The thermophilic bacterial strain MP 4 assigned to a new species, likely of the genus Alicyclobacillus, was isolated from geothermal soils on the NW slope of Mount Melbourne, Antarctica. These soils have high iron concentrations and the strain MP 4 requires iron additions for growth. Four mesophilic bacterial strains Paenibacillus validus MP 5, MP 8, and MP 10, and P. apiarius MP 7, isolated from the same site, need iron supply for growth depending on the medium. Growth temperature of thermophilic strain ranges from 42 to 70 degrees C, and that one of mesophiles from 25 to 44 degrees C. Thermophilic and mesophilic strains shared microenvironments with temperature of 42-44 degrees C and showed optima of pH values ranging from 5.5 to 6.0. The thermophilic strain MP 4 reached values of 10(6) CFU ml(-1) in aqueous soil extract from the NW slope of Mt. Melbourne, and 10(5) CFU ml(-1) in water extracts from other geothermal Antarctic areas (Mt. Rittmann and Cryptogam Ridge). Growth of thermophilic bacteria in aqueous extracts of the NW slope of Mount Melbourne soils caused a reduction of 50% of soluble iron content, which was recovered in bacterial biomass. These results suggest a possible involvement of the thermophilic strain MP 4 in iron bioavailability in these geothermal soils.