Calcium carbonate precipitation by cyanobacterial polysaccharides - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 60

group (pK a ¼ 3) and carboxyl-phosphoric groups
(pK a ¼ 3.8-4.6) dominate in all strains with
30-37% and 13-17% respectively, closely fol-
lowed by the hydroxyl groups (pK a ¼ 10) which
represent 20-22%. The small fraction was provided
by phosphoric groups (pK a ¼ 6.8-7.4) which was
similar in all strains with c. 10%.
The extracellular polysaccharides of picocyano-
bacteria are negatively charged at a pH range
between 6-7, which is typical for natural surface
water. Calcium cations can therefore be easily
attracted. On one hand, this reaction is important
for the nucleation of calcium carbonate; on the
other hand, calcium removal from solution leads
to reduction of saturation in respect to calcium car-
bonate and therefore, inhibits the precipitation.
CaCO 3 precipitation experiments clearly demon-
strated that extracellular polysaccharides of three
picocyanobacterial strains have a potential to pre-
cipitate calcium carbonate. Due to the dominance
of carboxyl groups, extracellular polysaccharides
of picocyanobacteria can play an important role in
metal cycling in aquatic systems. For that reason,
picocyanobacteria have a general potential for
applications such as reducing metal concentrations
at polluted sites. However, reaping this potential
requires further investigation of the EPS character-
istics and the development of techniques to effec-
tively cultivate picocyanobacteria.

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