Carboniferous coalbed rocks originating from the Lublin Coal Basin (South-East Poland) were investigated for the presence and activity of aerobic methanotrophic bacteria. Laboratory studies of samples collected from 914 to 1004 m below earth's surface and performed at 30 degrees C, 100% water capacity (WHC) and 10% v/v headspace CH4 revealed microbial methane oxidation which ranged from 0.13 to 0.75 mu M CH4 g(-1) day(-1). Methanotrophic activity (MA) was also investigated as a function of temperature (5-30 degrees C), moisture content (25-200% WHC) and substrate concentration (1-30% v/v CH4). The highest MA was recorded at 30 degrees C, 100% WHC and 20% CH4 v/v, indicating the mesophilic and microaerophilic character of methanotrophic bacteria (MB) present in the coalbed. Lack of detectable MA at 5 and 10 degrees C shows narrow temperature tolerance of MB, which may result from a long-term isolation of the bacterial habitat and stability of thermal conditions in the coalbed. Cryo-SEM revealed presence in the investigated rocks of bacteria (c.a. 2-2.5 mu m long and 0.5-1 mu m wide) and spore-like structures (0.5-1 mu m in diameter). These observations were confirmed by fluorescence in situ hybridization with probes Mg705 (5'fluoresceine), Mg84 (5'Cy3) and Ma450 (5'Cy5), which showed that enrichments in NMS medium contain types I and II MB of similar size. Identification performed with the use of DNA amplification of 16S rRNA targeted group specific primers confirmed the presence of bacteria belonging to Methylosinus, Methylomicrobium, Methylocystis and Methylocaldum. Here we show that coalbed rocks constitute a habitat for diverse MB. (C) 2013 Elsevier B.V. All rights reserved.