Chemical Technology May 2016

Rehabilitation in a time of coral bleaching by Gavin Chait

Divers consider the Great Barrier Reef, off the far northern coast of Queensland, Australia, to be one of the greatest destinations for viewing coral anywhere in the world. At 2 300 km long, the system is the largest living thing on earth, and it is dying.

T he Great Barrier Reef comprises thousands of reefs, over 600 types of hard and soft coral, and an almost endless variety of fish and sea-life. It is also worth over US$6 billion to Australia in tourism. The cause of the corals’ dying is coral bleaching. Here’s how it happens. Corals are tiny marine invertebrates living in vast compact colonies of polyps. A particular coral group forms a coherent structure and creates a recognisable pattern. The polyps are a few millimetres in width and maybe a centimetre or two in length with a hard exoskeleton secreted at its base. Over generations, these secretions build up on themselves and become vast reefs. The corals themselves depend for their survival on a symbiotic relationship with photosynthetic unicellular flagellates which live within their tissues. These are called zooxanthellae. Corals require sunlight so that the zooxan- thellae can photosynthesise, and water temperature that is not too warm or too cold. There are corals that don’t rely on this symbiotic rela- tionship, and they can grow at much greater depths and in the absence of sunlight. Recently, a vast and previously unknown reef was discovered beneath the very murky wa- ters where the Amazon river stretches out into the ocean. The colour you see in a healthy reef is mostly as a result of the zooxanthellae living in the tissues of coral polyps. When the water temperature changes too much, or the amount of light is diminished, the zooxanthellae die. This leads to dramatic colour loss across the affected reef. Coral bleaching indicates that the coral itself is not dead yet, but it is starving. If the zooxanthellae do not return, then the reef is doomed. The Great Barrier Reef has experienced bleaching events, of increasing severity, every few years co- inciding with the El Niño current, which raises temperatures. However, this current has become more frequent, and of a

higher temperature, as a result of global warming. In March of this year, Professor Terry Hughes of James Cook University, flew over 600 km of reef. More than 60 % of it was bleached. These bleaching events, while devastat- ing, also lead to major structural changes across the reef. A reef that fails to recover will be colonised by other types of algae and sea weeds. This impacts on the fish which used to depend on the reef and consequently leads to major species loss. This phenomenon is known as a ‘phase-shift’ to a reef dominated by macroalgae (as compared to the microalgae of the zooxanthellae). This does not always happen, however, and scientists have been attempting to figure out why. For starters, there is a very wide range of species differentiation between cor- als with some surviving post-bleaching events significantly better than others. They use the space created to colonise dying reefs and come to dominate. There are also herbivores which eat the macroalgae and hold back growth until the reef can recover. The question about how to encourage coral survival is more than just about tourists on their summer vacations. Over 500 million people in 100 countries depend on coral reefs for their food and livelihoods. One organisation working on coral restoration and reme- diation is the Coral Reef Targeted Research and Capacity Building for Management (CRTR) Program based at the University of Queensland. They rear nearly 10 000 nursery- grown colonies for reef transplants every year. The problem with the nursery-led approach of CRTR is that transplanted corals have a higher mortality rate and grow more slowly than do the reefs where they are cemented. Scientists’ concern is that they still know very little about the life-cycle and biology of specific species. In American Samoa, lagoons full of coral survive 35 °C

20

Chemical Technology • May 2016

Made with