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Microorganisms mainly from the domain of bacteria dominate life in the terrestrial subsurface up to a depth of several kilometers. Microbial life in that habitat is dependent on the availability of water and nutrients of biotic or abiotic origin. To produce biomass, bacterial primary producers are capable to use simple building blocks such as CO2, H2, CH4, reduced iron and sulfur, nitrate, phosphate as well as other salts and trace elements. Here, we describe the microbial content of several biofilm samples taken during the construction phase of a railway tunnel between the provinces of Styria and Carinthia (Koralm Tunnel). We could identify bacterial cells using light microscopy and used 16S rDNA based microbial profiling to characterize the bacterial community structure. Our results clearly show that the massive biofilms in tunnel drainage water were produced by a bacterial consortium typical for the deep continental subsurface biosphere. In many cases the majority of the consortium could be assigned to autotrophs, methanotrophs and methylotrophs. [ABSTRACT FROM AUTHOR]