Black Slate : Surface Alternation Due to Fungal Activity
Black slates of different oxidation states and graphite samples as pure carbon source were incubated with the white‐rot fungus Schizophyllum commune 4‐39 for 3 months. Investigations by SEM showed fungal affinity to rough surfaces. Oxidatively weathered, bleached black slate samples have more micropores and increased surface roughness than unweathered samples. Unpolished and polished bleached black slate samples were colonized best by fungal hyphae. The hyphae often entered pores. Etch pits induced by fungal activity could be observed which were then quantified by vertical scanning interferometry (VSI). The etch pits measured 3 – 4 μm in width and 180 – 200 nm in depth. In contrast to black slate samples, more etch pits were found on graphite surfaces. Moreover, they showed a more elongated morphology and branches reminiscent of fungal growth morphology. Under laboratory conditions, Schizophyllum commune was able to degrade about 0.03 μm3/μm2 rock material during 3 months of incubation. It could be shown that collapsed hyphae material remained in their etch pits even after sample preparation which was caused by strong fungal fixation to the rock surfaces. The higher carbon content and the “softness“ of pressed graphite thus allow for earlier fungal attachment and higher dissolution rates. Amounts of nitrogen could be proved by chemical analyzing techniques (x‐ray photoelectron spectroscopy and electron microprobe) at all incubated rock samples. Nitrogen increased eminently on rock surfaces due to fungal colonization. Amounts of nitrogen were detected both on hyphae and former hyphae attachment areas. This indicates the presence of fungus and/or fungal excreted proteins such as hydrophobins for attachment or enzymes for degradation. The accumulation of iron within fungal hyphae, which were grown on iron containing black slates, demonstrates microbial influence on metal mobility during rock alteration. Furthermore, biomineralization on incubated black slate and graphite samples could be observed. The biominerals were often spatially associated with fungal hyphae. Biominerals with amorphous or crystalline shapes varied between 3 and 100 μm in mean size. Chemical analyses suggested magnesium phosphate and calcium rich biominerals. Additionally, small lateral structures occurred at fungal hyphae, some of which extended into elongated structures attached to the rock surface or to other hyphae.