Biology Reference
In-Depth Information
of photosynthates from
Prochloron
. The contribution of
Prochloron
to the carbon demand of ascidians
differs from species to species (Koike and Suzuki, 1996) as exemplifi ed by
Dimemnum molle
which
cannot depend solely on photosynthates from its symbiont where as
Lissoclinum voeltzkowi
the carbon
demand can be fully met by
Prochloron
(Koike
et al
., 1993).
V. ALGAE
A) Diatoms:
Cyanobacteria form three types of symbioses with diatoms. These are formation of
microbial spheres, epiphytic and intracellular associations.
i)
Microbial spheres
:
Brehm
et al
. (2003) observed the formation of loose associations of bacteria,
cyanobacteria and diatoms leading to the formation of spherical objects known as microbial spheres.
In the fi rst phase, bacteria and diatoms come together and are held in a matrix. In the second phase,
cyanobacteria penetrate these spheres and arrange themselves on the surface. The formation of
such spheres and their proliferation in non-axenic cultures of
Phormidium
from North Sea microbial
mats by the entrapment of phototrophic bacteria and diatoms (especially
Navicula
) was observed.
Chemotactic responses are indicated for such an association and possible nutritional interactions
are indicated.
In another loose association, the presence of a number of diatoms (of the genera
Amphora
,
Berkeleya
,
Cymbella
,
Entomoneis
,
Epithemia
,
Lunella
,
Mastogloia
,
Nitzschia
and
Rhopalodia
) deep inside
the colonies of
Rivularia
growing in brackish waters of Baltic Sea has been noted. The advantages of
such association for diatoms can be protection from grazing, free mobility in the secreted mucilage
and supply of organic and inorganic nutrients (Snoeijis and Murasi, 2004).
ii) Epiphytic and endophytic associations
:
These two types are considered together here because of
their interchangeability from one type to the other and the early events associated with the discovery
of this symbiosis.
Richelia
is a short fi lamentous, heterocystous cyanobacterium with 4-10 vegetative
cells. A single terminal heterocyst is present that is slightly smaller in diameter than the vegetative
cells. The fi laments are slightly tapered and do not possess gas vesicles (Janson
et al
., 1995). Due to these
morphological features it is considered closer to
Calothrix
and in fact Lemmermann (1899) initially
identifi ed the epiphyte of
Rhizosolenia
as
Calothrix rhizosoleniae
. Subsequently, the endosymbiont
of
Rhizosolenia
was given the name of
Richelia
intracellularis
(Schmidt, 1901). The endosymbiotic
nature of
R
.
intracellularis
was confi rmed by Lemmermann (1905) not only in
Rhizosolenia
but also in
Hemiaulus
and its occurrence as epiphyte on
Chaetoceros
. Generic names
Calothrix
and
Richelia
, have
been used by Carpenter (2002) for the epiphyte and the endosymbiont, respectively. Reports on the
epiphytic nature of
R
.
intracellularis
growing on
Chaetoceros
(Janson
et al
., 1999) and
Bacteriastrum
(Villareal, 1992; Rai
et al
., 2000; Carpenter, 2002) exist in literature.
R
.
intracellularis
establishes as an
epiphyte by attaching to the spaces in between the cells in chains of diatom colonies of
Chaetoceros
.
The endosymbiotic nature of
R.
intracellularis
in diatom
Hemiaulus
spp. (Kimor
et al
., 1978; 1992;
Heinbokel, 1986; Villareal, 1994) and
Rhizosolenia
clevi
var.
communis
(Sundström, 1984) has been
described with two in the former and 2-4 fi laments inside the cells of the latter.
Rhizosolenia
does not
fi x nitrogen in the absence of its endosymbiont
R
.
intracellularis
(Villareal, 1987). Nitrogen fi xation
by
R
.
intracellularis
can fully support the needs of its host (Villareal, 1990, 1991). The division cycle
of
Rhizosolenia
-
R
.
intracellularis
symbiosis (Villareal, 1989) and a preliminary characterization of this
symbiosis
in vitro
(Villareal, 1990) have been reported.
The justifi cation for designating the epiphyte and endosymbiont by different generic names has
been examined by studying the genetic diversity of
R
.
intracellularis
. Janson
et al
. (1999) used PCR
