Corrosion of carbon steel is a potential hazard to oil and gas production infrastructure. The total annual cost of corrosion in the oil and gas production industry is estimated to be $1.372 billion in the USA (Simons, 2008). Over 20% of the total corrosion cost is caused by microbial induced corrosion (MIC) or biocorrosion (Javaherdashti, 2008). MIC of the carbon steel infrastructure in oil and gas production facilities is most often associated with the activity of sulfate-reducing bacteria (SRB). However, a different biocorrosion mechanism was presumably observed in the pipelines in the Putumayo Basin, southwest Colombia, South America and production water storage tanks in the Barnett Shale, north Texas, USA. Both sites typically experienced very specific types of corrosion and/or MIC. CO2 corrosion was considered as the corrosion mechanism in Putumayo Basin sampling sites, while SRB induced corrosion was presumed to be the main corrosion mechanism in Barnett Shale storage tanks. However, in the Putumayo Basin production water samples, the pH of all samples was circumneutral which didn’t support typical CO2 corrosion mechanism. The total dissolved iron ions species (ferrous and ferric species) concentration was 6 to 8 orders of magnitude higher than the equilibrium iron concentration with respect to solid-phase Fe(OH)3. The Shewanella genus, which is known to contain iron-reducing bacteria (IRB), comprised 30% of the microbes detected in Sucombio production water sample from the Putumayo Basin. In Barnett Shale production water samples, only one sample indicated it was impacted by the typical SRB corrosion mechanisms. The four samples lacked sulfate reducing activity and had no FeS precipitate or had sulfate reducing activity but FeS was only 14% to 21% of the corrosion solid phase products. These four samples had dissolved iron ions concentrations 2 to 5 times higher than the equilibrium iron concentration of solid-phase FeCO3 and contained bacteria orders with species that can act as both SRB and IRB. As such, the Putumayo and Barnett Shale results indicated that there were other microbial induced corrosion mechanisms in the sampling sites besides that facilitated by SRB. The research questions of this study are: 1) what caused the high total dissolved iron concentration in the production water samples from Putumayo Basin and Barnett Shale, and 2) can the high total dissolved iron concentration shift the function of microorganisms from sulfate respiration to ferric respiration. It is hypothesized that organic ligands chelated with Fe(III) in the ferric oxyhydroxide layer to form soluble ferric-ligands complexes, resulting in the high total dissolved iron concentration. The increased availability of dissolved ferric ions shifted the function of microorganisms from sulfate respiration to ferric respiration, thereby allowing microbes with iron reducing abilities to take advantage of the thermodynamic and kinetic gains in energy to thrive. Gas chromatography/mass spectrometry analysis showed a positive relationship between the concentration of organic metabolites and the total dissolved iron concentration in Putumayo production water samples, and a positive linear relationship was found between iron-chelating molecules and the total dissolved iron ions concentration in Barnett Shale production water samples. These results support the hypothesis that organic ligands chelated with Fe(III) and caused the high dissolved iron concentration. Evidence supporting the hypothesis that the high concentration of the total dissolved ferric ions shifted the microbial community to favor microbes capable of respiration ferric ions was supported by the atomic ratio of S/Fe in particulates collected from biofilms, as well as sulfate reduction assays and microbial community analysis results.