Biology Reference
In-Depth Information
housed is also formed in the absence of it. The cyanobiont fi xes nitrogen and transfers the fi xed
nitrogen to the host plant and in return the latter provides fi xed carbon to the cyanobiont. Thus in
these mutualistic associations the cyanobacteria constitute a driving force in the evolution of their
hosts (Usher
et al
., 2007).
I. SPONGES
The existence of sponges (Phylum-Porifera) can be dated back to Precambrian. As many as 9000
living sponge species are distributed on tropical reefs from lower to higher latitudes (Brusca and
Brusca, 1990). Sponges are known as fi lter feeders and pump large volumes of water through their
canal system (Reiswig, 1971, 1974; Pile
et al
., 1996). With reference to their nutrition and other features
they very much resemble Protozoa, since a number of amoeboid cells move freely in the sponge
matrix. Sponges harbour a diversity of prokaryotic and eukaryotic symbionts and they account
for 40% of their biomass (Vacelet, 1975; Vacelet and Donadey, 1977). The range of these symbionts
includes archaebacteria, heterotrophic bacteria, cyanobacteria, green algae, red algae, dinofl agellates,
cryptophytes and diatoms (Larkum
et al
., 1987; Santavy
et al
., 1990; Duglas, 1994; Preston
et al
.,
1996). It is also true for a single given species of sponge. As for example,
Theonella swinhoei
possesses
heterotrophic bacteria, unicellular cyanobacteria and fi lamentous heterotrophic bacteria (Bewley
et
al
., 1996). On the other hand, sponges of
Aplysina
spp. show different bacterial genera such as
Bacillus
sp.,
Micrococcus
sp.,
Arthrobacter
sp.,
Vibrio
sp.,
Pseudoalteromonas
sp. (Hentschel
et
al
., 2001). Likewise,
sponge
Rhopaloeides odorabile
is a good habitat for β-proteoacteria, γ-proteobacteria, cytophagas,
actinobacteria and green sulphur bacteria (Webster
et al
., 2001).
Cyanobacterial symbionts are known to occur in many sponge genera. Their location can be
extracellular in sponge tissue or intracellular in specialized vacuoles. Due to their growth, the
symbionts enable the sponges to compete for substrata with algae and corals in illuminated areas
(Wilkinson, 1983; Hinrichsen, 1997). Most common cyanobacterial symbionts belong to
Aphanocapsa
feldmannii
(Fremy) group (Feldman, 1933) which is present in the surface tissues of as many as 60
sponge species (belonging to 13 orders). Other cyanobacteria such as
Synechocystis
(Larkum
et al
.,
1988),
Oscillatoria
(Wilkinson, 1992),
Anabaena
(Larkum, 1999),
Cyanobacterium
(Webb and Mass,
2002) and
Synechococcus
(Hentschel
et al
., 2002; Usher
et al
., 2004a) have been reported from different
sponge genera. According to one estimate, the proportion of cyanobacterial biomass is equal to
that of sponge cells by meeting 50% of the sponge's energy budget and 80% of the sponge's carbon
budget through photosynthesis of cyanobionts (Wilkinson, 1983; Cheshire
et al
., 1997). However, of
the above cyanobionts, only two of them have been frequently reported from many sponge genera.
These are
Oscillatoria spongeliae
and
Synechococcus spongiarum
. The former has been recorded from
three species of sponges belonging to Dysideidae (Order Dictyoceratida), i.e.
Lamellodysidea
(formerly
Dysidea
)
herbacea
,
L
.
chlorea
and
L
.
granulosa
(Larkum
et al
., 1987; Hinde
et al
., 1994; Thacker and Starnes,
2003) whereas the latter has been reported from a wider variety of sponges. These are sponge genera
Xestospongia muta
(Petrosiidae, Haplosclerida; Gómez
et al
., 2002),
Aplysina aerophora
(Aplysinidae,
Verongida; Hentschel
et al
., 2002) and
Chondrilla nucula
(Chondrillidae, Chondrosida; Usher
et al
.,
2004b). Usher
et al
. (2004a) distinguished the
Synechococcus
spp. associated with sponges to be
different from the planktonic
Synechococcus
though as yet there are no evidences for the symbionts
to be host-specifi c.
Growth of sponges is generally measured in terms of dry weight and the area under cover
occupied by the sponges at the beginning of the experiment.
L
.
chlorea
lost both dry weight and
nearly 40% of the area under shading after 15 days of incubation where as in
X
.
exigua
the loss
