If you've been fortunate enough to swim through brightly coloured tropical coral reefs, you'd probably think that you are in one of the planet's most verdant environments. But you'd be wrong! Simon Davy explains that the crystal clear waters are a nutritional desert. Many of the algae that flourish on coral reefs survive on starvation diets, which have driven some of them to satisfy their nutritional needs by setting up home with a symbiotic partner. Most symbiotic algae solve the problem by nestling in the cells of their partner, but the red seaweed, Ceratodictyon spongiosum has taken the alternative approach, and draped a sponge around its branching fronds. Davy and his colleagues in Sydney wondered how these allies manage their nutritional trade, and are teasing apart the nutritional network that binds the partners together. In this issue of the J. Exp. Biol. they report that the sponge is able to satisfy its algal partner's hunger for nitrogen when other algae go hungry ().
Sponges are the most fantastic filter feeders. They can strain the last crumbs of food, even from the clearest of waters, to supply themselves with the essential proteins that they need to sustain life. Algae, on the other hand, depend on a supply of dissolved inorganic nitrogen from their environment. But if your environment includes a symbiotic partner, the nutritional equation becomes more difficult to calculate. Davy and his colleagues weren't daunted by the difficulties of untangling the relationship between the seaweed, Ceratodictyon, and its symbiotic partner, the sponge Haliclona cymiformis, even though neither species can survive alone in the wild.
Davy relocated his lab bench to the Great Barrier Reef to be close to the symbiotic partners' home, venturing out into the island's lagoon to collect fresh samples of the associated organisms for each day's experiments. First he decided to measure how much nitrogen each partner needed to maintain the duos combined daily growth rate of 0.83%. After measuring each partner's nitrogen content, he found that the sponge needed 0.167 mg N g-1day-1 to sustain its growth rate, while the alga needed 34% less. Davy knew that the sponge could probably filter enough plankton from the sea to satisfy its nitrogen demands, but there was little chance that the reef waters dissolved nitrogenous compounds could keep pace with the alga's growth. Was the sponge supplementing the alga's `diet'?
He measured the amount of ammonia that the sponge excreted by `switching'the alga off. Davy plunged the alga and its sponge partner into darkness, to stop the plant photosynthesising and absorbing the sponge's waste nitrogen. Meanwhile the sponge continued excreting waste nitrogen that the scientists could measure to see if the sponge's excretion rate would keep pace with the alga's growth rate. Davy was pleased to see that the nitrogen budget balanced;the sponge produced more than enough ammonia to satisfy the alga's demands.
Davy emphasises that this is still very preliminary work. He adds that`there are still many things that we don't know about nitrogen fluxes and metabolism in this symbiosis'. But one thing's for sure, the alga is certainly satisfied by sponging off its partner.
