Biology Reference
In-Depth Information
heterocyst envelope polysaccharides of A . cylindrica contain repeating units of one mannose and
three glucose residues linked by (β 1-3) glycosidic bonds with xylose and galactose residues being
present as side branches. Digestion of the heterocyst envelope polysaccharides by treatment with
(β 1-3) endoglucanase resulted in the release of glucose, a tri- (Man-Glc-Glc) and pentasaccharide
(Man-Glc-Glc-Glc and Xyl as side branch) and the backbones terminate in the pentasaccharide
subunit. Heterocyst envelope polysaccharides of Cylindrospermum licheniforme resemble those of
A . cylindrica in possessing terminal xylose and galactose residues as side branches that are absent
in the envelope polysaccharides of A . variabilis . However, the envelope polysaccharides from
A . variabilis and C . licheniforme in addition possess terminal arabinose residues (Cardemil and Wolk,
1981a). Purifi ed heterocyst preparations from A . variabilis synthesized envelope polysaccharides when
incubated in presence of C 14 -labelled mannose, arabinose and glucose residues and bacitracin inhibited
the incorporation of the sugar residues into the polysaccharides (Cardemil and Wolk, 1981b).
4) PERMEABILITY TO GASES
The heterocysts should be permeable to N 2 and at the same time restrict the entry of O 2 . What are the
probable routes through which the gases make their entry into the heterocysts? In the intact fi laments,
two routes are considered important for the entry of gases, the surface layers and through the polar
nodules and microplasmodesmata. During heterocyst differentiation the deposition of glycolipid
layer as a laminated layer and the envelope polysaccharides (that have a different composition
when compared to exopolysaccharides or sheaths) are the fi rst important developmental protective
strategies. Secondly, the absence of PSII remarbably decreases the generation of O 2 as a byproduct
of photosynthesis. Thirdly, the enhanced respiratory activity shown by the heterocysts enables to
scavenge O 2 from the site of nitrogen fi xation. Since the Hgls and envelope polysaccharides appear to
protect from the entry of gases, the microplasmodesmata that constitute the connecting link between
the adjacent vegetative cells appear to be the appropriate alternative. The permeability of N 2 and O 2
to the heterocysts has been assessed in Anabaena fl os - aquae by Walsby (1985). The heterocysts in this
organism retain gas vacuoles and the collapse of gas vacuoles to 50% of their initial level by pressure
diffusion of N 2 or O 2 was tested. The average permeability coeffi cient of the surface layers to N 2 and
O 2 was found to be 0.4 µm s -1 which is within the range that could be provided by few glycolipid
layers. On this basis, it was concluded that the main route for the entry of gases is through the surface
layers, because the permeability of the pore region to gases is considerably less. A consideration of
the metabolism of H 2 , N 2 and O 2 provided ratios ranging from 0.9 to 1.2 for C 2 H 4 to H 2 produced by
the isolated heterocysts of Anabaena sp. strain CA-V and the H 2 evolution occurred in two phases
one by a burst phase followed by a steady-state phase. But due to H 2 -simulated C 2 H 2 reduction, H 2
evolution was abolished. A persistent O 2 uptake (0.7 µmol of O 2 4 per mg dry weight per h) by the
isolated heterocysts (35% of the whole fi laments) suggested that O 2 impermeability is not a pre-
requisite for nitrogen fi xation by the heterocysts (Smith et al ., 1985). The fi rst direct evidence that the
Hgls and the Hep layer provide the requisite O 2 impermeability came from the studies of Murry and
Wolk (1989) who characterized mutants EF114 and EF116 of Anabaena sp. strain PCC 7120 isolated
by Wolk et al . (1988). EF114 defi cient in Hgls reduced acetylene strictly under anaerobic conditions.
EF116 that lost the cohesiveness of the envelope polysaccharide also lost the low affi nity respiratory
component associated with the heterocysts. The behavior of EF114 was also similar in this respect
(Murry and Wolk, 1989). The other evidence in favour of Hgls and envelope polysaccharide providing
the requisite O 2 impermeability was that of Kangatharalingam et al . (1992) who observed that
the thickness of especially heterocyst glycolipids of A . fl os-aquae gradually thickened with an
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