Biology Reference
In-Depth Information
amplifi cation of
hetR
gene sequence unique for heterocystous cyanobacteria for this purpose. The
host for epiphytic
R
.
intracellularis
was
Chaetoceros
sp. (two species collected from the Pacifi c Ocean).
Species of
Hemiaulus
showed variable number of cells in their fi laments as well as in the number of
endosymbiotic
R
.
intracellularis
fi laments. For example
H
.
hauckii
(with 5-6 cells per fi lament from
the Pacifi c Ocean) contained two
R
.
intracellularis
fi laments in each cell while
H
.
membranaceous
(with
10 cells per fi lament from Atlantic and Pacifi c Oceans) showed the presence of two
R
.
intracellularis
fi laments in each cell and
R
.
clevei
var.
communis
(from Pacifi c Ocean) contained four
R
.
intracellularis
fi laments in each cell. The amplifi cation products of
hetR
gene sequences of the epiphytic and
endosymbiotic
R
.
intracellularis
all belonged to the same group and there was no difference in the
sequences of
R
.
intracellularis
growing intracellularly in
H
.
membranaceous
either from Pacifi c or
Atlantic Oceans. A high degree of host specifi city was also deduced based on the divergence of the
sequences between symbionts from different genera.
Gómez
et al
. (2005) studied the distribution pattern of
R
.
intracellularis
as an epiphyte on
Chaetoceros compressus
from the Pacifi c Ocean. Colonies of
C
.
compressus
consisted of 10-16 cells
and 1-9 epiphytic fi laments of
R
.
intracellularis
. Variables of environment (temperature and salinity)
and nutrients (nitrate and phosphate) have been taken into account to assess the distribution of
Rhizosolenia
+
Richella
consortia. These studies have been extended to various depths (5 to 200 m)
and latitudes (at nine stations in between 30º 30' to 34º 15' N in May and at 10 stations in between
30º to 34º 20' N during July) of Pacifi c Ocean at Kurashio, Oyashia currents surrounding Japan
and the Celebes, Sulu and South China Sea. The occurrence of
R
.
intracellularis
as endosymbiont of
Rhizosolenia
clevei
and free-living
R
.
intracellularis
in the samples prompted the authors to suggest
that the free-living
R
.
intracellularis
fi laments might have originated due to their release from the
surface of the symbiotic diatom plasmolemma that lacked the frustule. Such nascently released
R
.
intracellularis
fi laments could colonize the scenescent cells in colonies of
C
.
compressus
lacking the
epiphytic
R
.
intracellularis
. Once
R
.
intracellularis
establishes on a single cell, it then spreads to other
diatom cells in the colony. These events have been correlated with the distribution patterns of the
two diatoms and
R
.
intracellularis
. The occurrence of
C
.
compressus
with its epiphyte is restricted to
the Indian and western Pacifi c Oceans whereas the endosymbiont
R
.
intracellularis
in other diatoms
is ubiquitous in warm oceans. The occurrence of
Richelia
-
Chaetoceros
consortia exclusively in the
periphery of the geographic proliferations of
C
.
compressus
coincided with the overlapping area of
the populations of asymbiotic
C
.
compressus
and
R
.
intracellularis
as an endosymbiont in
R
.
clevei
.
Further support for their fi ndings is derived from the work of Janson
et al.
(1999) who reported a
similarity of
hetR
gene sequences of endosymbiotic
R
.
intracellularis
in
R
.
clevei
and the epiphytically
growing
R
.
intracellularis
on
Chaetoceros
. Further, as free-living
R
.
intracellularis
needs a support (due
to lack of gas vacuoles in its cells) and as
Chaetoceros
cannot survive in oligotrophic waters both
fi nd it mutually benefi cial leading to the formation of
Richelia
-
Chaetoceros
consortia. Foster and Zehr
(2006) compared the utility of the molecular markers
nifH
,
hetR
and 16S rRNA sequences in the
characterization of diatom-diazotroph associations (DDAs).
iii) Role of DDAs in the open oceans
:
The nitrogen cycle has been recognized as an important intrinsic
component of the ocean ecosystem and is supposed to play a greater role in the response of oceans
to global environmental changes (Zehr and Kudela, 2011). DDAs and
Trichodesmium
constitute the
important components of the upper euphotic zone fl ora (White
et al
., 2007a,b; Dore
et al
., 2008). The
contribution of
R
.
intracellularis
to the enrichment of nitrogen status of tropical marine waters as
an epiphyte and endophyte of diatoms has been highlighted (Venrick, 1974; Carpenter
et al
., 1999;
Scharek
et al
., 1999a,b; Capone, 2001). According to one estimate, based on ARAs of samples assayed
