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11426 Bioenergetics Explains When and Why More Severe MIC Pitting by SRB Can Occur

Picture for 11426 Bioenergetics Explains When and Why More Severe MIC Pitting by SRB Can Occur
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Product Number: 51300-11426-SG
ISBN: 2011 11426 CP
Author: Dake Xu and Tingyue Gu
Publication Date: 2011
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The detection and prediction of microbiologically influence corrosion (MIC) have long been plagued by a lack of basic understanding of MIC mechanisms. In this work, bioenergetics was used to explain why and when sulfate reducing bacteria (SRB) become aggressive toward iron. The electrochemical standard reduction potentials of Fe2+/Fe and acetate + CO2/lactate are very close. This means that bioenergetically Fe oxidation is as favorable as lactate oxidation, except that the former does not provide organic carbons for cell growth. In the stationary cell growth phase, Fe oxidation when coupled with dissimilatory sulfate reduction generates energy in the form of ATPs that are used by SRB as maintenance energy. In this work experiments were carried out by subjecting established Desulfovibrio vulgaris (ATCC 7757) biofilms on carbon steel coupons to various degrees of carbon source starvation. It was found that when sessile SRB cells were starved of organic carbons, they turned to Fe oxidation and thus increased pitting weight loss. Moderately reduced carbon source caused deeper pits and a larger weight loss compared with full strength medium. Severely reduced carbon source caused wider but shallower pits and a larger weigh loss compared with moderately reduced carbon source. When the carbon source was dropped from the medium, weight loss decreased compared to that in full strength medium, probably due to a lack of a minimum amount of carbon needed for sufficient enzyme activities that are required for MIC. Biofilm morphology plays a critical role in MIC pitting. Loose biofilms allow better mass transfer of organic carbon to the iron surface, while dense biofilms restrict the mass transfer and thus can result in organic carbon starvation in the sessile cells that are close to the iron surface. Many NRB form very loose biofilms while SRB biofilms typically have a very dense morphology. This is probably why SRB tend to be aggressive while NRB usually are not. Our bioenergetics theory for MIC also explains in some cases why the same biofilm can be “docile” and all of sudden can become aggressive.

Keywords: MIC, Mechanism, SRB, NRB, Sulfate reduction, Bioenergetics
The detection and prediction of microbiologically influence corrosion (MIC) have long been plagued by a lack of basic understanding of MIC mechanisms. In this work, bioenergetics was used to explain why and when sulfate reducing bacteria (SRB) become aggressive toward iron. The electrochemical standard reduction potentials of Fe2+/Fe and acetate + CO2/lactate are very close. This means that bioenergetically Fe oxidation is as favorable as lactate oxidation, except that the former does not provide organic carbons for cell growth. In the stationary cell growth phase, Fe oxidation when coupled with dissimilatory sulfate reduction generates energy in the form of ATPs that are used by SRB as maintenance energy. In this work experiments were carried out by subjecting established Desulfovibrio vulgaris (ATCC 7757) biofilms on carbon steel coupons to various degrees of carbon source starvation. It was found that when sessile SRB cells were starved of organic carbons, they turned to Fe oxidation and thus increased pitting weight loss. Moderately reduced carbon source caused deeper pits and a larger weight loss compared with full strength medium. Severely reduced carbon source caused wider but shallower pits and a larger weigh loss compared with moderately reduced carbon source. When the carbon source was dropped from the medium, weight loss decreased compared to that in full strength medium, probably due to a lack of a minimum amount of carbon needed for sufficient enzyme activities that are required for MIC. Biofilm morphology plays a critical role in MIC pitting. Loose biofilms allow better mass transfer of organic carbon to the iron surface, while dense biofilms restrict the mass transfer and thus can result in organic carbon starvation in the sessile cells that are close to the iron surface. Many NRB form very loose biofilms while SRB biofilms typically have a very dense morphology. This is probably why SRB tend to be aggressive while NRB usually are not. Our bioenergetics theory for MIC also explains in some cases why the same biofilm can be “docile” and all of sudden can become aggressive.

Keywords: MIC, Mechanism, SRB, NRB, Sulfate reduction, Bioenergetics
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