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Ammonium excretion by a symbiotic sponge supplies the nitrogen requirements of its rhodophyte partner
1 School of Biological Sciences, A08, University of Sydney, New South Wales
2006, Australia
2 School of Biological Sciences and Biotechnology, Murdoch University,
Murdoch, Western Australia 6150, Australia
* Author for correspondence at present address: Institute of Marine Studies, University of Plymouth, Plymouth PL4 8AA, UK (e-mail: sdavy{at}plymouth.ac.uk)
Accepted 13 August 2002
Symbioses between sponges and algae are abundant in the nutrient-poor
waters of tropical reefs, yet very little is known of the nutritional
interactions that may promote this abundance. We measured nitrogen flux
between the sponge Haliclona cymiformis and its symbiotic partner,
the rhodophyte Ceratodictyon spongiosum, and assessed the potential
importance of this flux to the symbiosis. While the association can take up
dissolved inorganic nitrogen (DIN) as ammonium and nitrate from the
surrounding sea water, enrichment of the water with nitrate did not affect its
rates of photosynthesis and respiration. Much of the DIN normally assimilated
by the alga is waste ammonium excreted by the sponge. A nitrogen budget for
the symbiosis shows that the nitrogen required for algal growth can
potentially be provided by sponge catabolism alone, but that only a small
amount of nitrogen is available for translocation back to the sponge in
organic compounds. The stable isotope composition (
15N) was
consistent with our interpretation of the sponge supplying excretory DIN to
its algal partner, while the results also suggested that this DIN limits
nitrogen deficiency in the alga. If our observations are typical of
sponge—alga symbioses, then the supply of excretory nitrogen could be a
major reason why so many algae form symbioses with sponges on coral reefs.
Key words: symbiosis, sponge, rhodophyte, Haliclona cymiformis, Ceratodictyon spongiosum, nitrogen flux
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