Investigating the role of anaerobic hydrocarbon degrading and fatty-acid oxidizing bacteria on the corrosion of carbon steel

The anaerobic degradation of hydrocarbons is increasingly associated with the corrosion of carbon steel particularly in sulfate-rich environments.  However, some anaerobic hydrocarbon degrading bacteria can syntrophically couple with methanogens under sulfate-deplete conditions.  We hypothesized that syntrophic hydrocarbon metabolism or other electron donors will stimulate biocorrosion even in the absence of sulfate. To test this hypothesis, we grew an alkane-utilizing, sulfate-reducing bacterium Desulfoglaeba alkanexedens ALDC, with either sulfate or Methanospirillum hungatei JF-1 as electron acceptors, and tested the ability of the cultures to catalyze metal corrosion.  Axenically, D. alkanexedens produced a higher instantaneous corrosion rate (1/Rp = 10^-3 Ω-cm^-2) and produced more pits in carbon steel coupons compared to the syntrophic co-culture with the methanogen (1/Rp = 10^-4 Ω-cm^-2).  Since anaerobic hydrocarbon biodegradation pathways converge on fatty acid intermediates, the corrosive ability of a known fatty acid-oxidizing syntrophic bacterium, Syntrophus aciditrophicus was compared when grown in pure culture or in co-culture with a H₂-utilizing sulfate-reducing bacterium (Desulfovibrio sp. G11) or a methanogen (M. hungatei).  The instantaneous corrosion rates in the cultures were not substantially different (1/Rp = 10^-3 Ω-cm^-2) but the syntrophic, sulfate-reducing co-culture produced significantly more pits in coupons than other combinations of microorganisms.  Thus, if sulfate is available as an electron acceptor, the same microbial assemblages produce sulfide and low molecular weight organic acids that exacerbated biocorrosion.   Ongoing work aims to characterize other alkane-utilizing, sulfate-reducing bacteria, including Desulfococcus oleovorans Hxd3, with respect to their corrosive abilities via coupon profiling, chemical characterization of the extracellular polymeric substance (EPS), and transcriptomics.