Biology Reference
In-Depth Information
together. These results signify that the genus
Synechococcus
is not monophyletic as was stated earlier
by Urbach
et al
. (1998) and Honda
et al
. (1999). According to Robertson
et al
. (2001),
Synechococcus
sp.
strain PCC 6716 and
Synechococcus
sp. strain PCC 6717 should be classifi ed as members of a different
genus from that of
Synechococcus
sp. strain PCC 6301. These may represent early morphological types
of cyanobacteria and forerunners for the evolution of other cyanobacterial morphological types. In
this connection, it is important to know that
Synechococcus
sp. strains PCC 6716 and PCC 6717 and
S. elongatus
(Toray) have been isolated from hotsprings. However, as per Bergey's Manual of
Systematic Bacteriology,
Synechococcus
sp. strains PCC 6716 and PCC 6717 represent members of
the same species as
Synechococcus
PCC 6301. They retained the seven lineages described by Honda
et al
. (1999) and added an eigth one for the
S. elongatus
(Toray),
Synechococcus
sp. strains PCC 6716
and PCC 6717. Of the other groups, Group 6 is most complex and consists of three subgroups 6a,
6b and 6c. Group 6a comprises of
Synechococcus
isolates from saline lakes of Antarctica and strains
of
Synechococcus
from MC-A and MC-B (PCC 7805, WH 8103, and WH 8101; Vincent
et al
., 1999).
Group 6b includes strains from MC-B and
Cyanobium
cluster. Along with these, strains of MC-A
(
Synechococcus
PCC 7805 and WH 8103) and
Prochlorococcus marinus
also fall into this subgroup
(Honda
et al
., 1999; Vincent
et al
., 1999). Subgroup 6c includes three strains of
Synechococcus
sp. (PCC
6301, PCC 7942 and PCC7943) and species of
Microcystis
(
M
.
elebans
NIES 42 and
M
.
holsatica
NIES
43, now treated as members of
Synechococcus
) that lack gas vacuoles. They further preferred the
creation of new genera for the subgroups 6a, 6b and 6c that contain most of the
Synechococcus
isolates
and also for
Synechococcus
members forming group 8. They concluded that some of the members
of
Synechococcus
are misclassifi ed members ( such as PCC 7902 found in group 4, PCC 7309, PCC
7117, PCC 7002 found in group 5 and PCC 7335 present in group 7).
Fuller
et al
. (2003) identifi ed three new novel clades of
Synechococcus
on the basis of 16S rDNA
sequencing. One of these clades contains halotolerant isolates lacking PE and strains that are not
capable of utilizing nitrate as sole source of nitrogen. This clade is clustered with MC-A. Cloning
and sequencing of the two copies of 16S rRNA of genes present in
Synechococcus
sp. strain WH
7803 and a comparison of the same with
Synechococcus
sp. strain WH 8102 revealed them to be
identical.
In situ
community structure of marine
Synechococcus
populations in the Red Sea, at the
time of their abundance, predominantly was represented by genotypes of a single clade and these
are common representatives isolated into culture. These accounted for a very minor component of
Synechococcus
population diversity.
Prochlorococcus
contains divinylchlorophyll
a
(Chl
a
2
) and both
monovinyl and divinylchlorophyll
b
as the major photosynthetic pigment in contrast to chlorophyll
a
and phycobiliproteins that are typical of cyanobacteria (Chisholm
et al.
, 1988, 1992; Goericke and
Repeta, 1992). Two genetically and physiologically distinct ecotypes have been recognized that have
implications on their ecological distribution. High-B/A (previously designated as low-light adapted)
isolates have larger ratios of Chl
b
/
a
2
and are able to grow in extremely low radiances where low-B/A
isolates (previously designated as high-light adapted strains) are incapable of growth. Low-B/A
isolates have lower Chl
b
/
a
2
ratios and are able to grow maximally at higher light intensities where
high-B/A isolates are inhibited (Moore and Chisholm, 1999). On the basis of 16S rDNA sequences
strains of low-B/A ecotype are very closely related to each other (with 99% identity supported by
bootstrap values; Urbach
et al
., 1998; Moore and Chisholm, 1998; Rocap
et al
., 1999). At the same
time strains of high-B/A ecotypes have a lower degree of identity in their 16S rDNA sequence (97
to 98%) and are not monophyletic but at least form three independent branches (Rocap
et al
., 1999).
These also exhibit a high degree of sequence similarity to the
Synechococcus
strains
of
MC-A
.
