© 2002 Blackwell Science Ltd Microenvironments and microbial community structure in sediments water interface. Microbial mats are laterally compressed ecosystems in surface sediments, which support most of the major biogeochemical cycles within a vertical dimen- sion of only a few millimetres (Paerl and Pinckney, 1996). Consequently, the understanding of interactive influences between oxygen, sulphide, manganese, iron, redox po- tential, pH and microorganisms requires microtechniques capable of probing these interactions. We describe the application of such techniques to the sediment–water interface of Sælen lake, a semi-enclosed basin connected to the fjord system of the Norwegian coast. Sælen lake is a meromictic lake and exhibits a wide variety of oxygen conditions, from seasonal fluctuations in the shallow parts to permanent anoxia in the central basin. One notable advantage of using such an environ- ment, is that the burrowing activities of macro-organisms is minimal because few of these organisms survive anoxic conditions and, consequently, only chemical and micro- biological reactions can contribute to the chemical fluxes between sediment and the water column. The microbial ecology of this ‘model environment’ has been extensively studied previously (Indrebø et al., 1979a; b; Heldal et al., 1996; Øvreås et al., 1997). The aim of this study was to explore the potential of a combined chemical and micro- biological approach as part of a study of organic carbon oxidation processes in Saelen lake sediments. We were particularly interested in examining the participation of the microbial mat community in the Fe and Mn cycling at the sediment–water interface. Results Chemical characterization of the biofilm in surface sediment In October 2000, O 2 , H 2 S, pH and redox potential were measured across the sediment–water interface of Sælen lake and data corresponded to daylight conditions. The depth profiles are shown in Fig. 1A and B. Oxygen satu- ration increased from 45% in bottom water to 50% just above the sediment–water interface (Fig. 1A) and then decreased to 0% saturation at 1 mm depth into the sedi- ment (Fig. 1B). There was a 0.2 mm overlap between the O 2 and H 2 S profiles. pH showed a maximum of 7.53 near the sediment–water interface followed by a steep decrease just below. The redox potential was relatively high (+500 mV) in bottom water and in the 0–0.9 mm zone Environmental Microbiology (2002) 4(2), 97–105 S. P. C. Tankéré, 1 * D. G. Bourne, 1 F. L. L. Muller 2 and V. Torsvik 1 Departments of 1 Microbiology and 2 Chemistry, University of Bergen, Bergen, Norway. Summary The aim of this study was to explore the potential of a combined chemical and microbiological approach as part of a study of organic carbon oxidation pro- cesses in sediments. An assessment of microbiologi- cal diversity using molecular techniques was carried out in combination with high resolution chemical measurements at the sediment–water interface of a coastal lagoon affected by eutrophication in autumn 2000. There was a 0.2 mm overlap between the O 2 and H 2 S profiles. pH showed a maximum just above the sediment–water interface coinciding with an oxygen maximum, suggesting photosynthetic activity, and a minimum coinciding with the O 2 –H 2 S interface. The redox potential was high in bottom water and sur- face sediment, reflecting the presence of oxygen and oxides, and reached low values after a step-wise decrease at –18 mm. Reduction of Fe occurred within the biofilm at the O 2 –H 2 S interface and was mostly due to reduction by H 2 S. The elevated concentrations of dissolved Mn in the oxic water may have been caused either by in situ production within organic aggregates or lateral water flow from sites nearby at which Mn 2+ diffuses out of the sediment. Sequences related to sulphur chemolitotrophs were retrieved from the biofilm samples, which is consistent with the small overlap between O 2 and H 2 S observed in this biofilm. Although the resolution of techniques used was different, sequencing results were consistent with chemical data in delineating the same horizons according to redox, pH or ecological properties. Introduction Aquatic systems that receive large inputs of organic mate- rial, as frequently occurs in lakes, rivers, estuaries and coastal waters affected by eutrophication, are charac- terized by marked chemical changes in the sediment– *For correspondence. E-mail sophie.muller@im.uib.no (S.P.C. Tankéré); Tel. (+47) 55584638 Fax (+47) 55589671.