How a host cell signalling molecule modifies carbon metabolism in symbionts of the coral Plesiastrea versipora.

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 (14) C-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, (14)C-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.

Heterotrophy on ultraplankton communities is an important source of nitrogen for a sponge-rhodophyte symbiosis.

Grazing on ultraplankton by the sponge partner of an invertebrate/algal symbiotic association can provide enough particulate organic nitrogen to support the nitrogen needs of both partners. The previously unknown natural diet of the sponge in the Haliclona-Ceratodictyon association consists of bacteria and protozoans, which are rich sources of nitrogen. Retention of ultraplankton varied with season and time of day. During the winter there was an order of magnitude more nitrogen taken up than in summer. Time of day during each season also affected the amount of ultraplankton retained. In summer retention was higher at night whereas the opposite was true during winter. Overall, the Haliclona-Ceratodictyon association is able to meet its metabolic nitrogen demands through grazing on the naturally occurring water column community.

Ammonium excretion by a symbiotic sponge supplies the nitrogen requirements of its rhodophyte partner.

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 (delta(15)N) 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.