Microbes in caves: agents of calcite corrosion and precipitation - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 6

Microbes in caves: agents of calcite corrosion and precipitation

BRIAN JONES
Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta,
T6G 2E3, Canada (e-mail: [e-mail])

Abstract: Diverse biogenic and abiogenic processes produce calcite speleothems. From a bio-
genic perspective, cave microbes mediate a wide range of destructive and constructive processes
that collectively influence the growth of calcite speleothems and their internal fabrics. Destructive
processes include substrate breakdown by dissolution, boring and residue micrite production,
whereas constructive processes include microbe calcification, trapping and binding of detrital par-
ticles to substrates, and microbial induced calcite precipitation. Biogenesis can be established
from: (1) the presence of mineralized microbes; (2) fabrics, such as stromatolite-like structures,
that can be attributed to microbial activity; and/or (3) geochemical proxies (carbon and oxygen
isotopes, lipid biomarkers) considered indicative of microbe activity. Such criteria have, for
example, been used to demonstrate microbial involvement in the formation of pool fingers,
stalactites/stalagmites, cave pisoliths and moonmilk. Nevertheless, absolute proof of microbial
biogenesis in calcitic speleothems is commonly difficult because taphonomic processes and/or
diagenetic processes commonly mask evidence of microbial activity. The assumption that calcitic
speleothems are abiogenic, which has been tacitly assumed in many studies, is dangerous as there is
clear evidence that microbes thrive in most caves and can directly and indirectly influence calcite
precipitation in many different ways.

With their wondrous and complex morphologies
that develop in dark cave environs, speleothems
have long been the subject of scientific attention
and curiosity. The calcite crystals that form these
speleothems have been the focus of many studies
with emphasis commonly being placed on their
crystallography and the factors that controlled
their growth (White 1976; Kendall & Broughton
1978; Broughton 1983a, b, c). Such studies were
commonly based on the tacit assumption that the
calcite must be of abiogenic origin because it grew
in the dark environs of caves. Such an assumption
is false because microbes, including bacteria,
fungi, actinomycetes, and Archaea, thrive in most
caves despite the lack of light (e.g. Høeg 1946;
Claus 1955; Mason-Williams 1967) - a fact that
has been emphasized in many reviews (Barton
et al. 2001; Northup & Lavoie 2001; Barton 2006;
Barton & Jurado 2007; Barton & Northup 2007;
Mulec 2008; Taboro ˇ i 2008). Given the presence
of microbes in caves throughout the world, the
role that they play in the growth and development
of speleothems has become a topic of considerable
interest.
Caves can be divided into two parts: (1) the twi-
light zone, which is the transition between lightless
cave interior and the outside world; and (2) the
aphotic zone where there is no light. Cave walls in
the twilight zone are commonly covered with a
green, mucilaginous biofilm that is formed by
microbes that thrive in the light that penetrates
through the cave entrance. Although less obvious

in the aphotic zone, biofilms and their formative
microbes are present on cave walls and the spe-
leothems that adorn the cave. Calcite precipitation
in caves has commonly been directly or indirectly
linked to these microbial communities (e.g. Danielli
& Edington 1983; Cox et al. 1989a, b; Polyak &
Cokendolpher 1992; Cox et al. 1995; Cox et al.
2001; Melim et al. 2001). Potentially, microbes
can mediate destructive ('erosion' of Cunningham
et al. 1995) and constructive ('microbe-assisted
depositional forms' of Cunningham et al. 1995)
processes (Jones 2001). Although operational on a
micro-scale, these processes have the potential of
significantly impacting the growth and development
of speleothems.
Assessment of the role of microbes in caves lies at
the interface betweenmicrobiology and geology: the
former deals largely with determining the taxonomy
of the microbes and their life modes, whereas the
latter deals mostly with the destructive and construc-
tion geological processes mediated by the microbes.
This paper approaches the problemfroma geological
perspective with emphasis being placed on the roles
that microbes play in the growth, destruction, and
development of calcitic speleothems. It is based on
a comprehensive assessment of the literature and
information obtained from speleothems that adorn
various caves on theCayman Islands (Fig. 1). Collec-
tively, this informationprovides a critical assessment
of the influences that microbes have on speleothem
development and also highlights areas where our
knowledge is in its infancy. Although considerable