Biology Reference
In-Depth Information
Cylindrospermum
sp., and
F
.
muscicola
(Wolk, 1965; Kaushik
et al
., 1971; Fisher and Wolk, 1976; Nichols
and Adams, 1982; Herdman, 1987, 1988; Sarma and Khattar, 1992). However, in certain other forms
such as
Aph
.
fl os
-
aquae
and
Nostoc
PCC 7524, P-defi ciency did not stimulate the formation of akinetes
(Rother and Fay, 1979; Sutherland
et al
., 1979). Studies on
A
.
torulosa
revealed that the induction of
akinetes in P-defi cient conditions depended greatly on the source of nitrogen as nitrate-free cultures
were unable to fi x nitrogen and the cultures did not differentiate akinetes. However, nitrate enriched
cultures defi cient in P alone supported the formation of akinetes (Fig. 8). Thus P-defi cient nitrate
enriched cultures supported enhanced frequency of akinetes with subsequent sub-culturing in a
similar medium. These observations signify that (i) regulation of akinete differentiation in P-defi cient
cultures proceeds through the regulation of N-assimilating systems as nitrogen-fi xing and nitrate
utilizing cultures revealed no greater differences in P-uptake; and (ii) nitrate uptake by
A
.
torulosa
seems to be not dependent on P since nitrate uptake and GS activity did not differ in presence or
absence of P. The most signifi cant aspect of this study is the decrease of cellular P levels with a
concomitant increase of extracellular P in the medium (Sarma and Khattar, 1992).
(A)
(B)
Figure 8:
Pattern of akinete differentiation in
Anabaena torulosa
. Akinetes from nitrate-free medium with phosphate on 20th
day (A) and akinetes from nitrate medium without phosphate on 10th day (B). x1320 (From Sarma and Khattar, 1992).
ii) Nitrogen sources
:
Confl icting
reports
exist in literature on the role of nitrogen sources in inducing
akinete differentiation in cyanobacteria. Nitrate nitrogen favoured akinete formation in several
Anabaena
spp. (Canabaeus, 1929) and
A
.
torulosa
(Sarma and Malhotra, 1989; Sarma and Khattar,
1994) whereas the incorporation of nitrogen sources did not affect akinete differentiation in
A
.
cylindrica
(Wolk, 1965),
Nostoc
PCC 7524 (Sutherland
et al
., 1979) and other
Nostoc
strains (Rippka
et al
., 1979). On the other hand, the frequency of akinetes decreased in
A
.
doliolum
in presence of
nitrate and nitrite (Singh and Srivastava, 1968). However, nitrate enhanced early and en masse
akinete differentiation in
N
.
linckia
while ammonium chloride delayed and reduced the frequency
(Singh
et al
., 1972b). In nitrogen sources such as nitrate and nitrite,
A
.
torulosa
experiences extensive
fragmentation of the fi laments soon followed by the initiation of akinete development on the 4th
