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precipitation of calcium carbonates (e.g. Braissant
et al. 2007). In contrast, coral skeletons are
usually considered as biologically controlled depos-
its, with specific genes controlling the achievement
of the coral architecture, although a significant phe-
notypic plasticity due to environmental conditions is
also observed (Todd 2008). The main conclusions of
this general view are not contested here. There are
obvious structural differences between corals and
stromatolites such as the absence of centers of
rapid accretion (i.e. centres of calcification) in stro-
matolites, variations in the orientation of the c-axes
of aragonite within a single stromatolite lamina that
are not visible in corals, or the absence of Mg -
Si-rich laminae in corals. However, in the present
study we show that some textural features, which
have been attributed specifically to corals in the
past and interpreted as the result of a biologically
controlled process (e.g. clusters of nanocrystals
within the centres of rapid accretion, crystallo-
graphic alignment of aragonite crystals, striations
interpreted as growth bands), can also be found in
stromatolites.
Two alternative conclusions can be drawn from
this comparison: (1) biologically controlled miner-
alization processes may also play a role in the
formation of Satonda stromatolites, at least to a cer-
tain extent; or (2) these textural features for which
the precise mode of formation is unknown cannot
be used as signatures of bio-controlled processes
in corals. Using a reductionist but possibly heuristic
approach, we discuss below some of the similarities
and differences in the mineralogical textures of
corals v. stromatolites at the submicrometre-scale.
From a mineralogical point of view, it is interest-
ing to note that corals are assemblages of basic units
consisting of aragonite fibres, aragonite nanoglo-
bules (forming centres of calcification in scleracti-
nian corals), and organic polymers (associated
with the fibres as well as centers of rapid accretion,
e.g. Stolarski 2003; Cuif & Dauphin 2005). No
centre of rapid accretion (i.e. micrometre-sized clus-
ters of aragonite nanocrystals with a very precise
crystallographic alignment), was observed in
Satonda stromatolites. However, Satonda stromato-
lites show similar fibrous, single-crystal aragonite as
well as aragonite nanocrystals. Moreover, several
studies have previously shown that aragonite or
calcite nanocrystals are often associated with
organic polymers in stromatolites (e.g. Kawaguchi
& Decho 2002; K ¨hl et al. 2003; Kazmierczak
et al. 2004; Dupraz & Visscher 2005; Benzerara
et al. 2006; Kremer et al. 2008). The reason why
two types of aragonite morphologies (fibrous
v. micritic) are observed in these objects is not
clear. Several authors have proposed for Satonda
stromatolites as well as for corals that the micritic
texture in both coral and stromatolites may be the
result of precipitation in organic-enriched micro-
environments (centers of rapid accretion in corals,
or in living or partially degraded biofilms in stroma-
tolites), while fibres would form in an organic-poor
environment (e.g. Kempe & Ka´mierczak 1993;
Arp et al. 2003; Stolarski 2003; Kempe &
Ka´mierczak 2007). This proposal is supported by
observations that aragonite nanocrystals within
microbialites are surrounded by organic polymers
(e.g. Benzerara et al. 2006) and that the experimen-
tal precipitation of carbonates in an organic matrix
results in the formation of numerous and very
small grains (Sethmann et al. 2005; Aloisi et al.
2006). In stromatolites from the Bahamas, three
different types of microbial communities exhibiting
different physical structures were identified. Micri-
tic laminae composed of small aragonite fibres
and not nanospheroids were shown to be specific
to one of these communities (Reid et al. 2000;
Visscher et al. 2000; Petrisor et al. 2004). It
should be noted that in these open marine stro-
matolites, aragonite fibres can nucleate within
EPS-rich biofilms, balancing the idea that fibres
would always be associated with organic-deprived
environments (Reid et al. 2000; Visscher et al.
2000). Finally, it has been shown that different
EPS have various affinities for cations and may
produce various polymorphs of calcium carbonate
(e.g. Braissant et al. 2003), suggesting an additional
source
of
variety
in
the
texture
of
calcium
carbonates.
There are genuine single-crystal aragonite fibres
within stromatolites as observed in the present study
(e.g. Fig. 3). They share a preferential crystallo-
graphic orientation along the growth axis (c-axis),
perpendicular to the laminae, suggesting an abiotic
growth process that initiates from an underlying
surface. In contrast, some fibres may have formed
by the clustering of aragonite nanocrystals resulting
in what have been called mesocrystals, when nano-
domains share a common crystallographic orien-
tation. The existence of such mesocrystals in
stromatolites is suggested by the STXM polarization-
dependent images in the present study. Their pres-
ence in microbialites may have been overlooked in
the past and will have to be assessed more
thoroughly in the future. Interestingly, the aggrega-
tion of oriented vaterite nanocrystals with c-axis
normal to the bacterial cell wall has been observed
in cultures of Myxococcus xanthus during bac-
terially induced calcification (Rodriguez-Navarro
et al. 2007). This is one of the very few studies
showing such a pattern involving prokaryote cells.
In contrast, abundant mesocrystals have been
observed in several eukaryote biomineralizing
systems such as corals (Cuif & Dauphin 2005;
Przeniosło et al. 2008; Vielzeuf et al. 2008), includ-
ing oriented aragonite nanocrystals within centers of
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