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As first noted by Phillips & Self (1987), NFC
bundles could be the first step in the distribution
of NFC in soil pores. Indeed, with time, the collapse
of bundles due to various processes, such as weath-
ering and bioturbation, would lead to a random
distribution of NFC (mesh). Nanofibres on
bundles would then be a relict of the organic
sheath (assumed to be fungal in origin). The pres-
ence of mycelial strands is critical to understand
the origin of the bundles. Strands and bundles are
both organized as a tubular structure (Figs 3a &
5b) composed of sub-parallel components. They
have similar diameters: 2-30 mm on average for
the bundle and 8-80 mm for the whole fungal
strand. But the outer layer of the mycelial strand
is usually wide and often represents between a
third to a half of the strand section. Thus, only the
inner diameter should be considered in this case.
The outer layer is composed of hyphae with thick
cell walls. Their central hole is probably too small
to contain any NFC, whereas the inner part of the
strand contains wider hyphae that would have
enough room to allow the formation of a crystal
such as a needle.
One of the most important elements for fungal
growth is calcium. Indeed, it is implicated in the
apical growth control. Nevertheless, Ca
2þ
is con-
sidered as toxic when present in high concentrations
(Gadd 1993). Consequently, its concentration
within the fungal cell, and especially in the apex,
must be under strict control of the organism in
order to allow proper growth (Jackson & Heath
1993). Under hydrous stress conditions, the con-
centration of calcium could reach a high level,
close to saturation. As it has been suggested for
metal-oxalate (Whitney 1989; Gadd 1999), fungi
could induce the precipitation of carbonate, possibly
leading to a decrease of their internal calcium
content (Gadd 2007). This process is documented
for bacteria (Simkiss 1986; Schultze-Lam et al.
1996; Barton & Northup 2007). The inner layer of
the fungal cell wall is composed by a large
amount of chitin known to be a good template
for calcite precipitation (Manoli et al. 1997). Conse-
quently, nucleation of calcite crystals inside the
inner functional hyphae from mycelial strands
constitutes a serious hypothesis. The role of fungal
hyphae as a crystal nucleation enhancer has
already been suggested in the past (Went 1969;
Northup & Lavoie 2001; Gadd 2007). Any other
cell wall fibrous material or polymeric substance,
for example b (1-3) glucan or a glycoprotein, may
have the same effect (Burazerovic et al. 2007;
Shen et al. 2007). To conclude, all our observations
are recapitulated in a step-by-step hypothetical
model (Fig. 7), showing the potential relationships
between fungal organic matter and calcium carbon-
ate precipitation.
Conclusion
Considering previous hypotheses on the origin of
nanofibres (i.e. biogenic or purely physico-
chemical), the results presented here indicate that
nanofibres could also originate from the breakdown
of fungal hyphae, especially their cell walls. During
the decay of organic matter, microfibrils such as
chitin or b (1-3) glucan, are released from the
inner layer of the fungal cell wall. When these
organic nanofibres are exposed to mineralizing
pore fluids, they could undergo calcitic pseudomor-
phosis and/or be used as templates for calcitic pre-
cipitation. In the case of NFC bundles, which have
an intimate relationship with nanofibres, these nano-
features could indicate the relict of an organic
sheath. As interpreted by Phillips & Self (1987),
the implication of fungal strands in the genesis of
NFC is now better supported. In other words:
bundles could be the ultimate remains of the pres-
ence of a fungal strand. This hypothesis emphasizes
the important role of organic matter in carbonato-
genesis as well as the fundamental role of fungi in
the terrestrial carbon cycle.
The authors would like to thank Andr´ Villard andMich`le
Vlimant for their technical assistance for sample prep-
aration, especially for TEM purposes; Dr Massoud
Dadras, Dr Vladislav Spassov, and Mireille Leboeuf
from CSEM for their assistance in using electron
microscopy, and Dr. Thierry Adatte from UNIL for
X-ray diffraction analysis. This work is supported by the
Swiss National Science Foundation, Grant No. FN
205320-109497/1 and FN 205320-122171.
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