Biology Reference
In-Depth Information
C
HAPTER
3
C
HAPTER
AKINETES
I. AKINETE DIFFERENTIATION
141
II. FACTORS AFFECTING AKINETE DIFFERENTIATION
150
III. AKINETE GERMINATION
155
IV. AKINETE GERMINATION IN RELATION TO BLOOM DEVELOPMENT
157
Many fi lamentous cyanobacteria of the orders Nostocales
and Stigonematales
(corresponding to
subsections IV and V, respectively of Rippka
et al
., 1979) exhibit relatively simple morphology and the
vegetative cells differentiate into heterocysts and akinetes during growth. Based on this taxonomic
clustering and some biochemical and genetic similarities between these two cell types, akinetes
have been suggested to be the progenitors of heterocysts (Wolk
et al
., 1994). Akinetes have been fi rst
reported by Carter (1856) followed by other workers. The term 'akinetes' has been derived from the
Greek word 'akinetos' which means 'motionless' and represent thick-walled, resting cells (Adams
and Duggan, 1999). The discovery of Mesoproterozoic
Archaeoellipsoides
, microfossils of akinetes
(as long as 135 µm) of heterocystous cyanobacteria, indicated that the atmospheric oxygen levels
that persisted 1500 million years ago were well above 1% and these akinetes were comparable in
morphology with the present day bloom-forming
Anabaena
sp. (Golubic
et al
., 1995).
Archaeoellipsoides
has been reported from Newari locality of Mesoproterozoic Kheinjua Formation of the Semri
Group, Vindhyan Supergroup in India (Srivastava, 2005). An important group of substances such as
2-methylhopane hydrocarbons serve as indicators of cyanobacteria and in other words the origins
of oxygenic photosynthesis (Summons
et al
., 1999). In this connection, the akinetes of
N
.
punctiforme
are distinctive in possessing 2-methylbacteriohopanepolyol (2-MeBHP) lipids 34-fold higher than
vegetative cells and these are localized in the outer membranes. Since 2-MeBHPs are associated with
outer membranes of akinetes, functionally they can not be linked to oxygenic photosynthesis as has
been generally done to trace the origin of oxygenic photosynthesis in a geobiological perspective
(Doughty
et al
., 2009).
