In incubations containing Y. lipolytica or Byssochlamys sp. SW2, positive current developed within three days of inoculation, as pH decreased and acetate concentration increased, indicating B20-fatty acid metabolism (Fig. 2b and c). Y. lipolytica or Byssochlamys sp. SW2 biomass also increased in these incubations (Table 1). The positive current indicated electron transfer from the inoculated chamber (WE1) to the sterile chamber (WE2), as depicted in Fig. 1. Given the high O2 concentration that biodiesel can accommodate [3, 10, 44] and the relatively acidic conditions induced by microbial activities (Fig. 2b), the flow of electrons from inoculated to sterile chambers (positive current) indicates that the activities of Y. lipolytica or Byssochlamys sp. SW2 induce anodic conditions on WE1 and the half reaction:
while the abiotic half reaction:
was occurring on WE2, supporting mass loss on WE1. These patterns of current, metabolism, and corrosion are consistent with previous batch experiments examining corrosion by Byssochlamys sp. SW2 and Y. lipolytica and other organisms [30, 34, 35, 45]. While the current direction could be interpreted as electron transfer in a microeukaryotic bioelectrochemical system (i.e., microbial fuel cell[46, 47]), the microbial fuel cell systems deploy inert graphite electrodes. When used for MIC characterization, the SC-ZRA described in this work uses reactive carbon steel electrodes. Indeed, when positive current was detected, greater corrosion was detected on WE1 (Table 1), which is consistent with the R1 and R2 redox couples, as well as previous work to characterize MIC using SC-ZRA measurements[30, 34, 35].
In contrast to SC-ZRA incubations containing Y. lipolytica or Byssochlamys sp. SW2, minimal current was observed in incubations containing Wickerhammomyces sp. SE3, despite similar patterns of acetate production, pH change, and growth (Fig. 2 and Table 1). The SC-ZRA incubations containing Wickerhammomyces sp. SE3 suggest that microbial metabolism does not always correspond to development of current. In fact, corrosion rates and current in incubations containing Wickerhammomyces sp. SE3 were similar to the uninoculated control (Fig. 2a; Table 1), even though the incubations with Wickerhammomyces sp. SE3 showed microbial activity, evidenced by pH change, acetate and biomass accumulation, and FAME depletion (SI Fig. 1).
In a previous yearlong monitoring study, Wickerhammomyces sp. SE3 and Byssochlamys sp. SW2 were frequently found together in B20 storage tanks[23, 24, 36]. When a consortium of Wickerhammomyces sp. SE3 and Byssochlamys sp. SW2 was added to SC-ZRA incubations, the development of current was delayed in comparison to the incubation that included Byssochlamys sp. SW2 alone (Fig. 2a). This delay could be due to the rapid initial growth and metabolism of Wickerhammomyces sp. SE3 in comparison to Byssochlamys sp. SW2 [16, 45], whereby initially lower current is a reflection of the activities of Wickerhammomyces sp. SE3 alone, and the increase in current near the end of the incubation could be indicative of Byssochlamys sp. SW2 activity (Fig. 2a) and associated MIC. Indeed, similar WE mass loss patterns were observed in the SC-ZRA incubation containing Wickerhammomyces sp. SE3 and Byssochlamys sp. SW2 and the incubation that contained only Byssochlamys sp. SW2 (Table 1).
In batch incubations, metabolic patterns (Supplemental Information Tables SI1 and SI2) and corrosion rates were similar to those observed in SC-ZRA incubations. Higher corrosion rates were observed in batch incubations containing Y. lipolytica or Byssochlamys sp. SW2 compared to incubations with Wickerhammomyces sp. SE3 and sterile controls (Supplemental Information Tables SI1 and SI2), with no statistical difference (p-value < 0.05) between the corrosion rates of batch and SC-ZRA incubations, indicating that the SC-ZRA technique does not influence corrosion of the coupons and is only monitoring electron transfer patterns as the coupons corroded. Corrosion rates by Wickerhammomyces sp. SE3 in both the SC-ZRA incubations and current batch experiments are similar to previous batch experiments[36], further highlighting that microbial growth is not always an indication for risk of MIC.
Imaging of the metal surface was performed to determine the extent of pitting corrosion in the incubation. Coupons incubated with Y. lipolytica, Byssochlamys sp. SW2, and the Byssochlamys sp. SW2-Wickerhammomyces sp. SE3 consortium all have significantly higher pitting densities (pits/mm2) than the sterile control (Table 1).