Spawning Studies Investigate Bleaching Effects

A CRC research project on soft coral reproduction has found a new link between spawning and the 'bleaching' phenomenon which leaves stressed corals pale or dead. James Cook University doctoral candidate, Kirsten Michalek-Wagner, is investigating how bleaching affects the internal chemistry of soft corals and their ability to reproduce.

Kirsten Michalek-Wagner at Orpheus Island during a field trip to study survival mechanisms and reproduction systems of soft corals affected by raised water temperatures and increased ultraviolet light. (Photo: GBRMPA)

Coral bleaching events have been recorded on reefs throughout the world for more than 60 years and may be a natural aspect of reef communities. However, in the past ten years, awareness of bleaching events worldwide has increased dramatically. It has been proven in recent studies that high ultraviolet radiation and elevated temperatures are the main factors that trigger bleaching. This has sparked speculation about links between coral bleaching, ozone depletion and global warming.

How survival mechanisms and reproduction systems of soft corals are affected by raised water temperatures and increased ultraviolet light is the topic of Kirsten Michalek-Wagner's PhD research, made possible through a CRC scholarship.

Kirsten, a German-born marine biologist and chemist at James Cook University, is concerned about the possible effects of bleaching on soft corals living in the Great Barrier Reef. While Australia's coral reefs are in good shape, bleaching occasionally kills certain species of coral during summer months, and scientists don't know if anything can be done about it.

"Coral bleaching is different from the way your swimsuit fades in the sun," Kirsten says. "The coral becomes pale because it loses its zooxanthellae, the tiny algae living in the tissue of the coral polyp. Zooxanthellae not only provide the coral's colour but also most of the animal's nutrition. With bleaching, healthy corals expel their zooxanthellae guests in situations of stress brought about by high levels of ultraviolet radiation and elevated temperatures".

Bleaching can be lethal to large areas of coral. In years of the El Nino Southern Oscillation (ENSO) where normal water temperatures rise by two, or even three, degrees celsius, widespread mortality of corals has been recorded by scientists observing reefs around the world. In response to five major bleaching events on the Great Barrier Reef over the last 15 years, a number of studies have addressed the phenomenon. Yet, despite frequent bleaching in soft corals, which often make up a large percentage of cover on a reef area, research into bleaching has focused almost exclusively on hard corals.

Kirsten's studies comprise three main experiments: the effects of bleaching on the production of "secondary metabolites" in relation to defence, the symbiotic association of algae, and the reproductive capabilities of soft corals after bleaching occurs.

Although these metabolites are considered as 'by-products', they ensure fitness, protection and aid in reproduction for the coral. Interference in production of these metabolites could leave individuals more open to predation, algal overgrowth, or may result in a reduced reproductive output. "Since zooxanthellae probably contribute to the production of secondary metabolites, bleaching could compromise the survival of soft corals," Kirsten said.

"Bleaching is probably the same for both hard and soft corals, but there may be differences in how the two groups are affected," Kirsten says. "When bleaching occurs, the loss of zooxanthellae in soft corals may interfere with the production of the secondary metabolites, which are important for the fitness of the coral. The production of these compounds, which are at least partly produced by their zooxanthellae, is energetically very costly - but they get value for money".

When corals lose their zooxanthellae they also lose their major food source. They become in a sense 'run down' in a similar way to the human body becoming susceptible to infection when the immune system is put under severe stress.

"Corals are living animals and without their zooxanthellae they can still gain energy using their polyps to harpoon food. The coral still survives - but at a high cost Nutrition is poor, its defences are down and the coral may not be able to fulfil the primary means of survival - reproduction," Kirsten says.

Implications on the longevity of our coral reefs alarmed Kirsten and prompted her to design one experiment around the reproductive abilities of soft coral after bleaching.

In 1995, she began experiments with reproductively mature colonies of Lobophytum compactum, a common species of soft coral which is pizza shaped with lobe-like fingers. To simulate a natural bleaching event, Kirsten exposed different female colonies of Lobophytum in a laboratory to prolonged levels of ultra violet light and high water temperatures until 60 to 95% of the coral bleached. Numbers of zooxanthellae were counted before and after the treatments and the zooxanthellae repopulation of the coral was monitored until spawning.

Kirsten says soft corals are really quite hardy. "This particular coral (Lobophytum) survived with only five percent of its zooxanthellae and totally regained its zooxanthellae within three months."

A typical field visit to Orpheus Island is no picnic for researchers. The day starts at 6am with Kirsten loading dive gear and sampling equipment into the boat then on to her research area. Sample bags are numbered corresponding with the experiment and with individual corals to be studied. For Kirsten, corals are more than just numbers - they have personalties. She understands their current state of egg production by sampling branching lobes and examining the texture and colour of eggs.

"I love the smell of soft corals when they are exposed on the reef top at low tide. Although they look so sad, all limp and covered in mucus from being out of the water, the smell caused by the secondary metabolites is beautiful," Kirsten says. Her peers suggest that this must be an acquired taste.

Samples are taken from corals above and below water. Back at the lab, Kirsten `blends' them into a liquid and places a sample under the microscope to count the zooxanthellae. These figures are recorded and show the recovery rate of each coral.

Just before spawning, nets are placed over individual female coral colonies to collect the released eggs. The eggs are then removed and counted. Kirsten expects her `ladies', as she calls them, will reveal whether the loss of zooxanthellae has impaired production of the secondary metabolites and the health and survival rate of offspring.

The eggs are counted, measured and chemically analysed. "I look for any possible effects that bleaching has had on the secondary metabolite chemistry. The fertilisation rates and overall survival of the new coral larvae are monitored to see whether it has been a successful spawn," Kirsten said.

The larvae collected at Orpheus Island will be transported to James Cook University where Kirsten will nurture and monitor growth of the new corals. They can then be analysed to check whether their secondary metabolite chemistry has developed as well as their non-bleached counterparts. "I hope to bring the young corals back to the sea eventually, to see how they perform in the wild," she said.

Kirsten's research is the first step in understanding the effects of bleaching on soft coral communities - specifically how bleaching affects their secondary metabolite chemistry and, therefore, their reproductive abilities.

The total impact of the mass bleaching on the world's coral reefs is still not known. But one thing is for certain, without further research into this global phenomenon we cannot be confident our reefs, as we know them now, have a secure future.

By Tania Ashworth