Biology Reference
In-Depth Information
widely dispersed and that the conclusions of Ernst
et al
. (2003) are quite premature and not based on
appropriate population size required for sampling. They further showed the existence of at least six
to seven clusters of non-marine picocyanobacteria with in picophytoplankton clade which further
supports global dispersal of some closely related picocyanobacterial genotypes.
Cells of
Microcystis
are coccoid, tend to aggregate into colonies, possess gas vacuoles and are
present in amorphous mucilage or circumscribed sheath. Watanabe (1996) recognized fi ve different
morpho-species (
M
.
aeruginosa
,
M
.
novacekii
,
M
.
ichthyoblabe
,
M
.
viridis
and
M.
wesenbergii
) from
within the bloom-forming species of
Microcystis
on the basis colony, sheath characteristics and
the mode of arrangement of cells. The molecular and chemotaxonomic data point out that each of
these fi ve morphospecies is non-monophyletic and is quite similar genetically and biochemically.
Otsuka
et al.
(2001) proposed that these fi ve morphospecies should be merged into one species i.e.
M
.
aeruginosa
. A number of attempts have been made to distinguish toxic strains of
M
.
aeruginosa
from the non-toxic ones. These are based on 16S rRNA gene sequencing (Neilan
et al
., 1997; Lyra
et al
., 2001; Tillett
et al
., 2001), 16S rRNA-23S rRNA ITS sequencing ( Otsuka
et al
., 1999; Janse
et al
.,
2003),
cpcBA
-
IGS
sequences (Neilan
et al.
, 1995; Tillett
et al.
, 2001) and the microcystin (
mcy
) gene,
(Nishizawa
et al
., 1999; Tillett
et al
., 2001; Kurmayer
et
al
., 2002). These observations point towards
the absence of any relationship between genetic similarity and toxicity of a strain. This is more
evident when
mcy
was chosen as a marker for distinguishing between the toxic and non-toxic strains
where it was revealed that a single
mcy
genotype consisted of both toxic and non-toxic strains.
To overcome these inconsistencies, Tanabe
et al
. (2007) adopted MLST approach for phylogenetic
analysis of
Microcystis
based on seven housekeeping genes such as
ftsZ
(governing cell division
protein FtsZ),
glnA
(glutamine synthetase),
gltx
(glutamyl-tRNA synthetase),
gyrB
(DNA gyrase
subunit B),
pgi
(glucose-6-phosphate isomerase),
recA
(encodes recombination protein RecA) and
tpi
(triose phosphate isomerae). Phylogenetic trees constructed on the basis of NJ method revealed fi ve
clusters of
Microcystis
. Cluster A consisted of toxic strains, Cluster B had both toxic as well as non-
toxic strains, Cluster C , Cluster D and Cluster E possessed non-toxic strains but with the exception
of Cluster D where the strains possessed
mcyG
gene.
Based on the morphological criteria, ten species recognized in Europe [
M
.
aeruginosa
(Kützing)
Kützing,
M
.
viridis
(A. Braun in Rabenhorst) Lammermann,
M
.
wesenbergii
Komárek in Kontratieva,
M
.
novacekii
(Komárek) Compère,
M
.
icththyoblabe
(Kützing),
M
.
fl os-aquae
(Wittrock) Kirchner,
M
.
natans
(Lemmermann) ex Skuja,
M
.
fi rma
(Kützing) Schmidle,
M
.
smithii
(Kützing et Anagnostidis) and
M
.
botrys
(Teiling)] are resolved by 16S rRNA gene sequences with less than 1% divergence (Otsuka
et al
., 1998; Boyer
et al
., 2001). Phylogenetic trees of the strains in the genus
Microcystis
based on direct
sequencing of 16S rRNA-23S rRNA ITS region did not match with the morphological characteristics
defi ned for delineating the various species (Otsuka
et al
., 1999). In view of this, El Herry
et al
. (2008)
coined the term 'morphospecies' instead of species. The RFLP patterns of the 16S-23S rDNA ITS
regions of morphospecies of
Microcystis
(MCYS-LB01, MCYS-LB02 and LB03) isolated from Lebna
Dam in Tunisia exhibited a similar pattern suggesting that they represent intraspecifi c varaiations or
phenotypic variations but did not agree with previous assignments made in respect of MCYS-LB01
and LB02 to
M
.
aeruginosa
or
MCYS-LB03 to
M
.
wesenbergii
.
iii) Freshwater
Leptolyngbya
lineage (V lineage according to Wilmotte and Herdman, 2001):
The
genus
Leptolyngbya
was created by Anagnostidis and Komárek (1988) to include a large number of
Oscillatoria
spp. with trichomes up to 3 µm. Some of these strains have been included in LPP-group
B by Rippka
et al
. (1979).
Leptolyngbya foveolarum
Komárek 1964/112 and
L
.
boryanum
PCC 73110 that
cluster here share morphological and 16S rRNA gene sequence identity. In addition to
L
.
foveolarum
