Biomineralization of Fungal Hyphae with Calcite (CaCO3) and Calcium Oxalate Mono- and Dihydrate in Carboniferous Limestone Microcosms

The formation of biogenic fabrics in limestone by two fungi, Serpula himantioides and a polymorphic fungal isolate from limestone identified as a Cephalotrichum (syn. Doratomyces) sp., was investigated. The fungal cultures were grown in laboratory microcosms consisting of Carboniferous limestone and after 21 d incubation at 25°C, biomineralization of fungal filaments was observed. Environmental electron scanning microscopy (ESEM) and X-ray micro-analysis (EDXA) of crystalline precipitates on the hyphae of S. himantioides demonstrated that the secondary crystals exhibited different crystalline forms but were similar in elemental composition to the original limestone. Powder X-ray diffraction (XRD) of crystalline precipitates showed they were composed of a mixture of calcite (CaCO ₃ ) and calcium oxalate monohydrate (CaC ₂ O ₄ · H ₂ O). Analysis of crystals precipitated on the hyphae of the limestone isolate, using ESEM and EDXA, showed that the crystals exhibited similar morphological characteristics and elemental composition to the original limestone. XRD showed that they were composed solely of calcite (CaCO ₃ ) or of calcite with some calcium oxalate dihydrate (CaC ₂ O ₄ · 2H ₂ O). These results provide direct experimental evidence for the precipitation of calcite (CaCO ₃ ) and also secondary mycogenic minerals, on fungal hyphae in low nutrient calcareous environments, and suggest that fungi may play a wider role in the biogeochemical carbon cycle than has previously been appreciated.