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morphological features but chemical signatures of
the microbes are rarely, if ever, preserved. Thus,
identification in terms of extant taxa depends, to a
large extent, on the criteria first used to establish
the taxonomy of those taxa. As chemotaxonomic
characteristics are increasingly used to define new
microbial taxa, so does the probability of associat-
ing a mineralized microbe with an extant microbe
decrease. This is especially true in situations
where modern microbes are defined with virtually
no mention of their morphological attributes.
Indeed, when dealing with mineralized microbes it
is commonly difficult even to determine affinity to
one of the major microbial groups.
The notion that cave microbes can influence
calcite precipitation arises, at least in part, from
the fact that such microbes directly or indirectly
induce calcite precipitation when cultured in the
laboratory (e.g. Cacchio et al. 2003; Rautaray
et al. 2003; Cacchio et al. 2004; Rautaray et al.
2004). Even this must be treated with some
caution given the difficulty of exactly replicating
cave conditions in a laboratory setting (Chalmin
et al. 2008). Nevertheless, mineralized microbes
found in calcite speleothems have commonly been
implicated in their growth and development
through a wide variety of destructive and construc-
tive processes (e.g. Jones 2001; Melim et al. 2001;
Northup & Lavoie 2001; Barton & Northup 2007).
The mere presence of microbes, however, does
not guarantee that they actually played any role in
the formation of the surrounding calcite because
they may simply have been buried in the calcite
during precipitation (Polyak & Cokendolpher
1992; Forti 2001). Assessment of the exact role that
the microbes played in the calcite accumulation/
precipitation may be virtually impossible to deter-
mine given the complexity of the systems involved.
Even in controlled laboratory experiments it can be
difficult to determine the exact reason why a particu-
lar CaCO
3
polymorph was precipitated or why a
particular crystal form resulted.
Available evidence - be it from experiments,
from microbiological studies, or from geological
studies - clearly indicates that microbes may play
an important role in the growth and development
of calcitic speleothems. In most cases, however, it
is difficult to know if microbes played an active
role in calcite precipitation or were passive bystan-
ders that suffered accidental burial in the calcite
(Forti 2001). Interestingly, similar debates are
ongoing with respect to the role(s) that microbes
play in the growth and development of opal-A pre-
cipitates that are accumulating on the discharge
aprons of many springs (e.g. Konhauser et al.
2004). Absolute proof of the role that microbes
play in the formation of calcitic speleothems may
be impossible to attain given the microscale at
which the processes operate.
Despite the problems it should be possible to
infer, with a reasonable level of confidence, that
microbes were, in one way or another, instrumental
in the growth and development of speleothems. If
mineralized microbes are present, this can be
based on documentation and recognition of micro-
bes in numbers sufficient to have mediated any
process that is attributed to them (Jones 2001;
Melim et al. 2001), and recognition of structures
and fabrics (e.g. stromatolites, dissolution features)
that are consistent with known microbial activity
(Jones 2001; Melim et al. 2001). In the absence of
mineralized microbes, or if indirectly microbial
influences are suspected, proxies for microbial
activity must be obtained. Such proxies may
include: (1) lighter d
13
C isotopic signatures that
may result from microbial fractionation of C
(Melim et al. 2001; L
´
veill
´
et al. 2007);
(2) heavier d
18
O isotopic signatures that can be
ascribed to bacterial activity (Gradzinski 2001,
2003); (3) biomarker signals related to microbially
derived lipid groups (Blyth & Frisia 2008); or
(4) the presence of proteins in the calcite that were
inherited from the formative microbe (Rautaray
et al. 2003; Ahmad et al. 2004; Rautaray et al. 2004).
Implications for palaeoclimate studies
The calcite in stalagmites and stalactites is geneti-
cally linked to the groundwater from which it is
precipitated and should therefore incorporate geo-
chemical signals that can be related to the climate
of the area. This premise has led to the development
of various climatic proxies, including growth rates
as determined from annual growth laminae (Genty
& Quinif 1996; Qin et al. 1999; Proctor et al.
2000; Baker et al. 2008), stable isotopes (Bar-
Mattchews et al. 1999; Lauritzen & Lundberg
1999; McDermott et al. 1999), Sr isotopes (Banner
et al. 1996; Goede et al. 1998; Verheyden et al.
2000), and trace elements (Roberts et al. 1998; Fair-
child et al. 2001; Huang et al. 2001; Treble et al.
2003; Borsato et al. 2007). These studies tacitly
assumed that the calcite crystals in the stalagmites
Fig. 8. (Continued) that are probably actinomycetes spores encased with calcite. Note calcified mucus in background
(arrow). (i) Possible calcified mucus (arrows) coating surface of calcite crystals. ( j) Calcified, partly collapsed filaments
embedded in micrite matrix. (k) General view of micrite lamina with no obvious microbes. Square and white letter
'l' indicates position of Fig. 8l. (l) Poorly preserved collapsed filaments embedded in micrite groundmass.
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