark. After incubation, bacterial activity was stopped by slicing sub-cores at 1 cm increments and transferring sediment layers into 50 mL plastic centrifuge tubes filled with 20 mL zinc acetate (20 % w/w). Triplicate 'killed' controls were produced from additional sediment of the same MUC core and microbial activity was first terminated with zinc acetate before the addition of radiotracer to the centrifuge vial. All samples were frozen at -20ºC until analysis, when sulfate reduction rates were determined following the cold chromium distillation procedure.
