Analysis of biofilms formed by static culture of bacterial communities in the Inba floodway (Lake Inbanuma-Tokyo Bay).

Microbiologically influenced corrosion (MIC) is a major threat to marine infrastructure, yet the bacterial communities on metal surfaces before corrosion remain poorly understood. In this study, environmental water collected across a salinity gradient (freshwater-brackish-seawater) in the Inba floodway (Lake Inbanuma to Tokyo Bay, Japan) was treated with the extremely low-nutrient R2A medium, after which stainless steel was immersed in the treated water. After 1 week of static incubation, biofilms were formed on stainless steel immersed in the water samples. The early-stage biofilm-forming bacterial communities on stainless steel surfaces were profiled by 16 S rRNA gene V3-V4 amplicon sequencing, and then functional inference was performed. Flavobacterium, Pseudomonas and Comamonas were detected as dominant in biofilms formed using freshwater and brackish water, while Agrobacterium, Bacillus, and Ralstonia were identified in biofilms formed using seawater. Notably, classical MIC-associated sulfate-reducing bacteria (SRB), including Desulfobacter, Desulfopila and Desulfotomaculum, were absent from all bacterial communities, which indicated that anaerobic, sulfate-reduction-driven MIC pathways had not yet been established. However, Pseudomonas and Bacillus, which are capable of causing MIC on stainless steel through aerobic extracellular electron transfer mechanisms independent of sulfate reduction, were already present in the initial biofilms. These results indicate that the initial bacterial communities are distinct from classical SRB-dominated MIC communities but may nonetheless harbor other communities capable of initiating corrosion through alternative, oxygen-independent-of-sulfate mechanisms, underscoring the value of monitoring both SRB and non-SRB MIC-associated communities from the earliest stage of biofilm formation.