High diversity of nitrifying bacteria and archaea in biofilms from a subsea tunnel

Microbial biofilm formation can contribute to the accelerated deterioration of steel-reinforced concrete structures and significantly impact their service life, making it critical to understand the diversity of the biofilm community and prevailing processes in these habitats. Here, we analyzed 16S rRNA gene amplicon and metagenomics sequencing data to study the abundance and diversity of nitrifiers within biofilms on the concrete surface of the Oslofjord subsea road tunnel in Norway. We showed that the abundance of nitrifiers varied greatly in time and space, with a mean abundance of 24.7 ± 15% but a wide range between 1.2% and 61.4%. We hypothesize that niche differentiation allows the coexistence of several nitrifier groups and that their high diversity increases the resilience to fluctuating environmental conditions. Strong correlations were observed between the nitrifying families Nitrosomonadaceae and Nitrospinaceae, and the iron-oxidizing family Mariprofundaceae. Metagenome-assembled genome analyses suggested that early Mariprofundaceae colonizers may provide a protected environment for nitrifiers in exchange for nitrogen compounds and vitamin B₁₂, but further studies are needed to elucidate the spatial organization of the biofilms and the cooperative and competitive interactions in this environment. Together, this research provides novel insights into the diverse communities of nitrifiers living within biofilms on concrete surfaces and establishes a foundation for future experimental studies of concrete biofilms.