Biology Reference
In-Depth Information
must confer some kind of selective advantage to the cells. Actually, several
studies have shown that the presence of EPS can protect cyanobacteria
against dehydration and UV radiation (
Ehling-Schulz, Bilger, & Scherer,
1997
;
Ehling-Schulz & Scherer, 1999
;
Garcia-Pichel & Castenholz, 1991
;
Hill, Peat, & Potts, 1994
;
Potts, 1994
,
1999
,
2004
;
Shaw et al., 2003
;
Tamaru,
Takani, Yoshida, & Sakamoto, 2005
;
Wright et al., 2005
). In addition, it has
been hypothesized that the EPS may play an important role in preventing
the direct contact between the cells and toxic compounds, notably heavy
metals and/or sequestering metal cations (or other nutrients) that are essen-
tial for cell growth but are present at low concentrations in the environment
(
Parker, Schram, Plude, & Moore, 1996
;
Sutherland, 1999
). Overall, the role
of these polymers seems to differ from strain to strain, being dependent
on the physichochemical characteristics of the natural habitat or culture
medium in which the organism grows.
Over the past decades, a considerable amount of efforts have been placed
into the characterization of cyanobacterial EPS. The data gathered revealed
that these polymers possess distinctive characteristics compared to those
produced by other bacteria. Indeed, the EPS produced by cyanobacteria
comprise a large number of different monosaccharides, with the major-
ity of the polymers described possessing 6-13 different sugars. This feature
contrasts with the polymers synthesized by other bacteria and macroalgae
that usually contain around four (
De Philippis & Vincenzini, 1998
;
Fischer,
Schlosser, & Pohl, 1997
;
Pereira et al., 2009
;
Stengel, Connan, & Popper,
2011
). Cyanobacterial EPS also frequently contain two different uronic
acids (generally glucuronic and galacturonic) and sulphate groups, which
are unusual in other bacterial EPS (
Pereira et al., 2009
;
Sutherland, 1994
),
building a polymer particularly rich in negatively charged groups. In addi-
tion, many cyanobacterial polymers have a stronger hydrophobic behaviour,
which is conferred by the presence of ester-linked acetyl groups (up to
12% of EPS dry weight), peptidic moieties and deoxysugars such as fucose
and rhamnose (
Neu, Dengler, Jann, & Poralla, 1992
;
Pereira et al., 2009
;
Shepherd, Rockey, Sutherland, & Roller, 1995
). Altogether, these char-
acteristics make cyanobacterial EPS very promising for biotechnological
applications, such as the removal of heavy metals from polluted waters or
as thickening, suspending or emulsifying agents (
De Philippis, Colica, &
Micheletti, 2011
;
Pereira et al., 2009
). Despite the increasing interest on the
cyanobacterial EPS and the growing awareness of their potential for bio-
technological applications, the pathways leading to the production of these
polymers remain scarcely known, limiting their manipulation and industrial
