Biology Reference
In-Depth Information
iii)
Supply of Reductant
iv)
Assimilation
v)
Transport
vi)
Carbon metabolism
vii)
Amino acid transport and diazotrophic growth
viii)
Hydrogen metabolism
15) OTHER FUNCTIONS
254
Members of the orders Nostocales (except Oscillatoriaceae) and Stigonematales corresponding to
Sections IV and V of Rippka
et al.
(1979) differentiate specialized cells known as heterocysts during
nitrogen-defi cient growth conditions. The importance of the presence or absence of heterocysts for
taxonomic purposes was realized long time back by Bornet and Flahault (1886) who divided the
order Hormogonales into Homocystaceae and Heterocystaceae. Although once considered to be a
“botanical enigma” (Fritsch, 1951), the heterocysts have now emerged as a suitable prokaryotic model
for understanding cellular differentiation and pattern formation in multicellular cyanobacteria and
we are at a threshold of understanding the web of molecular events involved in their differentiation
(Adams and Duggan, 1999; Adams, 2000; Wolk, 2000; Zhang
et al
., 2006; Kumar
et al
., 2010; Flores
and Herrero, 2010). Thus the development of heterocyst from its antecedent vegetative cell proceeds
through very important morphological, structural, biochemical and genomic changes culminating
in the production of a slightly enlarged cell with microoxic internal atmosphere. Such a cell after
formation can neither revert to a vegetative cell nor divide but can only fi x nitrogen. That is why it
is often designated as a terminally differentiated cell.
1) MORPHOLOGY
The heterocysts differ from vegetative cells in being slightly larger, double-walled, pale yellow or
hyaline (colourless) with homogeneous cell contents and are connected to the vegetative cells by
means of the glistening nodule/granule-like structures known as polar nodules/granules. The
heterocysts are generally round (as in species of
Anabaena
,
Nostoc
,
Gloeotrichia
and
Rivularia
) or
rectangular (as in species of
Aulosira
,
Scytonema
and
Hapalosiphon
). They differentiate in intercalary
position at regularly spaced intervals all along the length of the fi lament, with the intervening number
of vegetative cells being fairly constant. In general, the inter-heterocyst distance is occupied by 8
to 10 vegetative cells in species of
Anabaena
and
Nostoc
(Fig. 1A,B,E and F). Besides the intercalary
heterocysts, the terminal cells of the fi laments in these genera also differentiate into heterocysts.
Thus there are intercalary and terminal heterocysts. However, in genera belonging to Rivulariaceae
(
Rivularia
and
Gloeotrichia
) there is a single basal heterocyst (Fig. 1C). The terminal and basal
heterocysts are connected with adjacent vegetative cells by a single polar nodule. Exceptionally, in
certain species of
Anabaenopsis
(
A. circularis
and
A. arnoldii
) the intercalary heterocysts are paired
(Fig. 1D). Species of the genus
Cylindrospermum
are characteristic in possessing terminal heterocysts,
one on either side of the fi lament (Desikachary, 1959). In branched members of Stigonematales
