with electron-donating organotrophic anaerobic microorganisms. C. SRB interacting in direct contact with electron-donating organotrophic anaerobic microorganisms. D. Special SRB exploit metallic iron as electron source at the outer surface of a pipeline. E. Special SRB exploit metallic iron as electron source inside of a pipeline. F. Speculative possibility of pyritization of FeS (FeS + H2S → FeS2 + 2H+ + 2e−) as a direct electron source for sulfate reduction