Biology Reference
In-Depth Information
Figure 3:
Pleurocapsa
sp. from nature. Magnifi cation bar represents 5 µm. Picture courtesy G. L. Tiwari, Department of Botany,
University of Allahabad, Allahabad-211002, India.
(iii) the genetic diversity of collected strains within a culture may be decreased by selective culturing
conditions; (iv) the species concept depended on the taxonomist and (v) the conservative approach
of a number of taxonomists to adopt the new molecular methods. A number of proposals have been
made for unifi cation of nomenclature in case of botanical and bacteriological codes (Oren, 2004; Oren
and Tindall, 2005; Hoffmann
et al
., 2005).
Hoffmann
et al
. (2005) presented a classifi cation scheme under ICBN on a polyphasic approach
by taking into account morphological, ultrastructural, ecological and genetic relationships (mainly
16S rRNA gene sequences). This is an attempt to merge the Botanical as well as Bacteriological
Codes. The Cyanobacteria are given the rank of a Class and designated them as Cyanoprokaryota
with three sub-classes Synechococcineae, Oscillatorineae and Nostocineae. The Synechococcineae
with two orders Synechococcales and Pseudoanabaenales consists of both coccoid and fi lamentous
forms, respectively. The second sub-class Oscillatorineae comprises coccoid (Chroococcales) and
fi lamentous forms (Orders Phormidiales and Oscillatoriales). The unifi cation of heterocystous
cyanobacteria into one sub-class Nostocineae with a single order Nostocales is a unique feature. The
inclusion of Prochlorophyta in cyanobacterial classifi cation system and the absence of distinction into
coccoid and fi lamentous forms at the highest sub-class level are the highlights of this classifi cation. The
distinction into sub-classes is based on arrangement of thylakoids and the presence of differentiated
cells. Chroococcales with unicellular or colonial members are characterized by radial arrangement
of thylakoids. The non-heterocystous fi lamentous forms have been divided into three orders based
on the nature of fi laments and the arrangement of thylakoids. The Oscillatoriales possess large
fi lamentous forms with radial arrangement of thylakoids while Pseudoanabaenales consist of
thin fi lamentous forms in which the thylakoids are arranged parallel to the cell surface. The order
Nostocales possesses fi lamentous, heterocystous, unbranched and branched members.
