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appear to be characteristic of settings high in dis-
solved inorganic carbon (Arp et al. 1998, 2003),
and may be traceable in the fossil marine record as
well (e.g. Archaean 'cuspate microbialites' with ver-
tical supports and draping laminae; Sumner 1997).
The transfer suggested above requires annota-
tions because marine and non-marine (particularly
freshwater) ecosystems are commonly regarded as
almost unrelated:
(1) Hydrochemical principles such as ion activity
products defining mineral saturations and pH buffer-
ing by weak acids and their conjugate bases are valid
in all aqueous settings. The crucial point for a transfer
from non-marine to marine settings is that different
concentrations of chemical species have to be
included in model calculations by using computer
programs accounting for ionic strength, ion pairing,
and pH-dependent dissociations (e.g. PHREEQC;
Parkhurst & Appelo 1999).
(2) Seawater composition changed significantly
during the Precambrian as well as during the Pha-
nerozoic (Hardie 1996; Lowenstein et al. 2001;
Habicht et al. 2002; Berner 2004) and present-day
seawater calcite and aragonite supersaturation is
lower than during most periods of the Phanerozoic
(e.g. Riding & Liang 2005). Consequently, various
non-marine calcifying biofilm systems may be
more suitable to elucidate factors and mechanisms
in the formation of fine-grained and skeletal fossil
marine stromatolites than the coarse-agglutinated
stromatolites of the present-day marine.
(3) While the detected prokaryote species
(cyanobacteria and - as far as cultivated - bacilli,
arthrobacteria, pseudomonads, flavobacteria) in the
karstwater streams find their closest relatives in
other freshwater or soil habitats (e.g. Claus & Ber-
keley 1986; Jones & Keddie 1992; Taton et al.
2003; Bernardet & Nakagawa 2006), their function
in biogeochemical cycles should nonetheless be the
same in the marine and non-marine (e.g. oxygenic
photosynthesis by cyanobacteria, degradation of
high-molecular-weight organic compounds by
flavobacteria). Major differences exist with respect
the potential effect of bacterial sulphate-reduction
between freshwater and marine stromatolite for-
mation. Sulphate-reducing bacteria have to date
not been detected in significant numbers in the
investigated karstwater stream biofilms - which
may either reflect methological failure or the com-
bination of high O 2 and low SO 22 . However,
the effect of sulphate-reduction on CaCO 3 pre-
cipitation in marine settings is controversially
discussed (Bosak & Newman 2003; Aloisi 2008)
and may rather relate to removal of kinetic inhi-
bitor SO 22 than to increasing CO 22 . Indeed,
future studies combining results from sulphate-
rich and sulphate-poor microbialite-forming set-
tings will likely provide substancial insights in
formation mechanisms and palaeoenvironment of
fossil microbialites, rather than focussing on single
settings alone.
General conclusions
(1) Independent from the initial Ca /alkalinity
ratio, loss of CO 2 goes hand in hand with an increase
in calcite supersaturation, approaching saturation
index maximum values of c. 1.0, not surpassed
due to concomitantly increasing calcite precipi-
tation. Stream waters with higher concentrations
of Mg and SO 22 reach higher calcite saturation
values than stream waters with low Mg and
SO 22 concentrations. Physicochemical precipi-
tation seems to be most effective in the Erasbach
rivulet, where inhibiting Mg and SO 22 are
lowest and a self-build tufa-canal largely composed
of spar-cemented moss plants forms. Generally,
there is no detectable impact of biofilm photosyn-
thesis on bulk water chemical composition, neither
seasonal nor diurnal.
(2) For the cyanobacteria, the most dominant
photoautotrophic organisms in the biofilms on tufa
surfaces, a culture-independent molecular approach
showed that microscopy of resin-embedded biofilm
thin sections underestimated the actual diversity of
cyanobacteria, i.e. the six cyanobacteria morpho-
types were opposed to nine different lineages of the
16S rDNA phylogeny. The same morphotype may
even represent two genetically distant cyanobacteria
and the closest relatives of tufa biofilm cyanobacteria
may be from quite different habitats. A drilling core
5 cm deep into the tufa rock revealed that the vast
majority of cyanobacteria was in fact located
within the biofilm on the tufa surface, but a few cya-
nobacteria (obviously with an endo- or chasmolithi-
cally life style) were recovered even from deeper
layers below the biofilm. The diversity of biofilm
diatoms was even higher than that of the cyanobac-
teria at the studied exemplar site, i.e. 13 diatom
species opposed to 9 cyanobacterial lineages.
(3) Based on cultivation, the non-phototrophic
prokaryotic community of the stream waters,
which is largely derived from adjacent soils, signifi-
cantly differs from that of the tufa-forming biofilms.
Based on clone numbers from tufa stromatolite
laminae below the Cyanobacteria-dominated tufa-
surface biofilm, an enormous variety of taxa were
identified which are likely to represent a highly
diverse metabolic and morphological spectrum of
bacteria. However, only a few strictly anaerobic
prokaryotes were detected.
(4) Lectin-binding analysis indicate the presence
of at least three structural EPS glycoconjugate
domains: cell (cyanobacteria/diatom/bacteria)-
associated, as well as sheet-like and cloud-like EPS
domains. Seasonal and spatial variability of these
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