Biology Reference
In-Depth Information
Akinetes of cyanobacteria have been shown to be tolerant to desiccation (Yamamoto, 1975;
Sutherland
et al
., 1979; Rai and Pandey, 1981; Sili
et al
., 1994; Baker and Bellifemine, 2000) and seven
year-old desiccated akinetes exhibited germination (Sili
et al
., 1994). Sediments as old as 64 years
revealed the presence of viable akinetes (Livingstone and Jaworskii, 1980). The thermal tolerance
of akinetes, represented by their germination capacity after heat treatments, varied from species
to species. The germination potential of
Anabaena vaginicola
akinetes was almost completely lost
after heat treatment at 55°C for 8 min or 60°C for 2 min (Rai and Pandey, 1981) while the akinetes
of
Anabaena
circinalis
survived a heat treatment of 45°C for 60 min (Baker and Bellifemine, 2000).
Akinetes of
Anabaena
dried in sunlight (10 h) and by heat treatment (60°C for 50 h) germinated only
up to 40% and 15%, respectively (Hori
et al
., 2003). Akinetes of
A
.
circinalis
have been reported to
be susceptible to ultraviolet radiations (Fay, 1988). Dried akinetes of
Anabaena cylindrica
exposed
to desiccation (28 d) and extreme low temperature (-80ºC) survived better than those subjected
to vacuum (0.7 x 10-3 KPa) and high temperature (80ºC) under the laboratory conditions. Some
proportion of the akinetes also survived for a period of 10 d in low Earth Orbit and also simulated
conditions of Mars without UV radiations (28 d) (Olsson-Francis
et al
., 2009).
Due to their resistance to various extremes in environmental variables, the akinetes of
cyanobacteria have been equated with bacterial endospores (Nichols and Carr, 1978; Nichols and
Adams, 1982). Since they are not heat-resistant, some others considered them equivalent to cysts of
Azotobacter
due to their metabolic activities and rapid growth on return to favourable environmental
conditions (Sutherland
et al
., 1979). A considerable wealth of information has accumulated on their
ultrastructure, formation and metabolic activities and factors regulating their formation (Fogg
et
al
., 1973; Nichols and Carr, 1978; Adams and Carr, 1981; Pandey and Talpasayi, 1981; Nichols and
Adams, 1982; Herdman, 1988; Sarma, 1993; Adams and Duggan, 1999).
I. AKINETE DIFFERENTIATION
1) Genera studied
:
A number of cyanobacteria such as
Anabaena circinalis
(Fay
et al
., 1984; van
Dok and Hart, 1996),
Anabaena cylindrica
(Wolk, 1965; Simon, 1977);
Anabaena
.
doliolum
(Singh,
1967; Singh and Srivastava, 1968; Singh
et al
., 1972b; Tyagi, 1974),
Anabaena fertilissima
(Reddy and
Talpasayi, 1981),
Anabaena iyengarii
(Agrawal and Singh, 2000),
A
.
oscillarioides
(Rother and Fay, 1979),
Anabaena
torulosa
(Sarma and Swarn Kanta, 1979; Sarma and Malhotra, 1989; Ahuja
et al
., 2008),
Anabaena
variabilis
(Singh
et al
., 1972b),
Anabaenopsis arnoldii
(Reddy, 1983),
Aphanizomenon
fl os-aquae
(Wildman
et al
., 1975; Thompson
et al
., 2009),
Aph
.
ovalisporum
(Sukenik
et al
., 2009),
Cylindrospermum
licheniforme
(Fisher and Wolk, 1976; Hirosawa and Wolk, 1979a,b),
Cylindrospermum majus
(Singh
and Sinha, 1965),
Cylindrospermopsis
raciborskii
(Moore
et al
., 2004, 2005),
Fischerella
muscicola
(Singh
et al
., 1972a),
Nodularia spumigena
(Pandey and Talpasayi, 1980)
Nostoc
linckia
(Singh
et al
., 1972b),
Nostoc
PCC 7524 (Sutherland
et al
., 1979, 1985a,b);
Nostoc punctiforme
ATCC 29133 (Argueta and
Summers, 2005),
Nostoc
spongiaeforme
(Thiel and Wolk, 1983),
Nostoc
sp. (Ahluwalia and Kumar,
1983) and
Nostochopsis lobatus
(Agrawal and Singh, 2000),
Scytonema fritschii
(Sarma and Ghai, 1998)
and
Westiellopsis prolifi ca
(Agrawal and Singh, 2000) have been employed for investigations on the
pattern of akinete differentiation.
2) Position of akinetes
:
The akinetes are formed either adjacent to or away from the heterocysts. In
some species of
Anabaena
and
Nostoc
the initiation of akinete differentiation starts midway between
two heterocysts and proceeds in both directions approaching the heterocysts (Fig. 1 A, B; Fig. 2 A, B).
In species of
Cylindrospermum
(Fig. 3 A),
Gloeotrichia
(Fig. 2 D) and certain other species of
Anabaena
and
Nostoc
(Fig. 1A, B), cells adjacent to the heterocysts differentiate into akinetes. However, the
