The temperature dependence of oxic minerahzation processes in perenn~ally cold coastal sediments from Arctic Svalbard, Norway, was determined in short-term incubations at -1 to 44'C and compared to similar incubations with warm temperate sediment. For oxygen respirat~on, nitrogen mineralization, and nitrification, adaptations to low temperature were evident with the microbial comrnunities from Svalbard. Oxygen respiration rates showed the same temperature dependence at all sites around Svalbard, with relatively high rates at O°C and a linear 3-to 4-fold increase from 0°C to a mean optimum temperature of 19.ZoC, whereas rates in the temperate sediment were close to zero at O°C and had optimum at 30 to 40°C. The temperature dependence of nitrogen mineralization was comparable to that of oxygen respiration, and C:N mineralization ratios in the Svalbard sediments were stable at 6 to 8 below 20°C. Thus, low temperature did not affect carbon and nitrogen mineralizatlon differentially. The most prominent adaptation to low temperatures was observed for nitrification, which had a mean optimum temperature of 14.0°C at Svalbard and decreased rapidly in rate at higher temperatures. In the warm temperate sediment the nitrification optimum was near 40°C. The catalytic efficiency of the nitrifying communities from Svalbard, at their in situ temperature, was as high as that reported for cornmunities from temperate regions. This implied that thermal adaptation fully compensated for direct temperature effects on this metabolism.

We have explored the sulfur metabolism and accompanying fractionation of sulfur isotopes during the disproportionation of elemental sulfur by seven different enrichments and three pure bacterial cultures. Cultures were obtained from both marine and freshwater environments. In all cases appreciable fractionation accompanied elemental sulfur disproportionation, with two ranges of fractionation observed. All cultures except Desulfobulbus propionicus produced sulfide depleted in 34S by between 5.5 and 6.9 per mil (avg of 6.3 per ml) and sulfate enriched in 34S by between 17.1 and 20.2 per mil (avg of 18.8 per .mI). The narrow range of fractionations suggests a conserved biochemistry for the disproportionation of elemental sulfur by many different marine and freshwater bacteria. Fractionations accompanying elemental sulfur disproportionation by Db. propionicus were nearly twice as great as the others, suggesting a different cellular level pathway of sulfur processing by this organism. In nearly every case pyrite formation accompanied the disproportionation of elemental sulfur. By using sulfur isotopes as a tracer of sulfur source, we could identify that pyrite formed both by the addition of elemental sulfur to FeS and from reaction between FeS and H,S. Both processes were equally fast and up to 104-lo5 times faster than expected from the reported kinetics of inorganic pyrite-formation reactions. We speculate that bacteria may have enhanced rates of pyrite formation in our experimental systems. The organisms explored here have different strategies for growth and survival, and they may be active in environments ranging from dissolved sulfide-poor suboxic sediments to interfaces supporting steep opposing gradients of oxygen and sulfide. A large environmental range, combined with high bacterial numbers, significant isotope fractionations, and a possible role in pyrite formation, make elemental sulfur-disproportionating bacteria potentially significant actors in the sedimentary cycling of sulfur compounds.Over the past 10 years newly discovered bacterial metabolisms of the sulfur cycle have considerably altered our view of sedimentary sulfur cycling. Bacteria conducting thiosulfate and sulfite disproportionation were first described in 1987 (Bak and Pfennig 1987), with high bacterial numbers of up to 2 X lo6 cells ml-l also reported (Jorgensen and Bak 1991). Both thiosulfate and sulfite disproportionation are completely anaerobic processes producing sulfate and sulfide as endproducts. Radiotracer experiments suggest a considerable role for thiosulfate disproportionation in marine and freshwater sediments (Jorgensen 1990a,b; Jgrgensen and Bak 1991). Thamdrup et al. (1993) discovered that marine sediments enriched with only elemental sulfur and iron oxides produced bacterial cultures whose livelihood is coupled to the disproportionation of elemental sulfur to sulfate and sulfide.-_ 1 Present

Small-subunit ribosomal RNA (SSU rRNA) has several characteristics making it a good candidate biomarker compound: it is found in bacteria, archaea and eukaryotes; it is quickly degraded extracellularly, hence SSU rRNA extracted from a sample probably derives from the currently active population; it includes both conserved and variable regions, allowing the design of capture probes at various levels of phylogenetic discrimination; and rRNA sequences from uncultured species can be classified by comparison with the large and growing public database. Here we present a method for isolation of specific classes of rRNAs from mixtures of total RNA, employing biotin-labelled oligonucleotide probes and streptavidin-coated paramagnetic beads. We also show that the stable carbon isotope composition of Escherichia coli total RNA and SSU rRNA reflects that of the growth substrate for cells grown on LB, M9 glucose and M9 acetate media. SSU rRNA is therefore a promising biomarker for following the flow of carbon, and potentially nitrogen, in natural microbial populations. Some possible applications are discussed.