In the elevated plateaus of Carajás province in Brazil, ferruginous duricrusts, locally known as canga, protect the underlying weathered banded iron formations (BIF) - iron ore deposits. The processes that govern canga formation are still unclear but include recurrent partial dissolution and recrystallization of goethite through iron reduction and oxidation. Recent studies1,2 suggest past and present biogeochemical cycling of iron within canga occurs at the surface or near-surface environment.
Field observations of exposed canga surfaces located near the banks of perched or ephemeral lakes identified surfaces encrusted with a mm-thick dark mat that, at first glance, could be mistaken for an oxidised coating. However, a combination of electron microscopy and metagenomics reveals the nature of the pervasive organic mat covering these goethite-cemented surfaces. A symbiotic community of cyanobacteria and fungi are acting as primary colonizers on this harsh, nutrient-poor substrate. In this mutualistic interaction, functionally resembling that of lichen; the hardy fungi shelters bacteria from UV damage, dry weather and exceptionally hot surface temperatures, whilst the cyanobacteria and their secretions provide a carbon source. The ‘thick’ bacterial biofilm might also act as an adherence agent for the larger heterotrophs.
This epilithic community seems to contribute to canga evolution in two opposing processes: (1) the bio-weathering of the surface through excretion of extracellular metabolites, such as organic acids; and (2) the bio-protection of weathered fragments through stabilization and trapping of fine particles. The weathering processes are both chemical and physical, with evidence of iron mobilized in the biofilm, and clear signs of cracking due to microbial activity. Underneath the live biofilm, multiple layers of fossilized bacteria are detected, implying an active growth front with a continuous weathering and formation cycle at the canga surface.
Understanding the contribution of microorganisms to the biogeochemical cycling of iron in canga is crucial when formulating post-mining biotechnological rehabilitation strategies, e.g., slope stabilisation, for global iron ore sites.