Dissimilatory sulfate reduction (DSR) is a key process in the anaerobic degradation of organic matter in marine sediments. The overall reaction can be written as 2 CH 2 O + SO 4 2-→ 2 HCO 3 -+ H 2 S.Sulfate-reducing bacteria reduce sulfate to sulfide, which either remains in solution as hydrogen sulfide or precipitates as various forms of metal (mostly iron), mono-and disulfides, or elemental sulfur (ES). These combined inorganic end products of sulfate reduction are termed total reduced inorganic sulfur (TRIS). The importance of DSR as a terminal electron acceptor process in marine sediments has been established through measurements of sulfate reduction rates (SRR) by using carrier-free radiolabeled 2-radiotracer into the undisturbed sediment followed by incubation for hours to days. Incubations may be terminated by freezing or transferring the radiolabeled sediment into 20% (w/v) zinc acetate solution. By comparing the activity of the radiolabeled TRIS to the total sulfate radiotracer a reduction rate can be calculated as shown below in Eq. 1.(1) a TOT t where SRR is the sulfate reduction rate (nmol cm -3 d -1); [SO 4 ] is the sulfate concentration in the porewater of the sediment sample (mmol L -1 ); P SED is the porosity of the sediment (mL porewater cm -3 sediment); a TRIS is radioactivity of TRIS (counts per minute [cpm] or decays per minute [dpm]); a TOT is total radioactivity used (cpm or dpm); t is incubation time in days; 1.06 is the correction factor for the expected isotopic fractionation (Jørgensen and Fenchel 1974); 1000 is the factor for the change of units from mmol L -1 to nmol cm AbstractThe separation and detection of extremely small amounts of radiolabeled reduced sulfur species for the determination of very low sulfate reduction rates (SRR) with the 35 SO 4 2-radiotracer was improved by optimization of the entire distillation and detection process. By reducing the amount of background radioactivity, the threshold from which turnover of radiotracer can be detected was lowered considerably. Reduction of the background radioactivity was achieved by (1) reducing cross-contamination between distillations by modifying the distillation setup, and (2) preventing an unidentified 35 S-containing compound that greatly contributes to the background from reaching the final trap by lowering the distillation temperature. These improvements allow the measurement of low SRR, shorter incubation times, and the use of less radiotracer. Experiments with pure sulfur minerals and a variety of sediments verified that the efficiency of the new method is equal to the hot single-step chromium reduction method, but with a greatly improved reproducibility.

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The non-selective channel for monovalent cations of cultured brown adipocytes was studied concerning its permeability to alkali metal ions, NH4+, Tris+, Ca2+, and Ba2+. Experiments were done by means of the patch clamp technique using inside-out patches. With symmetrically increasing sodium concentrations the ion fluxes saturated. They are described by a dissociation constant (KNa) of 155 mmol/l and a maximum single channel conductance of 50 pS. Permeabilities were determined in relation to those for sodium yielding values of 0.80 for potassium and 1.55 for ammonium. The complete permeability sequence for ammonium and the alkali metals is: NH4+ greater than Na+ greater than Li+ greater than K+ greater than or equal to Rb+ congruent to Cs+ . Ca2+ and Ba2+ as well as the buffer ion Tris+ are not able to pass the channel measurably. It is shown that the conductance behaviour of the non-selective channel is not sufficiently described by the Goldman-Hodgkin-Katz theory. Deviations from independence are saturation with increased activity of the permeant ion and non-linear current voltage relations in symmetrical solutions. A simple two barrier model with one binding site in the center of the electric field is shown to be more appropriate.