Impact of nitrate on the structure and function of bacterial biofilm communities in pipelines used for injection of seawater into oil fields

We studied the impact of NO₃^- on the bacterial community composition, diversity, and function in in situ industrial, anaerobic biofilms by combining microsensor profiling, ¹⁵N and ³⁵S labeling, and 16S rRNA gene-based fingerprinting. Biofilms were grown on carbon steel coupons within a system designed to treat seawater for injection into an oil field for pressurized oil recovery. NO₃^- was added to the seawater in an attempt to prevent bacterial H₂S generation and microbially influenced corrosion in the field. Microprofiling of nitrogen compounds and redox potential inside the biofilms showed that the zone of highest metabolic activity was located close to the metal surface, correlating with a high bacterial abundance in this zone. Upon addition, NO₃ ^- was mainly reduced to NO₂^-. In biofilms grown in the absence of NO₃^-, redox potentials of <-450 mV at the metal surface suggested the release of Fe²⁺. NO ₃^- addition to previously untreated biofilms induced a decline (65%) in bacterial species richness, with Methylophaga- and Colwellia-related sequences having the highest number of obtained clones in the clone library. In contrast, no changes in community composition and potential NO₃^- reduction occurred upon subsequent withdrawal of NO₃^-. Active sulfate reduction was below detection levels in all biofilms, but S isotope fractionation analysis of sulfide deposits suggested that it must have occurred either at low rates or episodically. Scanning electron microscopy revealed that pitting corrosion occurred on all coupons, independent of the treatment. However, uniform corrosion was clearly mitigated by NO₃^- addition.