Microbes in caves: agents of calcite corrosion and precipitation - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 17

cavities are applicable to caves. They showed that
systematic changes in the temperature, relative
humidity, and light levels were reflected in the
biofilms and thereby identified three zones:
1. Entrance zone where the microclimate is
strongly influence by the outside climate and
light levels were high and microclimatic con-
ditions fluctuated widely throughout the year.
Algae, cyanobacteria, crustose lichens, and
mosses generated the abundant mucilaginous
biofilms found in this zone;
2. Intermediate zone where light levels were low
and microclimate conditions underwent only
moderate oscillations. Although the algal bio-
films are thinner and contain less mucilage
than those in the entrance zone, the microbial
communities are formed of cyanobacteria and
green algae; and
3. Deep zone where light levels ranged from very
low to zero but microclimate variables were
relatively stable. Only a few taxa are found
in this zone, including Geitleria calcarea,
Loriella osteophila, various bacteria, and
filamentous fungi.
Besides controlling the species consortia, the
light and relative humidity gradients also appear to
influence the nature of the biofilm (Rold´n et al.
2004). Thus, biofilms formed of mucilaginous and
dark coloured cyanobacteria were gradually
replaced by biofilms formed of calcified filaments
with colourless sheaths as the light levels decreased
and the relative humidity became more stable. It is
important to note, however, that distance from the
cave entrance may not be the only factor that con-
trols the nature of the biofilm. The amount of light
reaching a substrate, for example, also depends on
the orientation of the cave wall relative to the cave
entrance. Substrates facing the cave entrance will
be under the influence of incoming light whereas
substrates that face towards the cave interior will
be in a shadow zone and hence receive less light.
The type of biofilm that develops on a particular
substrate may also be partly controlled by the
water retention capabilities of the host rock (De
Winder et al. 1989), the ability of a microbe to
attach itself to a particular rock type (Guillitte &
Dreesen 1995; Hern´ndez-Marin´ et al. 2001), and
possibly, interactions between species (Costerton
2000).
The biofilms found on cave walls in the twilight
zone are important because they mediate a wide
range of destructive and constructive geological
processes that can have a significant impact on
the host substrates (Jones 1995). Microbialites
with stromatolite-like laminations formed of alter-
nating CaCO 3 (calcite and aragonite) and kerolite
(Mg-rich silicate) found on cave walls in the twilight

zone of caves on the north coast of Kauai, Hawaii,
are produced by biofilms (L ´ veill ´ et al. 2000).

Microbial processes in the aphotic zone

The aphotic zone, defined simply as that part of the
cave where there is no light, encompasses many
different habitats, ranging from the phreatic realms
in the cave pools, to flowing streams, to areas under
the influence of dripping waters, to bare dry surfaces.
The different processes that operate in these settings,
be they biogenic or abiogenic in nature, operate in
accord with local conditions. These contrasting set-
tings and their associated microbial processes are
herein illustrated by considering the growth and
development of stalactites and stalagmites, pool
fingers, cave pisoliths, and moonmilk.

Stalactites and stalagmites

The growth of stalactites and stalagmites through
the precipitation of calcite has commonly been
viewed as an abiogenic process (e.g. Kendall &
Broughton 1978; Broughton 1983a, b, c) even
though the presence of microbes on stalactites and
stalagmites has long been known (e.g. Palik 1960;
Nagy 1965; Went 1969; Hasselbring et al. 1975).
That microbial communities grow and thrive on
stalactites and stalagmites in caves throughout the
world has been amply demonstrated in many
recent studies (Groth et al. 2001; Basker et al.
2005; Barton 2006; Baskar et al. 2006; Baskar
et al. 2007; Baskar et al. 2009). Accordingly, it
has been suggested that the microbes may play an
important role in the growth of stalactites and/or
stalagmites (Jones & Motyka 1987; Mulec et al.
2007).
Lamina in some speleothems have been like-
ned to microbialites/stromatolites because they:
(1) contain mineralized microbes (Fig. 6d, e);
(2) contain organic inclusions; (3) locally form
outward expanding columns (Fig. 6a-c); and
(4) include precipitates akin to those found with
terrestrial microbialites (Jones 2001; Baskar et al.
2007; Baskar et al. 2009). Jones & Motyka (1987)
suggested that microbes contributed to the growth
of stalactites as they became calcified, provided
micrite through the breakdown of calcified fila-
ments, and trapped and bound sediment to the

substrate on which they were growing.

Pool fingers

Pool fingers, which are up to 2 cm in diameter and
50 cm long, were first identified from Lechuguilla
Cave New Mexico, USA by Davies et al. (1990).
Although no longer in water, Davies et al. (1990)