AKINETES - Cyanobacteria - page 155

akinete phase does not support the view that the akinetes serve as the progenitors of heterocysts as
stated in the beginning of this Chapter but there seems to be considerable evolutionary divergence
between the akinetes and heterocysts (Campbell et al ., 2007).
Despite the availability of a lot of literature on akinete differentiation on a number of
cyanobacteria under varied environmental and nutritional conditions, a single trigger for akinete
formation under the diverse conditions examined has not been identifi ed.
III. AKINETE GERMINATION
The akinetes of certain cyanobacteria are reported to germinate too readily in the same medium
of their formation (Fay, 1969b; Singh et al ., 1972b; Fogg et al ., 1973; Rother and Fay, 1977) or upon
transfer to fresh medium (Miller and Lang, 1968; Stulp and Stam, 1982; Sutherland et al ., 1985a). In
the akinetes of A . variabilis JS/07 an expanding electron dense layer, present in between the outer
cell wall and the akinete coat, usually swells causing bursting of the spore coat during germination.
Such akinetes usually release uniformly mostly single cells that undergo division into two cells,
of which one cell differentiates into a heterocyst (Braune, 1980). Similarly, Skill and Smith (1987)
noted synchronous akinete germination in Anabaena PCC 7937 and Nostoc PCC 6720 in which the
germlings extrude from their envelopes as single cells. If germination is taking place in a medium
free of combined nitrogen, the fi rst heterocyst differentiates even when the germling is three cells
long as in Nostoc PCC 7524 (Sutherland et al ., 1985b), Anabaena PCC 7937, Nostoc PCC 6720 (Skill and
Smith, 1987) and Cyanospira capsulata (Sili et al ., 1994). During the fi rst few hours of germination,
the akinetes of A . doliolum undergo a change of colour from brown to blue-green and the contents
withdraw from the spore wall. This single cell structure undergoes division leading to the formation
of a four-celled germling, the terminal cell of which usually gets differentiated into a heterocyst
(Rai et al ., 1988). Three modes of release of germlings from the germinating akinetes have been
noticed. The most common mode is the formation of pore at one end through which the germling
is released out, represented by a single cell or 2-3 celled-structure, while the spore coat remains
intact (Skill and Smith, 1987; Sili et al ., 1994) This mode of germination of akinetes has been noted in
No. spumigena (Fig. 3 B) and S . fritschii (Fig. 7 K, L; Sarma and Ghai, 1998). In the second mode of
release the spore coat breaks into two equal halves at the mid point as noticed in case of A . doliolum
(Singh and Srivastava, 1968; Rai et al ., 1988) and G . ghosei (Singh et al ., 1972b). Complete dissolution
of the spore coat constitutes the third mode of release of germlings. The emergence of four-cell
germlings due to a rupture of akinete envelope of Cylindrospermopsis raciborskii was preceded by an
initiation phase of elongation followed by endospore separation from the akinete wall and division
to give rise to a germling (Moore et al ., 2004).
A requirement of light for germination of akinetes has been found in majority of the cyanobacteria
such as A . circinalis (van Dok and Hart, 1997; Thompson et al ., 2009), A . cylindrica (Yamamoto, 1976),
A . doliolum ( Singh and Sunita, 1974; Rai et al ., 1988), A . iyengarii (Agrawal and Singh, 2000), A . torulosa
(Sarma and Malhotra, 1989), A . variabilis (Braune, 1979), Anabaena verrucosa and Aph . fl os-aquae (Kim et
al. , 2005), Anabaena PCC 7937 and Nostoc PCC 6720 (Skill and Smith, 1987) Nostoc PCC 7524 (Chauvat
et al ., 1982; Sutherland et al ., 1985a,b), No . spumigena (Myers et al ., 2010), N . lobatus (Agrawal and
Singh, 2000), C. raciborskii (Moore et al ., 2005) and W . prolifi ca (Agrawal and Singh, 2000) examined
with the exception of akinetes of Anabaena azollae that germinated in dark in presence of fructose
(Neely-Fisher et al ., 1989). A continuous exposure to light for 24 h has been found to be necessary
for germination of No . spumigena akinetes with red light (620 to 665 nm) necessary for initiating
germination. Mostly, light of low intensity (0.5 µE m -2 s -1 ) has been found to be favourable for akinete