Table Of ContentReference: Biol. Bull. 198: 29-33. (February 2(1(10)
Parasitic Diatoms Inside Antarctic Sponges
GIORGIO BAVESTRELLO1-*, ATTILIO ARILLO2, BARBARA CALCINAI2.
RICCARDO CATTANEO-VIETTI2 CARLO CERRANO2 ELDA GAINO3
. , ,
ANTONELLA PENNA4 AND MICHELE SARA2
.
[stitiito di Science del Mure University di Ancona, Via Brecce Blanche. 1-60131 Ancona. Italy;
Dipartimento per lo Studio del Territorio c delle sue Risorse Universita di Geneva, Viale Benedetto
XV, 5 1-16132 Genova, Italv; and Dipartimento di Biologia animate ed Ecologia Universita di
Perugia, Via Elce di Sotto 1-06123 Perugia, ltal\; and 4lstituto di Biochinu'ca Universita di Urbino.
1-60118. Italv
Antarctic sponges may host large populations ofplank- benthic diatoms, and these microorganisms are actively
tonic and bentliic diatoms. After settling on the sponge. taken up by the pinacoderm and thus enter the sponge
these diatoms enter its body through pinacocytes (1) and choanosome (1). This couldbe an adaptive supplementation
form, there, large mono- orpaiici-specific assemblages. Yet of the food supply in the Antarctic ecosystem, because
the total amount ofcarbohydrates in the invaded sponge diatoms produce extracellular polysaccharides (8, 9) that
tissue is inverselv correlated with that ofchlorophyll-^.. We sponges might use as energy substrates. On the other hand,
suggest, therefore, that endobiont diatoms utilise the prod- the integrity ofthe diatom shells, the presence ofcytoplasm
ucts ofthe metabolism oftheir host as an energy source. inside the shells, and the formation of large mono- or
This is the first evidence indicating that an endobiotic pauci-specific assemblages inside a single sponge suggest
autotrophic organism mayparasiti-e its animal host. More- that diatoms are able to complete their entire biological
over, this unusualsymbiotic behavior couldbe a successful cycle inside the sponge body.
strategy that allows the diatom to survive in darkness. The aim ofthis study was to investigate the energy source
Heterotrophic bacteria, autotrophic cyanobacteria, that symbiotic diatoms might provide to Antarctic sponges.
zoochlorellae, and zooxanthellae are common symbionts To this end, we evaluated the variation in the total carbo-
in Porifera, where they may actually constitute most of hydrate content of sponges as a function of the biomass of
the sponge tissue. Sponges can also harbor fungal popu- endobiotic diatoms; biomass was estimated from chloro-
lations, the significance of which has been poorly inves- phyll-o concentration.
tigated (2). The association of sponges with autotrophic We examined 39 specimens representing 17 species of
symbionts occurs mainly in tropical (3) and temperate sponges (Table 1 ). The material wascollectedatTerraNova
m
waters (4); it is a fruitful strategy, allowing the sponges Bay (Ross Sea), between 100 and 120 depth, during the
to utilize the symbionts as a food source complementary XIII Italian Antarctic Expedition (Austral Summer 1997-
to filter feeding (5, 6). 1998). The sponges were prepared for ultrastructural study
Diatoms in sponge tissues have rarely been recorded (7), as follows. Immediately after collection, small pieces from
but such associationsare widespread in Antarctic waters (1 ). each specimen were fixed for 2 h in 2.5% glutaraldehyde in
In fact, most sponges from Terra Nova Bay (Ross Sea, artificial seawater (ASW). The fixed tissues were rinsed,
Antarctica) host large populations of both plunktonic and stored in ASW, and then dehydrated in a graded series of
ethanol concentrations. Critical point drying was achieved
R*eTcoeiwvehdom29cJournreesp1o99n9d;enaccecesphtoeudld5bNeoavdedmrbesesred1.99E9-.mail: bavestrellofs1 ewixtahmianedCOw,ithPabaisPhhilCipPsDEM75051a5ppsacraatnunsi.ngSaelmepcltersomiwcerroe-
popcsi.unian.it scope, and several species of diatoms were observed (Fig.
29
H
30 G. BAVliSTRKU.O Al..
I..I.I. 1
Chlorophyll-aconcentration andcarbohydratecontentforthesponge
specimensanalyzed
Chlorophyll-a Total sugars
Specimens dwaf) (mg/g d\val)
Artemisina rubulosa
DIATOMS PARASITIC ON SPONGES 31
Figure 1. Diatoms living inside Antarctic sponges, (a) Several specimensofFragilariopsiscitrta (arrows)
and a Fragilariopsis sp. disposed under the exopinacoderm of Tedania charcoti (Scale bar, 50 /j.m (b)
Fragilariopsissp.(arrows)embeddedinsidethesamesponge(scalebar. 50jim). (c)LargecentricPorosira sp.
(arrows) and numerous pennate Achnanthes sp. (arrow heads) in Pseudosuberites antarcticus (scale bar. 100
/uni). (d) Transmission electron micrograph of Fragilariopsis citrta inside the sponge body showing intact
cytoplasm (c) and thylocoid (t) (scale bar, 10 /u,m). s, sponge spicules.
facultative heterotrophy. Mixotrophy, a combination of not only by photosynthesis, but also by utilizing organic
autotrophy and heterotrophy, is widespread among cer- carbon derived from their host. Although some Antarctic
tain protists and, in particular, among flagellates such as diatoms are considered among the most shade-adapted mi-
haptophytes (13), dinoflagellates (14, 15, 16, 17), cryp- croalgae (23), the mixotrophy assumed in this report ex-
tophytes (18), and diatoms (19, 20, 21). Previous docu- plains the surprising presence of dense diatom populations
m
mentation of mixotrophy in diatoms was obtained by inside sponges living at 100-120 depth, where the level
observations in vitro. of irradiance is very low (24).
The relationship between myxotrophic diatoms and Ant- Finally, the mechanism that causes a sponge to actively
arctic sponges can be considered as a sort of symbiosis incorporate diatoms is unclear. However, some studies (22)
shifting to parasitism: i.e., the algae can get nourishment. have revealed a tendency of demosponges to incorporate
32 G. BAVESTRELLO tT AL
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