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or were to some extent completely covered by
calcium carbonate crystals (Fig. 11h, i). A clear
association
Physicochemical microgradients and
mass balance
of
calcium
carbonate
crystals
with
diatoms
was
not
detectable
during
any
of
the
In order to evaluate the effects of biofilm activity
on microenvironmental physicochemistry at tufa
stromatolite surfaces, pH, O
2
,Ca
2þ
and CO
22
microsensor profiles were measured (Bissett et al.
2008a; Shiraishi et al. 2008a, b). These measure-
ments were conducted in situ at Deinschwanger
Bach and Westerh
¨
fer Bach, and ex situ, in the lab-
oratory, using tufa stromatolite pieces removed
from their natural site (Fig. 12). In-situ measure-
ments
sampling periods.
Conclusions
In summary, the lectin-binding analysis in tufa
biofilms suggests that: (1) Detected EPS glyco-
conjugates can be distinguished into three major
structural domains. Based on their morphological
appearance glycoconjugates closely associated
with phototrophic and heterotrophic microorgan-
isms were found. However, at the moment a clear
separation of bacterial and diffuse EPS glycoconju-
gates is not possible via lectin-binding approach;
(2) Calcium carbonate crystals were detected in all
three structural EPS domains. Crystals nucleated
as embedded structures in diffuse or sheet-like
EPS glycoconjugates or were associated with EPS
glycoconjugates of cyanobacterial origin. It is
assumed that heterotrophic degradation of EPS
below the phototrophic biofilm part (i.e. major
zone of primary production) causes a break-down
of the inhibitory effect of EPS, thereby permitting
calcite overgrowth on existing calcite crystals (neo-
morphism) within calcite supersaturated pore
waters. In order to proof this hypothesis, pure
culture studies under controlled environmental con-
ditions are necessary. In addition, further studies on
in situ structure and function of diatom EPS
are required to elucidate their assumed inhibitory
effect
and
laboratory
experiments
showed
similar results.
Microgradients
Under illumination, pH and O
2
and CO
22
con-
centrations increased from the water column to
the biofilm surface, while Ca
2þ
concentration
decreased. Consequently, calcite supersaturation
calculated from Ca
2þ
and CO
22
microprofiles
exhibited a strong increase from 8-fold in the
water column to 37-fold at the biofilm surface
(Fig. 12). Under these conditions, CaCO
3
is precipi-
tated, as indicated by radioactive isotope (
45
Ca
2þ
)
uptake studies (Bissett et al. 2008a). In turn, under
darkness, pH, O
2
and CO
22
decreased toward the
biofilm surface, while Ca
2þ
increased mariginally.
As a result, calcite supersaturation decreased from
8-fold in the water column to 6-fold at the biofilm
surface (Fig. 12). Incubation experiments using
45
Ca
2þ
indicate that no CaCO
3
is precipitated
under darkness (Bissett et al. 2008a). Indeed, at a
biofilm-free limestone surface used as a control
on
calcite
nucleation
in
the
karstwater
streams.
Fig. 12. pH, O
2
,Ca
2þ
and CO
22
microprofiles at tufa biofilms measured in situ (Deinschwanger Bach, left) and ex
situ (Westerh¨ fer Bach, middle). As a control, pH and Ca
2þ
microprofiles measured at a biofilm-free limestone substrate
are shown on the right hand side. Open circles indicate light profiles, and closed circles indicate dark profiles.
Saturation state (V calcite) calculated from Ca
2þ
and CO
22
profiles is also shown. From Bissett et al. (2008a), with
permission of the American Society of Limnology and Oceanography, and Shiraishi et al. (2008a), with permission of
Taylor & Francis Ltd.
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