In order to investigate the food chain and energy balance of the chemolithoautotrophically-based ecosystem of the sulphur spring in Movile Cave as a model system for extraterrestrial life, a first sampling campaign was done. Microbial diversity and activity were analysed by MPN-enumeration methods and microcalorimetry, respectively. In addition, a speciation of the inorganic sulphur compounds by HPLC and IC techniques was performed. Metabolic activities were predominantly connected with thick microbial mats floating on the water surface of the cave. These mats showed an aerobic heat evolution of about 1200 μW/g and contained about 500 μmol/g elemental sulphur. In contrast, other samples collected from cave water, sediment and rock exhibited only activities of maximal 40 to 60 μW/g and contained only up to 2.5 μmol/g elemental sulphur. As the main primary producers aerobic and facultatively anaerobic sulphur oxidisers were identified at high numbers, occasionally exceeding 10⁷ CFU/g. Methylotrophic bacteria were present in all samples at up to 10⁶ CFU/g, indicating the important role of C1 metabolism for the cave ecosystem. Although reduced sulphur species were biologically oxidised to sulphuric acid, the pH values of the samples ranged from 6.5 to 8.2 due to the high buffering capacity of the cave walls, which consisted mainly of limestone. Surprisingly, not only neutrophilic but also extremely acidophilic bacteria were detected. Sulphate reducers were present in both aerobic and anaerobic zones. The data presented suggest a close interdependence of sulphur oxidation and reduction as well as carbon dioxide and other C1 compound metabolism in the most biologically active zone of the cave ecosystem, i.e. the floating microbial mats.