High Taxonomic Diversity in Ship Bilges Presents Challenges for Monitoring Microbial Corrosion and Opportunity To Utilize Community Functional Profiling.

One of the key areas in which microbially influenced corrosion (MIC) has been found to be a problem is in the bilges of maritime vessels. To establish effective biological monitoring protocols, baseline knowledge of the temporal and spatial biological variation within bilges, as well as the effectiveness of different sampling methodologies, is critical. We used 16S rRNA gene metabarcoding of pelagic and sessile bacterial communities from ship bilges to assess the variation in bilge bacterial communities to determine how the inherent bilge diversity could guide or constrain biological monitoring. Bilge communities exhibited high levels of spatial and temporal variation, with >80% of the community able to be turned over in the space of 3 months, likely due to disturbance events such as cleaning and maintenance. Sessile and pelagic communities within a given bilge were also inherently distinct, with dominant exact sequence variants (ESVs) rarely shared between the two. Taxa containing KEGG orthologies (KOs) associated with dissimilatory sulfate reduction and biofilm production, functions typically associated with MIC, were generally more prevalent in sessile communities. Collectively, our findings indicate that neither bilge water nor an unaffected bilge from within the same vessel would constitute an appropriate reference community for MIC diagnosis. Optimal sampling locations and strategies that could be incorporated into a standardized method for monitoring bilge biology in relation to MIC were identified. Finally, taxonomic and functional comparisons of bilge diversity highlight the potential of functional approaches in future biological monitoring of MIC and MIC mitigation strategies in general. IMPORTANCE Microbially influenced corrosion (MIC) has been estimated to contribute 20 to 50% of the costs associated with corrosion globally. Diagnosis and monitoring of MIC are complex problems requiring knowledge of corrosion rates, corrosion morphology, and the associated microbiology to distinguish MIC from abiotic corrosion processes. Historically, biological monitoring of MIC utilized a priori knowledge to monitor sulfate-reducing bacteria; however, it is becoming widely accepted that a holistic or community-level understanding of corrosion-associated microbiology is needed for MIC diagnosis and monitoring. Before biology associated with MIC attack can be identified, standardized protocols for sampling and monitoring must be developed. The significance of our research is in contributing to the development of robust and repeatable sampling strategies of bilges, which are required for the development of standardized biological monitoring methods for MIC. We achieve this via a biodiversity survey of bilge communities and by comparing taxonomic and functional variation.