Applicability of yeast extract in Postgate culture medium for microbiologically influenced corrosion (MIC) tests

This study was initiated to investigate the influence of Yeast Extract (YE), a component not naturally present in produced water, on in-vitro experiments of Microbiologically Influenced Corrosion (MIC) in a modified Postgate’s Medium B with 32 g/L NaCl (MPB). The concern was that MPB’s inclusion YE could potentially skew the results of corrosion studies. In the laboratory, static MIC tests containing postgate with and without YE were incubated for 25 days at two temperatures, 20 and 38 °C, after which analyses were performed: H₂S concentration, corrosion rate, maximum pitting penetration rate, and microbial community analysis.

A significant reduction in H₂S production was observed when YE was omitted. However, an increase in corrosion was observed, suggesting that H₂S production and sulfate reduction might not be the most critical metrics for MIC. The maximum pitting penetration rate, determined by extrapolating the depth of the deepest pit on a coupon over one year, showed an increase with the omission of YE, but the results were not statistically significant. 16S rRNA amplicon sequencing of liquid samples revealed less bacterial DNA in samples without YE. The omission of YE caused a dramatic shift in the microbial community, with a significant increase in the relative abundance of Spirochaetaceae and a decrease in Desulfovibrionaceae at 20 °C. At 38 °C, the omission of YE led to an increase in the relative abundance of several families not usually associated with increases in microbial corrosion.

The study emphasized the importance of a multiple lines of evidence (MLOE) approach for diagnosing MIC, as relying solely on corrosion morphology or the presence of microorganisms can lead to false conclusions. The addition of YE to Postgate media, while beneficial for bacterial growth, seemed to inhibit the corrosion that researchers and operators aim to simulate experimentally. Excluding YE appeared to foster a more realistic microbial community, suggesting its omission in future studies. These findings question the inclusion of YE in the Postgate medium for corrosion tests and call for further research into the role of specific bacterial families in MIC, the impact of microbial diversity on MIC, and the potential role of unknown organisms in MIC. The study also emphasizes the need for a more comprehensive approach to diagnosing MIC, incorporating multiple lines of evidence.