Biology Reference
In-Depth Information
The composition of heterocyst glycolipids (Hgls) and envelope polysaccharides has been
investigated in great detail. Heterocysts contain polar glycerolipids that are common with vegetative
cells. These include monogalactosyl diacylglycerol, digalactosyl diacylglycerol, phosphatidylglycerol
and sulphoquinovosyl diacylglycerol which contain mainly C
16
and C
18
fatty acyl substitutes. On
the other hand, the heterocyst glycolipids (Hgls) are localized in the laminated layer and they are
suggested to provide an impervious layer for gases like O
2
(Winkenbach
et al
., 1972). The major
Hgl fractions contain glycosides of C
26
and C
28
polyhydroxyalkanes and glucose esters of C
26
and
C
28
hydroxyfatty acids (Bryce
et
al
., 1972; Lambein and Wolk, 1973). Investigations on biosynthesis
of Hgls in
A
.
cylindrica
by the incorporation of sodium acetate-[1-
14
C] revealed the existence of the
label in long chain mono-, di-, and trihydric alcohols with non-saponifi able glycosides (monohydric
alcohols) occupying the major fraction in the young (60 h old) cultures. In the older cultures
(234 h) the radioactivity was distributed among the three fractions by almost equal percentages
(Abreu-Grobois
et al
., 1977). Similar studies on
A
.
cylindrica
confi rmed a simultaneous increase in
the activity of key enzymes involved in the biosynthesis of polyhydroxyalkanes and hydroxyfatty
acid moieties of Hgls during heterocyst differentiation (Krepski and Wolk, 1983). In the members of
Nostocaceae, the presence of triols and the corresponding C-3 ketones as aglycones has been reported.
Accordingly, the structure of the Hgls of the marine cyanobacterium
Nodularia harveyana
, established
by spectroscopic and chemical methods, have been shown to be 1-(O-α-D-glucopyranosyl)-3R, 25R-
hexacosanediol, 1-(O-α-D-glucopyranosyl)-3S, 25R-hexacosanediol and 1-(O-α-D-glucopyranosyl)-
3-keto-25R-hexacosanol (Soriente
et al
., 1992). However, the Hgls of
Cyanospira rippkae
have been
found to be 1-(O-α-D-glucopyranosyl)-3R, 27R-octacosanediol and 1-(O-α-D-glucopyranosyl)-27
keto-3R-octacosanol (Soriente
et al
., 1993). Gambacorta
et al
. (1998) reported the existence of tetrols
in
Scytonema hofmanni
,
Calothrix desertica
,
Chlorogloeopsis fritschii
and
Fischerella muscicola
. Triols and
the corresponding glycosides as in Nostocaceae were found only in
Microchate
sp. and
Tolypothrix
tenuis
.
Bauersachs
et al.
(2009) examined the distribution of glycolipids in 34 axenic strains of
cyanobacteria with the help of high performance liquid chromatography coupled to electrospray
ionization tandem mass spectrometry and stated that the heterocystous members of the families
Nostocaceae and Rivulariaceae are characteristic in possessing glycolipids with sugar moieties
glycosidically bound to long chain diols, triols, keto-ols and keto-diols. Their absence in the
unicellular and fi lamentous non-heterocystous cyanobacteria affords a chemotaxonomic criterion
for distinguishing these members. With an increase in growth temperature, a decrease in keto-ols
and keto-diols relative to the contents of diols and triols is suggestive of an adaptation for protecting
nitrogenase from being inactivated while selectively permitting diffusion of molecular nitrogen
into the heterocysts. The presence of heterocyst-specifi c glycolipids in the Pleistocene and Eocene
Arctic sediments is of great ecological and evolutionary signifi cance on the role played by these
cyanobacteria in maintaining nitrogen balance in the past in these ecosystems (Bauersachs
et al
.,
2010). Awai
et al
. (2009) summarized the current status of our knowledge on the Hgls and the role
played by them in the heterocystous cyanobacteria with an emphasis on the relationship of the
enzymes mediating their synthesis with polyketide synthases.
The envelope polysaccharides account for 62% of the heterocyst wall in case of
A
.
cylindrica
and
are composed of glucose (Glc), mannose (Man), galactose and xylose (Xyl) as the subunits with the
fi rst sugar as the major component (Dunn and Wolk, 1970). While the aminosugars are absent in the
heterocyst envelope polysaccharides, isotopic labelling studies indicated that the content of murein
(as assessed by the content of muramic acid) of the antecedent vegetative cell is retained during
heterocyst differentiation (Dunn
et al
., 1971). Studies of Cardemil and Wolk (1979) revealed that the
