Calcitic nanofibres in soils and caves: a putative fungal contribution to carbonatogenesis - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 221

Calcitic nanofibres in soils and caves: a putative fungal
contribution to carbonatogenesis

SASKIA BINDSCHEDLER 1 , L. MILLI ` RE 1 , G. CAILLEAU 1 , D. JOB 2 &
E. P. VERRECCHIA 1 *
1 Institut de G´ologie et de Pal´ontologie, Universit´ de Lausanne, Anthropole,
1015 Lausanne, Switzerland
2 Institut de Biologie, Universit´ de Neuch ˆtel, Rue Emile Argand 11,
2007 Neuch ˆ tel, Switzerland
*Corresponding author (e-mail: [e-mail])

Abstract: The origin of soil mineralized nanofibres remains controversial. It is attributed to either
biogenic factors or physicochemical processes. Scanning electron microscope and transmission
electron microscope observations show that nanofibres could originate from the breakdown of
fungal hyphae, especially its cell wall. It is hypothesized that during the decay of organic
matter, cell wall microfibrils are released in the soil where they are exposed to mineralizing
pore fluids, leading to their calcitic pseudomorphosis and/or are used as a template for calcite pre-
cipitation. When associated with needle fibre calcite bundles, nanofibres could indicate the relict of
an organic sheath in which calcite has precipitated. This paper emphasizes the important roles of
both organic matter and fungi in carbonatogenesis, and consequently in the soil carbon cycle.

Natural nanofibres have been observed in various
environments such as subtropical and temperate
soils (Verrecchia & Verrecchia 1994; Cailleau
et al. 2005) and cave deposits such as moonmilk
(Borsato et al. 2000; Ca˜averas et al. 2006). They
are often associated with Needle Fibre Calcite
(NFC; Verrecchia & Verrecchia 1994; Borsato
et al. 2000). The aim of this study is to provide new
insight into the processes at the origin of nanofibres.
In order to differentiate between organic and
mineral nanofibres, the term 'organic nanofibre'
will be used for nanofibres whose organic nature
has been determined by analytical methods. The
term 'mineral nanofibre' will be used for: (i) for
nanofibres observed in scanning electron micro-
scopy (SEM), in absence of specific labelling (see
'Materials and methods' section); and (ii) for nano-
fibres diffracting the electron beam under trans-
mission electron microscopy (TEM). The term
nanofibre alone refers only to a shape or an object
and therefore used for morphological descriptions.

structure composed by a dense nanofibre scaffolding
(Borsato et al. 2000; Richter et al. 2008); (ii) a
straight macro-structural alignment (3-5 mm wide
and .70 mm long) of unordered nanofibres obser-
ved close to an organic filament (possibly actino-
mycetes, cyanobacteria, or fungi; Benzerara et al.
2003); and (iii) 3 mm wide and .50 mm long fila-
ments interpreted as 'calcified filaments with
needles in grain-coating needle mat' have also
been observed by Jones & Ng (1988).

Filamentous organisms and structures
in soils and caves

Filamentous organisms living within the soil or in
caves must be heterotrophic organisms. Algae and
cyanobacteria are photosynthetic organisms and
thus are present only at the soil surface or in rock
fractures near a light source. Indeed, in mineral sub-
strates that are far away from any light, these organ-
isms are absent due to the lack of their energy
source. Accordingly, filamentous fabrics present in
these environments could be fungi, filamentous
bacteria (in soils and caves mostly actinomycetes),
and roots (Paul & Clark 1996; Gobat et al. 2003).
Taking into account their sizes and morphologies
summarized in Table 2, fungi are the most suitable
organisms associated with nanofibres and NFC.

Previous work on mineral nanofibres

Since 1980, many authors have reported mineral
nanofibres from various environments (Table 1).
The four following authors have specifically
observed organized structures related to mineral
nanofibres: (i) filamentous, ramified, microscopic