AKINETES - Cyanobacteria - page 146

(Sarma et al ., 2000). A high respiratory activity was noted in the akinetes of A . cylindrica (Fay 1969b;
Herdman, 1987), N . spongiaeforme (Thiel and Wolk, 1983) and A . torulosa (Ahuja et al ., 2008) but it was
reduced in case of the akinetes of Nostoc PCC 7524 (Chauvat et al ., 1982) and A . doliolum (Rao et al .,
1984). Nitrogenase activity was negligible in the akinetes of A . cylindrica (Fay, 1969b) and those of
S . fritschii (Sarma and Ghai, 1998). A sharp decline in nitrogen fi xation was noted in case of akinete
development of Anabaena sp. (Ahluwalia and Kumar, 1982). Singh and Kashyap (1988) have detected
low rates of respiration, evolution of O 2 and lack of photosynthetic pigments along with a lowering
of enzymes of inorganic nitrogen metabolism in the mature akinetes of F. muscicola. Akinetes of N .
spongiaeforme are metabolically active capable of synthesizing protein and lipid and respire in the
dark and also evolve oxygen in light (Thiel and Wolk, 1983).
6) Pattern of akinete differentiation : In diverse cyanobacteria akinetes may differentiate adjacent
to heterocysts as in A . cylindrica (Wolk, 1965), C . licheniforme (Fisher and Wolk, 1976; Hirosawa and
Wolk, 1979a) and other akinete-forming Anabaena strains (Rippka et al ., 1979) or midway between
two heterocysts as noted in A . doliolum (Tyagi, 1974), certain Nostoc strains (Rippka et al ., 1979), Nostoc
PCC 7524 (Sutherland et al ., 1979), A . torulosa (a halotolerant strain; Fernandes and Thomas, 1982)
and N . punctiforme ATCC 29133 (Meeks et al ., 2002). After such initiation, cells on either side start
developing into akinetes and the gradients of maturation reach the two heterocysts so that in a short
time all the cells tend to get differentiated into strings of akinetes. In cultures grown in presence of
nitrate or ammonia that lack heterocysts, the differentiation of akinetes occurred randomly in the
fi laments (Meeks et al ., 2002). The akinete formation in A . torulosa is initiated simultaneously in all
vegetative cells and cells away from the heterocysts (5th to 6th cell in between two heterocysts in
nitrate-free medium) always matured fi rst. Thus the lag in the formation of akinetes is dependent
on the lag in the maturation period (Sarma and Swarn Kanta, 1979) in contrast to the lag in the
initiation of akinete formation in A . cylindrica (Simon, 1977). Wolk (1965, 1966) fi rst advocated that
heterocysts play a key role in akinete differentiation in A . cylindrica since vegetative cells adjacent
to heterocysts always fi rst differentiated into akinetes. A lack of such relationship between the two
cell types was subsequently reported (Eberly, 1966; Hill, 1970; Burger, 1974; Wildman et al ., 1975;
Sutherland et al ., 1980). Due to the fact that ceratin amino acid analogues such as 7-azatryptophan
altered the pattern in the differentiation of heterocysts, some workers were interested to examine
whether akinete differentiation depended on pattern of heterocysts or is independent of heterocysts.
A common control mechanism for the differentiation of heterocysts and akinetes in Nostoc PCC 7524
has been proposed by Sutherland et al. (1979) on the basis of alteration in the position of akinetes due
to the presence of 7-azatryptophan and canavine. However, in Anabaena sp. the position of akinetes
has been altered in presence of 7-azatryptopohan and canavine. In A . torulosa the differentiation of
akinetes always occurred away from the heterocysts in nitrate-free cultures. In a number of nutrients
tested, the same pattern of akinete differentiation occurred as in nitrate-free medium but in nitrogen
sources like nitrate, nitrite and carbon sources such as acetate or citrate the trichomes fragmented
releasing heterocysts into medium and few-celled trichomes had undergone akinete differentiation
in the absence of heterocysts at high frequency ( Fig. 5; Sarma and Malhotra, 1989). In S. fritschii
during akinete differentiation either in nitrate-free or nitrate medium a second round of heterocyst
differentiation precedes akinete development and the cultures undergo a perceptible change of
colour from blue-green to brown. Upon maturity, the akinetes acquired thick envelopes and were
seen in elongated series interrupted by the dead necridia which resulted due to the crumpling of
the newly developed heterocysts (Figs. 6, 7).