Biology Reference
In-Depth Information
B
A
C
D
E
F
Figure 6:
Morphological features of
Scytonema fritschii
in cultures. (A) colonies of
S
.
fritschii
on nitrate-free agar medium; (B),
(C) and (D) types of false-branching in S.
fritschii
; (E) an intercalary heterocyst; (F) a terminal heterocyst. The magnifi cation
bar in all pictures represents 10 µm (From Sarma and Ghai, 1998).
akinetes (Lang, 1967). Most of the akinetes of cyanobacteria reveal the presence of gylcogen and CGP
granules. Exceptionally the akinetes of
Cyanospira rippkae and C. capsulata
lack CGP granules (Sili
et al
.,
1994). The isolation of an arginine biosynthesis mutant of
N
.
ellipsosprum
was unable to accumulate
CGP but even in its absence akinetes formed were devoid of CGP granules (Leganes
et al
., 1998).
Various investigations carried out on
A
.
torulosa
revealed that (i) glycogen accumulation during
akinete formation is a light-dependent process and no glycogen formation was found in darkness
or in cultures supplemented with DCMU in light (Sarma and Swarn Kanta, 1979); (ii) glycogen was
maximum at the onset of akinete initiation that continued to increase during maturation period; (iii)
inorganic nitrogen sources, carbon sources or their combinations that stimulated akinete formation
also supported enhancement in the content of glycogen (Swarn Kanta and Sarma, 1980) and (iv) of
the 21 amino acids L-methionine, L-tyrosine, L-glycine and L-histidine supported the formation of
maxium glycogen content with concomitant formation of akinetes (Sarma and Swarn Kanta, 1980).
The relative amounts of glycogen and CGP were compared in cells undergoing akinete formation
in
A
.
torulosa
(Sarma and Khattar, 1986a,b). Sutherland
et al
. (1985b) suggested that internal reserves
in akinetes of
Nostoc
PCC 7524 were not necessary for germination since carbon dioxide fi xation
commenced immediately in light. Since microcycle akinete formation bypasses vegetative growth
of
A
.
torulosa
, it is likely to depend on the internal reserves of glycogen and CGP. This is evident by
the decreased levels of the two reserves from akinete stage to microcycle akinete formation (Sarma
and Malhotra, 1989).
