• host cell signalling molecules;
  • Plesiastrea versipora ;
  • starch;
  • Symbiodinium ;
  • triacylglycerol

Cnidarian cell signalling remains poorly understood. This study has expanded our knowledge of the cell signalling molecule host release factor (HRF) from the coral Plesiastrea versipora. We have now confirmed that HRF is present in coral host cells that lack intracellular algae. Previous studies showed that HRF stimulates the release of photosynthetic products (mainly glycerol) from Symbiodinium algae, thus providing the host with carbon; glycerol release was accompanied by reduced synthesis of algal triacylglycerols and starch. In this study, we have shown that supplying glycerol to algae incubated with HRF does not restore normal triacylglycerol and starch synthesis. Release of 14C-labelled products from algae may continue after photosynthesis ceases, although at a much lower rate. When algae were placed in the dark for 4 h with HRF following 2 h of photosynthesis in seawater, 14C-labelled products were released, but at ≤ 15% of the amount released during 2 h of photosynthesis with HRF. HRF did not stimulate the release of compounds derived from a nonphotosynthetic source. The response of Symbiodinium from P. versipora to HRF has been compared with the response of Symbiodinium algae from Tridacna maxima, Heliofungia actiniformis, Aiptasia pulchella and Pocillopora damicornis to both their own HRF and to P. versipora HRF. Algae from P. versipora showed the highest response to both P. versipora HRF and to the other hosts’ HRF. Further purification of P. versipora HRF suggests that HRF is a peptide with an acidic pI. We propose that HRF will provide a useful tool for the study of carbon metabolism.