Geology Reference
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Fig. 5. SEM photomicrographs. (a, b) Samples from A¨nsa (Spain). (a) Fungal strand associated with cotton-ball-like
NFC (star). Note the absence of the inner hyphae (arrow), which seem to have already undergone breakdown. (b)
Close-up of another fungal strand showing the narrow and thick-walled hyphae from the outer part (black arrow). Note
that the internal wide thin-walled hyphae are absent (white star). (c, d) Swiss Jura Mountains samples. (c) Close-up of a
decaying hypha from a grain coating sample showing the release of cell wall fibrous material (e.g. chitin, b-glucan) from
its cell wall (arrow). (d) Remains of a putative fungal hyphae on needle fibre calcite. Two textures can be distinguished,
an outer one that appears to be smooth (black arrow) and an inner one that appears to be composed of nanofibres (white
arrow). Note also the presence of a mesh of nanofibres along the other calcite needles in this picture (star). Sample from
cotton-ball like NFC associated with decaying organic matter.
such as mycelial strands (Fig. 5a, b). Two layers
are visible during the breakdown of the hyphal
cell wall (Fig. 5d). It is known that the inner wall
layer is composed of a hard microfibril framework
(theoretically 10-25 nm in diameter, Carlile et al.
2001) made of chitin and b (1-3), b (1-6) glucan.
Based on this fact and the recognition of the
organic nature of some nanofibres (Fig. 6c, d), the
organized meshes of nanofibres are considered as
the result of a slightly destructive decay of the
fungal fibrous cell wall material. At this stage, the
organized meshes are interpreted as the first step
in the breakdown of the fungal hyphae cell wall,
whereas the non-organized mats represent the ulti-
mate state of decay and reworking of this organic
matter. In other words, the nanofibres could be inter-
preted as organic in origin and being the result of an
incomplete decaying of fungal matter. During early
diagenesis, their calcitic pseudomorphosis (Cailleau
2005; Cailleau et al. 2005) and/or their role as a
template for calcitic precipitation results from the
release of the nanofibres in the soil environment,
followed by their exposure to mineralizing pore
filling fluids. This could explain why non-organized
meshes (interpreted as the oldest decay product)
are often composed of nanofibres of calcite, due
to the longer exposure time to soil fluids. To con-
clude, the release of nanofibres may represent a
partly destructive decay of the fibrous cell wall
material.
Moreover, this interpretation has an important
implication for NFC origin. The observation of
NFC inside organic sleeves and the presence of
small mats of nanofibres on bundles of NFC
(Fig. 5d (star); Cailleau et al. 2009) suggest a large
contribution of organic matter for their genesis.
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