Microbiologically influenced corrosion of carbon steel was assessed in a laboratory environment simulating the deep geological repository of radioactive waste. A dense and diverse biofilm was formed on the surfaces of steel in biotic systems without concrete. Addition of nutrients favored biofilm formation and altered the bacterial community; most distinctly, the relative abundance of Alphaproteobacteria decreased, and Deltaproteobacteria or Betaproteobacteria became more abundant, when nutrients were available. Nutrient amendment also increased the corrosion rate and changed the composition and resistance of corrosion products (mostly FeS, Fe2O3, or Fe(OH)2). Presence of concrete inhibited the corrosion of steel and hindered the biofilm formation on steel. Only sparse biofilm consisting of known alkaliphilic bacteria was detected. In the presence of concrete, the corrosion rate was consistently radically decreased, as the properties of the surface deposits (mostly CaCO3) were different from those in the other systems.