ABSTRACT

The understanding of microbiologically influenced corrosion (MIC), specifically the role different species of microorganisms play in the process, is continuously evolving. Recently, bacteria beyond typically implicated sulfate reducing bacteria (SRB) and acid producing bacteria (APB) have been found to influence corrosion. Sulfur oxidizing bacteria (SOB) populations, for example, have been linked to the corrosion of concrete and steel. As with the other MIC organisms found in gas industry samples, modern molecular techniques targeting functional gene DNA are the most accurate ways to identify and quantify the abundance of SOB. We studied the abundance and diversity of the soxB gene in oil and gas industry samples using sequencing and phylogenetic analysis. We have developed a quantitative polymerase chain reaction (qPCR) assay to detect and quantify SOB through the amplification of the soxB subunit of the thiosulfate-oxidizing gene complex. The ability to accurately quantify the SOB in environments where MIC is suspected will give a more complete understanding of the process.

INTRODUCTION

The total cost of corrosion is estimated to 2.5 trillion USD worldwide.1 One contributor to this enormous cost is the effect microorganisms have on the deterioration of metal, concrete, and plastic structures. The oil and gas industry is hit particularly hard by Microbiologically Influenced Corrosion (MIC) wherein MIC is attributed to an estimated 20-30% of internal corrosion.2 Corrosion control methods, which include microbiological monitoring, can potentially save 15-35% of the total cost of corrosion1. The organisms historically implicated in this type of corrosion are sulfate reducing bacteria (SRB) and acid producing bacteria (APB).3 However, an in depth understanding of the corrosive mechanisms of APB and SRB does not offer a complete understating of MIC as a whole. Due to advancements in molecular technology, a number of other organisms have been identified as corrosion causing.3

The destructive tendencies of SOB on concrete have been well documented in the waste water treatment industry.4,5 The deterioration of concrete occurs via biogenic sulfuric acid attack where sulfuric acid is created from a multi-step process involving SRB and SOB. SRB growing in the anoxic zone at the bottom of waste water treatment equipment reduce sulfate and other sulfur compounds present in the waste water into H2S. The gaseous H2S rises and seeps into the concrete above where oxygen is present. SOB genera Thiobacillus, Thiomonas, and Aciditiobacillus are then able to grow on the aerobic section of the equipment and oxidize the H2S into acidic sulfur compounds which will quickly degrade concrete.5 SOB have also been implicated in the corrosion of metals. Beech and Campbell6 conducted a study on accelerated low water corrosion of carbon steel. In this study, black biofilms containing both SRB and SOB were found on sites of corrosion. This study found that higher rates of corrosion were more closely related to high SOB concentrations than they were to high SRB concentrations. Geissler et. al3 performed an in depth Next-gen DNA sequencing study on bacterial diversity in oilfields. They found SOB made up 4-5% of the total population of bacteria analyzed. The most common SOB genera found in the study were Arcobacter, Sulfurimonas, and Paracoccus. Due to the increasing number of documented instances of SOB corrosion, and the realization that these bacteria are present within the gas and oil industry, the need for a fast and reliable method of SOB quantification that can be integrated into field monitoring is apparent.

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