Seasonally dynamic ecosystems within offshore wind power monopile foundations support microbially influenced...

Microbially influenced corrosion (MIC) is a widespread problem in the maritime sector causingsevere economic damage. Foundation structures (monopiles) of offshore wind power systemsappear particularly prone to MIC. The hollow structures trap seawater and thus provide only limited exchange with the ambient waters. These special environmental conditions inside monopiles play a pivotal role for shaping the microbial community and respective biogeochemical processes. Currently, corrosion protection measures inside offshore monopiles only address electrochemical corrosion, likely resulting from a lack of knowledge of the microbial processes and taxa involved in MIC in these specific environments.

Here we report on two in situ incubation experiments conducted inside a monopile structure located in the German North Sea. Mild steel (S355), as commonly used for monopiles, was incubated inside the largely enclosed structure in surface near and bottom waters for 3 and 12 months, respectively. Parallel to in situ incubations, temporal and spatial geochemical and microbial water column profiling was performed. Steel biofilms were analyzed with a suite of scanning electron and 3D microscopy, quantification of extracellular polymeric substances, and RNA-based 16S rRNA gene profiling, correlated with environmental parameters.

Geochemical water column parameters demonstrated strong seasonal vertical stratification inside the monopile. In summer months, anoxia (O₂ = 0 μM) can develop in bottom waters and biogenic methane and highly corrosive, toxic hydrogen sulfide in bottom waters accumulated. Compared to water column samples, biofilms of the steel incubations at different depths exhibited lower microbial diversities. Microbial taxa commonly associated with MIC like Feoxidizing bacteria (e.g. Hoeflea), sulfate reducing bacteria (e.g. Desulfobacteraceae) and methanogenic archaea (e.g. Methanococcaceae) as well as pitting corrosion gave indications for the onset of MIC processes during the incubation of the steel coupons. In summary, the highly dynamic environmental parameters of the water column and duration of the exposure of steel structures significantly influenced the microbial community composition on steel surfaces and thus may also influence the potential for MIC-related material degradation inside monopiles that are not protected against corrosion.