Macroscopic growths at geographically separated acid mine drainages (AMDs) exhibit distinct populations. Yet, local heterogeneities are poorly understood. To gain novel mechanistic insights into this, we used OMICs tools to profile microbial populations coexisting in a single pyrite gallery AMD (pH ∼2) in three distinct compartments: two from a stratified streamer (uppermost oxic and lowermost anoxic sediment-attached strata) and one from a submerged anoxic non-stratified mat biofilm. The communities colonising pyrite and those in the mature formations appear to be populated by the greatest diversity of bacteria and archaea (including ‘ARMAN’ (archaeal Richmond Mine acidophilic nano-organisms)-related), as compared with the known AMD, with ∼44.9% unclassified sequences. We propose that the thick polymeric matrix may provide a safety shield against the prevailing extreme condition and also a massive carbon source, enabling non-typical acidophiles to develop more easily. Only 1 of 39 species were shared, suggesting a high metabolic heterogeneity in local microenvironments, defined by the O₂ concentration, spatial location and biofilm architecture. The suboxic mats, compositionally most similar to each other, are more diverse and active for S, CO₂, CH₄, fatty acid and lipopolysaccharide metabolism. The oxic stratum of the streamer, displaying a higher diversity of the so-called ‘ARMAN’-related Euryarchaeota, shows a higher expression level of proteins involved in signal transduction, cell growth and N, H₂, Fe, aromatic amino acids, sphingolipid and peptidoglycan metabolism. Our study is the first to highlight profound taxonomic and functional shifts in single AMD formations, as well as new microbial species and the importance of H₂ in acidic suboxic macroscopic growths.

The corrosion of mild steel in chloride-rich atmospheres is a highly topical issue. The formation of the oxyhydroxide akaganeite (β-FeOOH) in this type of atmosphere leads to a notable acceleration of the steel corrosion process. The scientific literature contains many references to outdoor marine atmospheric tests, but so far has failed to clarify two basic matters in relation to akaganeite: the environmental conditions necessary for its formation, and its morphological characterization. Research has been performed at three atmospheric corrosion stations located at Cabo Vilano wind farm (Camariñas, Spain) at different distances from the shoreline (332, 590, and 2,400 m), with chloride deposition rates of 390, 74, and 29 mg/m2/day, respectively, with the exposure of mild steel specimens for 1 year. This paper reports the environmental conditions that generally led to the formation of akaganeite: an annual average relative humidity of around 80% or higher, and simultaneously, an annual average chloride deposition rate of approximately 60 mg/m2/day or higher. Rigorous characterization of akaganeite was performed by x-ray diffraction, scanning electron microscopy/energy dispersive spectroscopy, and transmission electron microscopy/selected area electron diffraction.