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growth. Their experiments showed that Ca-
carbonate precipitation took place during the
phase of exponential growth. From these exper-
iments, Castanier et al. (1999) developed the
notion of the carbonatogenic yield, which is
defined as the ratio between the weight of pro-
duced solid Ca-carbonate and the weight of
organic matter input.
† Rautaray et al. (2003) produced different types
and different crystal forms of CaCO 3 when
they added Fusarium sp. (fungus) and Rhodo-
coccus sp. (actinomycete) to an aqueous CaCl 2
solution and incubated it at 27 8C. In these exper-
iments, Ca came from the solution whereas the
CO 2 came from the microbes. Fusarium sp.
yielded cruciform-shaped calcite crystals
whereas Rhodococcus sp. produced rounded
crystals of vaterite. Experiments by Ahmed
et al. (2004) and Rautaray et al. (2004) produced
similar results. Solutions inoculated with Fusar-
ium oxysproum (fungi) produced CaCO 3 crystal-
lites that formed a circular superstructure
composed of calcite with minor amounts of
vaterite whereas Trichothecium sp. (fungi)
produced plate-shaped crystals of calcite with
preferential growth along the crystal edges
(Ahmad et al. 2004). Similarly, solutions with
Verticillium sp. (fungi) yielded circular-shaped
nanocrystals (70-100 nm diameter) of vaterite
whereas Thermomonos sp. (actinomycete)
resulted in the growth of plate-shaped calcite
and aragonite crystals, 20-30 nm thick, on the
mycelia and extracellularly (Rautaray et al.
2004). Fourier transform infrared (FTIR) ana-
lyses showed that all of these precipitates had
proteins incorporated in the crystal frameworks
with the types of protein varying in accord with
the mineral polymorph and crystal type.
Indeed, all of these studies suggested that the
proteins secreted by the microbes controlled
the type of precipitate and crystal morphology.
† Groth et al. (2001) tested the influence of various
microbes (mainly actinomycetes), obtained from
Grotta dei Cervi, Italy on mineral precipitation
by culturing them on a variety of different
media. These experiments showed that the acti-
nomycetes produced different types of minerals
(calcite, vaterite) and crystal forms that varied
in accord with the carbon source and the salts
that were present.
† Using bacteria (mostly Bacillus and Arthrobac-
ter) isolated from Stiffe cave, Italy, Ercole
et al. (2001) and Cacchio et al. (2003) showed
that many of them were capable of mediating
CaCO 3 precipitation. Although rhombohedral
calcite crystals (Cacchio et al. 2003, figs 4-6)
were the main precipitates, minor amounts
of vaterite were
experiments. Subsequently, Cacchio et al.
(2004) conducted experiments using eleven
strains of bacteria (Kocuria spp., Acinetobacter
spp., Bacillus sp.) isolated from Cervo cave,
Italy. Calcite produced in association with
Kocuria sp. was in the form of spheres whereas
Acinetobacter spp. produced crystal aggregates
in globular forms. These experiments showed
that all of the bacteria induced calcite precipi-
tation with some also producing vaterite.
† Hemispherical vaterite aggregates were pro-
duced in laboratory cultures of Acinetobacter
spp. that Sanchez-Moral et al. (2003) prepared
using microbial isolates from Altamira cave,
Spain. These spherulites, 5-20 mm in diameter,
had a radial internal structure, and were in some
cases, hollow. Crystals only formed when the
Ca acetate/Mg acetate ratio was .1. Cultured
Rhodococcus sp. produced spherulitic crystals
formed of monohydrocalcite. Precipitation of
metastable vaterite is favoured by various
factors including the presence of organic sub-
stances such as amino acids or mucopolysacchar-
ides (Manoli &Dalas 2000) and phosphorus-rich
media (Katsifara & Spanos 1999).
† Baskar et al. (2006) showed that Bacillus thurin-
giensis and B. pumilis, isolated from stalactites
collected from a cave in Dehradin Valley,
India, mediated the growth of calcite under lab-
oratory conditions. By varying the temperatures
of incubation, they showed that 25 8C was the
optimum temperature for precipitation.
Microbial processes in the twilight zone
The twilight zone, found around the entrance to the
cave, is the transition between normal light con-
ditions outside of the cave and the dark cave
interiors (Cox 1977; Cox & Marchant 1977;
Hoffman 1989). Geographical location and cave
configuration influence the climatic conditions
in the twilight zone with temperature, humidity,
and light gradients being especially important
(Hoffman 1989). That microbes thrive in the twi-
light zone is readily apparent from the green
colour of the biofilms that coat the cave walls. The
formative microbial communities of these biofilms
change as light levels decrease towards the interior
of the cave (Hoffman 1989; Rold´n et al. 2004).
Thus, green algae and cyanobacteria thrive on the
well-illuminated substrates near the cave entrance
(Dobat 1968) whereas the darker interior areas are
characterized by fewer microbes (Hoffman 1989).
Rold`n et al. (2004) examined phototrophic bio-
films from three limestone cavities in the karstic
Garraf massif of Spain. Although not caves in the
strictest sense, the trends they discovered in those
also produced in some
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