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shown for various diatom cultures by Wustman
et al. (1998), were not detected in the tufa-forming
biofilms investigated.
A combined staining approach of bacterial cells
and EPS glycoconjugates revealed that especially
the cloud-like EPS glycoconjugates were densely
colonized by bacterial cells (Fig. 11f). If bacteria
are located outside of such a cloud-like structure it
seems likely that the EPS glycoconjugates are
degraded by bacteria. Whereas if these EPS glyco-
conjugates were produced by bacteria, single cells
or cell clusters should be located inside the EPS gly-
coconjugates. In the future the origin of these cloud-
like EPS structures will be investigated in more
detail in order to clarify their function in terms of
mineral formation within tufa-forming biofilms.
Fucose and amino sugars (N-acetyl-glucosamine
and N-acetyl-galactosamine) are main constituents
of the detected EPS glycoconjugates produced by
cyanobacteria (¼ cell associated EPS) and sheet-
like EPS glycoconjugates, according to Sharon &
Lis (2003) and the information about lectin speci-
ficity provided by the supplier (Vector Inc., EY
Laboratorities, Inc.).
Diffuse lectin-specific EPS material not associ-
ated with cells suggested sialic acids, amino
sugars and galactose as predominating compounds
(Zippel et al., submitted). Detection of EPS in calci-
fying communities by using single or a few
fluorochrome-labelled lectins in combination with
CLSM has been shown by various authors. For
instance, EPS glycoconjugates secreted by Schizo-
thrix ep. were detected after ConA-FITC staining
and were associated with carbonate sand grains
(e.g. Kawaguchi & Decho 2002). In a study on the
architecture and structure of stromatolites from a
Mediterranean stream it was shown that an exten-
sive glycoconjugate network (stained with
succ-ConA-FITC) was present throughout the car-
bonated structures (Sabater 2000). Other exopoly-
saccharides produced by cyanobacteria isolated
from Polynesian microbial mats (Kopara) were
characterized by chemical analysis (Richert et al.
2005). It was found that both capsular and released
EPS consisted of 7 - 10 different monosaccharides
with neutral sugars dominating. For example,
fucose, xylose and glucose were present in all EPS
fractions. Glucosamine residues were found in the
released EPS of strain Rhabdoderma cf. rubrum,
and in both fractions of Chroococcus submarinus.
In both streams studied, Westerh¨fer and
Deinschwanger, the seasonal variability (April,
July, and October 2007 and 2008) of EPS gly-
coconjugates within the tufa-forming biofilms
was low, based on a panel of six selected lectins
(AAL_Alexa488; HMA_Alexa488; PNA_FITC;
LEA_FITC; LcH_FITC; and WGA_FITC). Sur-
prisingly,
conditions in terms of relevant nutrients (P, N) in
the stream water, the lectins investigated showed
in most cases the same binding pattern in both tufa-
forming biofilms (Zippel et al., submitted). This
implies that, independent of the trophic status of
the habitats, the dominant EPS structures consisted
more or less of the same glycoconjugates. Neverthe-
less, the overall occurrence of lectin-specific EPS
glycoconjugates was up to 50% higher in the
nutrient-poor stream (WB). A detailed analysis of
structural EPS composition in combination with
competive inhibition assays revealed that at least
sialic acid-specific components of diffuse EPS as
well as N-acetylglucosamine-specific compounds
produced by cyanobacteria were up to 50% higher
under nutrient limited condition in Westerh¨fer
Bach (Zippel et al., submitted). On the one hand,
this could be caused by higher cyanobacterial EPS
production under nutrient-limited conditions. A
stimulation of carbohydrate synthesis caused by
N- or/and P-limitation was also found for
example, in unicellular cyanobacteria isolated
from hypersaline habitats (de Philippis et al.
1993), in benthic diatoms or intertidal mudflats
(Stal 2003). It was highlighted that the increased
exudation of EPS was caused by unbalanced
growth and represented an overflow metabolism.
On the other hand, it might be possible that higher
metabolic activity of the heterotrophic community
induces an increased degradation of the EPS com-
pounds under nutrient-enriched conditions.
A large variety of bacterial morphotypes was
detected by laser scanning microscopy as shown in
Figure 11d - f. Single cells of filament-, coccoid-
as well as rod-shaped bacteria colonized either the
diffuse EPS matrix or discrete EPS structures like
the empty sheaths of filamentous cyanobacteria.
Furthermore, different bacterial morphotypes were
detected within network-like EPS structures
(Fig. 11e) which sometimes covered extended
areas of the uppermost biofilm layer. In addition,
dense bacterial colonies were observed either as
clusters which completely covered the diffuse EPS
structures (Fig. 11f) or in the form of connected
bands on top of the diffuse EPS matrix (image not
shown). It is possible that the diffuse EPS matrix
may be produced to some extent by the colonizing
bacteria. Nevertheless, bacteria which colonize the
surface of diffuse EPS glycoconjugates are likely
to be involved in their degradation.
A combined application of the lectin-binding
approach and calcium carbonate staining with
calcein revealed that calcium carbonate crystals
were detectable within all defined structural EPS
domains (Fig. 11g - i). In some cases, crystals of
different size were closely associated with empty
sheaths of cyanobacteria (Fig. 11g). In addition,
sheet-like and cloud-like EPS structures contained
despite
the
different
hydrochemical
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