We study the biochemistry of cell-cell signals that maintain a stable symbiosis between intracellular algae (zooxanthellae) and their marine invertebrate hosts. Coral bleaching is one event that is evidence of disruption to coral symbioses.
We have isolated 2 signalling compounds from the tissue of the coral P. versipora that modify carbon metabolism in isolated algae. Surprisingly these cell signals are found in coral cells which lack algae.
The first one is called host release factor (HRF) and it stimulates the release of photosynthetic compounds from algae. It also reduces synthesis of algal starch and triglycerides suggesting that it acts to divert carbon from further algal metabolism to the host. It is a small molecule (Mr of 500-1,000) which has a high absorbance at 215 nm suggesting that it is probably a peptide. It also stimulates photosynthate release from the algae of several other symbioses including zoanthids, anemones and two other corals, as well as cultured algae from a jellyfish.
The second signalling molecule is called photosynthesis inhibiting factor (PIF) which partially inhibits photosynthesis and is one of the very few naturally occurring regulators of photosynthesis. That is, it does not kill the algae. It is also a small molecule (Mr of 500-1,000) with a high absorbance at 215 nm, suggesting that it too is probably a peptide. We believe that these molecules play important roles in controlling algal carbon metabolism thus enabling the coral host to receive photosynthate for its own nutrition, and also regulating the numbers of intracellular algae to maintain a steady state population.
Recently we have discovered that the cell signals HRF and PIF are not confined to a coral but are also present in the symbiotic sponge Haliclona cymiformis which lives in symbiosis with the macro red alga, Ceratodictyon spongiosum. These sponge cell signals are of a similar size to those isolated from the coral P. versipora. The sponge HRF stimulates the release of succinate from its own freshly isolated C. spongiosum in contrast to the cell signal HRF from the coral P. versipora, which stimulates the release of glycerol from its own microalgae, Symbiodinium. However, what is really exciting is that the sponge cell HRF also stimulates the release of glycerol from Symbiodinium. That is, the sponge cell HRF can act both on its own macro algal symbiont as well as micro algae from a quite different type of symbiosis. These results suggest that these cell signals are more widely spread than previously thought and that both coral and sponge HRF modify a similar step in algal carbon metabolism, although the released carbon compound may vary according to the type of alga.
We are also studying the effects of pollutants on sessile marine organisms and have developed assays for the early detection of pollution damage in P. versipora. This research will help to prevent irreversible damage to marine organisms in a variety of habitats and also help to devise better management strategies of coral reefs. Zoanthids (Zoanthid robustus in particular) have also proved to be a suitable model for studying the effects of oxidative damage to animal tissue.
|This shows a dissected zoanthid (4cm long) with the mouth region on the left and the stolon on the right. The dark colour is due to the symbiotic algae which line the mesenterial filaments. After exposure to copper the zoanthids contract which is a sign of protein cross linking caused by oxidative damage.|
Photo: Malcolm Ricketts