Geology Reference
In-Depth Information
2000) and had an average concentration of 0.95-
0.35 mM g
21
of bacteria. The carboxyl group is a
basic component of several extracellular polymers
constituents (Wingender et al. 1999). For compari-
son, the carboxyl sites on the bacterial cell surface
were reported to have pK
a
values of 5.17 and 5.25
(Cox et al. 1999), 4.85+0.31 or 4.98+ 0.16
(Dittrich & Sibler 2005) and the humic acid to
have a pK
a
value of 4.2 (Brassard et al. 1990).
The pH range of the 5.6-6.0 sites may be attributed
to carboxyl or phosphoric groups (Comte et al.
2006c). The average concentration of this site was
0.64 mM g
21
of bacteria. The site at pK
a
6.8 may
be attributed to the phosphoric sites, e.g. triprotoic
phosphoric acid has the second dissociation con-
stant pK
a
value of 6.82. Phosphate groups can
exist in several different forms: inorganic forms of
phosphate such as orthophosphate and its oligomers,
and organic species in the form of phosphate mono-
and diesters. The concentration of this site was
0.41 mM g
21
bacteria for PCC 7942. A site at pK
a
7.8-8.2, which was likely attributed by the sulphy-
dryl groups, was identified (Hornback 1998). The
sites at pK
a
9.4 and 10 had a concentration of 0.09
and 0.71 mM g
21
bacteria for PCC 7942 was
likely attributed by phenolic or amine (Cox et al.
1999). These compounds are abundant in humic
substances and amino sugars. Among the five
binding sites for polysaccharides of PCC 7942,
the carboxylic site has the highest concentration
of 1.23 mM g
21
bacteria. Liu & Fang (2002)
studied the electrostatic characteristic of binding
sites of EPS and found a pK
a
of 6.0 for car-
boxylic/phosphoric groups, a pK
a
of 7.0-7.4 for
phosphoric groups, a pK
a
of 9.4-9.8 for amine/
phenolic groups.
The corresponding results for extracellular poly-
saccharides extracted from Syn. Red illustrated in
Figure 2a-b. The five sites found were comparable
to those found in the samples of PCC 7942 strain.
The concentrations of binding sites at pK
a
¼ 3-4.2
and 10 were highest. The results of the potentio-
metric titrations for Syn. Green are very similar to
those for strain PCC 7942 and Syn. Red (Fig. 3a).
Table 1 summarized the modelled concentrations
of binding sites in polysaccharides extracted from
three strains. PCC 7942 and Syn. Green have the
same total excess charge whereas it was five times
higher for Syn. Red. It was c. 3.7 mM g
21
bacteria
for PCC 7942, 15.0 mM g
21
bacteria for Syn. Red
and 4.1 mM g
21
of bacteria for Syn. Green. For
comparison, the sheath of gram-negative Calotrix
has 0.18 mM and Calotrix cells have 1.46 mM g
21
of bacteria (Phoenix et al. 2002). Borrok et al.
(2005) compared the buffering capacity of 36 differ-
ent bacterial species, and it was found to be around
3.2 mM g
21
of dry bacteria or 0.32 mM g
21
of
wet bacteria.
The total concentrations of the electrostatic
binding sites found in this study were comparable
to those reported for natural organic matters
(4-24 mM g
21
) using the same method (Bird &
Wyman 2003). In this study, titration was carried
out starting at pH ¼ 3. The pH in natural water
normally ranges between 6 and 9. At pH 6 only
binding sites at pK
a
¼ 4.4-4.8 carry negative
charge. Therefore, the charges carried at pH 6 in
the three strains were 21 meq g
21
, 25 meq g
21
and 2 1 meq g
21
assuming the absence of sites
carrying positive charge. These values are lower
than 215.7 to 21.2 meq g
21
- EPS measured for
three anaerobic sludges (Jia et al. 1996). This
could be due to the negligence of those sites at
pK
a
below 3 in this study.
The deprotonation constants obtained here (see
Table 1) represent functional groups similar to
Table 1. Deprotonation constants and concentrations of corresponding binding sites in EPS extracted from
three strains
Functional
groups
EPS of picocyanobacteria
PCC 7942
Syn. Red
Syn. Green
L
T
(mM g
21
bact.)
L
T
(mM g
21
bact.)
L
T
(mM g
21
bact.)
pK
a
pK
a
pK
a
Carboxylic
3/3.8-4.6
0.98/0.35
3/4.2
2.94
3/4.4-5.4
0.86/0.38
Carboxylic-
phosphoric
5.6-6.0
0.64
5.2-6.4
2.65
6
0.52
Phosphoric
6.8
0.41
6.8-7.4
2.76
6.8
0.82
Sulfhydryl
7.8-8.2
0.48
8.2
1.43
8
0.43
Amin-
phenolic
9.4
0.09
-
-
9.2
0.21
Hydroxyl
10
0.71
10
3.06
10
0.93
Total
3.66
14.97
4.15
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