Biology Reference
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was insignifi cant. However, the biomass of the cyanobiont in
L
.
chlorea
(as measured in terms of
chlorophyll content) did not differ very much from controls. These studies help us in understanding
the relationship between the symbionts. The cyanobiont
O
.
spongeliae
contributes to the overall
development of its host in a mutualistic association whereas
S
.
spongiarum
may be commensals
that draw resources from its host without signifi cantly affecting sponge mass. But when
X
.
exigua
is under shade it is likely that
S
.
spongiarum
is consumed by its host (Thacker, 2005).
Sponges growing in Great Barrier Reef of Australia have been examined for the nature of
cyanobionts, ultrastructure and pigment composition.
O
.
spongeliae
has been detected in
Dysidea
herbacea
and in many sponge species. Two more unidentifi ed
Oscillatoria
species have been found
in an unidentifi ed sponge species and the ascidian
Trididemnum miniatum
. Along with
Oscillatoria
sp., the latter contained
Prochloron
as well. All the three
Oscillatoria
species could be distinguished
on the basis of thylakoid arrangement (Larkum
et al
., 1987). Usher
et al
. (2006) characterized the
unicellular cyanobionts of marine sponges from Australia and the Mediterranean by transmission
electron microscopy, cell shape and size and thylakoid arrangement. The cells of
S
.
spongiarum
are
oval and the turns of thylakoids increase from 1 to 5. The cells are located in the outer wall of sponges
C
.
nucula
,
C
.
australiensis
and
Ircinia
variabilis
. Cells of
A
.
feldmannii
that occupy the matrix of sponges
I.
variabilis
and
Petrosia
fi ciformis
are almost spherical and the turns of thylakoids increase from 2.5 to 6.
The cells of
Synechococcus
spp. are oval and have a spiral thylakoid with 2-3 turns and the cells occupy
top few mm of sponge
Haliclona
sp. Thus the cell size and shape of
S
.
spongiarum
and
Synechococcus
spp. are similar suggesting that these cyanobionts are morphologically indistinguishable though
derived from different geographic locations or hosts. They could only be identifi ed on the basis of
number of turns of the thylakoids. However, sponges
Cymbastela marshae
and
I
.
variabilis
revealed
the presence of symbionts
Oscillatoria
sp. and
Aphanocapsa raspagaigellae
, respectively which could
readily be identifi ed both by size and ultrastructural features. One interesting feature noticed is that
in
C. australiensis
, the cyanobiont is reported to be transmitted vertically, i.e. through sponge eggs
(Usher
et al
., 2001). Moreover, attempts to culture sponge-associated cyanobacteria have not been
successful. In addition, the absence of these species in a free state in water samples suggests that the
cyanobionts may not survive outside their hosts (Usher
et al
., 2004b). Histological and ultrastructural
studies on sponge
Tethya orphei
(Demospongiae), collected from Arì Athol coral stones of Maldives
Islands, revealed the presence of
O
.
spongeliae
in the cortical region and penetrated deeply inside the
choanosomal region overlapping with siliceous spicule bundles. The proliferation of the cyanobiont
was so extensive that it could be vertically transmitted from sponge to sponge which confi rms that
the association is mutualistic (Gaino
et al
., 2006).
The molecular marker that has revolutionized the understanding of microbial ecology is 16S
rRNA gene amplifi cation of microbial samples. Besides understanding the phylogenetic relationships,
distribution patterns and diversity of microbes in the environment, this approach has an added
advantage in bypassing the requirement of culturing the microbes. A number of workers have
utilized this molecular marker for understanding the distribution and specifi city of the symbionts
that occur in various sponge species. Hentschel
et al
. (2002) investigated the diversity of symbiotic
microbial communities of 190 sponge species collected from all over the world by 16S rDNA
sequences. A total of 14 monophyletic sponge-specifi c clusters belonging to different bacterial
divisions have been observed of which seven sequences from cyanobacteria inhabiting sponges
Aplysina aerophoba
and
T. swinhoei
could be divided into two clades, i.e.
Synechococcus
/
Prochlorococcus
and
Pleurocapsa
. Webb and Mass (2002) found coccoid cyanobacteria in
Mycale
(
Carmia
)
hentscheli
by epifl uorescence microscopy. The amplifi cation of 16S rRNA genes of these organisms revealed
the presence of four closely related clones which had a high (8%) sequence divergence. The clones
