1 INTRODUCTION
It has long been recognized that certain microorganisms accelerate the corrosion of carbon- and stainless steel. This phenomenon is referred to as microbially influenced corrosion (MIC) (Javaherdashti, 2008). Under aerobic conditions, microorganisms can promote or inhibit the corrosion of metallic iron (Fe0) through oxygen respiration (Zarasvand and Rai, 2014). A specific group of sulfate-reducing bacteria (SRB) accelerates Fe0 corrosion in anaerobic environments (Enning and Garrelfs, 2014), with recent evidence indicating that the direct electron transfer from Fe0to SRB via extracellular electron transfer is mediated by outer-membrane cytochromes (Beese-Vasbender et al. , 2015; Deng et al. , 2018; Dinh et al. , 2004; Venzlaff et al. , 2013). Some hydrogenotrophic methanogens that reduce carbon dioxide to methane (CO2 respiration) have been found to be capable of causing MIC on Fe0 (Daniels et al. , 1987; Moriet al. , 2010; Uchiyama et al. , 2010) by secreting specific [NiFe] hydrogenase, thereby catalyzing the oxidation of Fe0 to ferrous ion: Fe0 + 2H+ → Fe2+ + H2(Tsurumaru et al. , 2018). In addition to SRB and methanogens, three facultatively anaerobic nitrate-reducing bacteria (NRB), namelyParacoccus denitrificans , Bacillus licheniformis , andPseudomonas aeruginosa , have been found to enhance the corrosion of carbon steel in the presence of nitrate (Ginner et al. , 2004; Jia et al. , 2017; Till et al. , 1998; Xu et al. , 2013). Shewanella oneidensis MR-1, notable for its diverse respiratory capabilities also stimulated Fe0 corrosion via nitrate respiration (De Windt et al. , 2003; Miller IIet al. , 2018).
We previously isolated a bacterium that corroded Fe0concomitantly with the reduction of nitrate to nitrite under anaerobic conditions (Iino et al. , 2015a). This strain was classified asProlixibacter denitrificans sp. nov., and is the first Fe0-corroding NRB belonging to the phylumBacteroidetes (Iino et al. , 2015a). However, only two strains, P. bellariivorans strain F2T (Holmeset al. , 2007) and P. denitrificansMIC1-1T (Iino et al. , 2015a), have been isolated thus far. To explore the potential environmental functions and diversity of Prolixibacter strains, we were interested in isolating more Prolixibacter strains, and characterizing their ability to corrode Fe0 under anoxic conditions. In this study, four strains belonging to the genus Prolixibacterwere newly isolated, and their Fe0-corroding activities were examined. We found that some but not all theProlixibacter strains enhanced the corrosion of Fe0. The basis for the phenotypic differentiation between the Prolixibacter strains was also investigated.