Coral cell-signal for algal control

19 March 2013

Corals need to control the growth of their intra-cellular algae, lest the algae become parasitic. In a paper recently published in FEBS Journal, School of Biological Sciences researcher Dr Adrienne Grant, has demonstrated how a local coral's cell signal exerts that control.

The symbiotic relationship between corals and their intra-cellular algae is mutually beneficial. The algae get a safe place to live and the coral gets the products of algal photosynthesis. To maintain this happy relationship the coral needs to control the growth of the algae. This is where Adrienne's research comes in. "We have shown that the widespread and long-lived coral, Plesiastrea versipora, has developed an efficient and highly active cell signal that controls the carbon metabolism of its symbiotic algae," explained Adrienne. That cell signal is called host release factor, abbreviated to HRF.

HRF from this coral acts to stimulate the release of glycerol from the algae. "The glycerol is produced by the algae during photosynthesis and is a precursor for a number of carbon compounds used for both plants and animals." By using this HRF cell signalling system, the coral can use the glycerol for its own purposes and, at the same time, leave less glycerol available for the algae. "If the algae use all the products of photosynthesis for themselves, then they may grow very fast, and become parasites."

Small-knob coral produce host release factor to control the algae within
Small-knob coral produce host release factor to control the algae within

Plesiastrea versipora, or small-knob coral, is a hard coral with a wide distribution within the Indo-Pacific, including the south coast of Australia. It is one of a small percentage of corals (15%) that transmit their algae maternally. That is, the symbiotic algae are passed through maternal inheritance (i.e. via the eggs) rather than being taken up and released from the surrounding environment. "An efficient HRF cell signal is a useful tool for animal hosts that pass on their algae solely through maternal inheritance."

Exactly how HRF acts to stop the algae making use of its own photosynthetic products is still to be discovered. "While glycerol is the main product released from algae, some amino acids are also released. My view is that as a result of HRF's initial action, there are several steps in which a synthesized compound builds up because it is blocked from further processing, and is then released from algae," mused Adrienne. "But this aspect really needs to be argued by carbon biochemists."

One of the puzzling things to come out of this research is that HRF was found in corals that had lacked algae for almost two years. If the sole role of HRF was in controlling release of algal photosynthesis products, you would not expect to find it in corals without algae. So what does this mean? "I believe that HRF is a cell signal that is present in most, if not all, animals," Adrienne explained, "and that HRF has a role in both animal and plant carbon metabolism. But because it is difficult to purify, no one has yet found it in non-symbiotic organisms."

Purification of HRF is described by Adrienne as the most difficult part of this study. "It is very difficult to purify hydrophilic (water soluble) cell signals," she explained. "Interference from seawater salt is a major problem and retaining biological activity during purification is also very difficult." But Adrienne thinks it is worth the effort. "The methods that we have used for the isolation of the water soluble HRF will have a wider application for drug discovery." Especially if it is as widespread as Adrienne suspects. Her plan now is to use her methods to detect HRF in other organisms.