Wednesday, 24 October 2012

Genes of the Seven Wonders of the World: The Great Barrier Reef

Genes of the Seven Wonders of the World: The Great Barrier Reef

The East Coast of Queensland is home to one of the Seven Wonders of the World – The Great Barrier Reef (GBR). It’s the world’s largest coral reef, covering more than 3000 000 square kilometers of sea. It is the only living ecosystem that can be seen from outer space and is home to many marine animals. It is made up of approximately 3000 coral reefs.  

Unfortunately, the coral that constitutes for the reef is under threat due to many human stressors on the environment. These include overfishing, pollution, and land run off.  To some degree, these can be prevented in order to maintain the integrity of the reef. However, one of the most prominent factors concerning the reef is the increasing temperature of the sea and reduction in pH. And this isn’t just a threat to the Great Barrier Reef alone; it is a threat to all off the reefs on a global basis.  Why is increasing temperature a problem? Well, even just a few centigrades difference in temperature can contribute to a condition where the coral becomes pale in colour, commonly referred to as “coral bleaching.” 

Figure 1
Figure 1 represents three different phases of coral. Healthy coral is surrounded by the symbionts, zooxanthellae and polyps, which are microorganisms beneficial to the coral. They protect the coral skeleton and help it to produce food and 90% of the energy required for growth and reproduction. In return, the coral host provides these microorganisms with protection and a continuous supply carbon dioxide through respiration, which is important for photosynthesis.   

When a coral has experienced a bleaching event it looses zooxanthellae.  Two things can happen depending on the fitness level of the coral. It can reestablish its zooxanthellae effectively recovering and physically increasing its tolerance to heat stress. However, the coral experiences a reduction in growth and reproductive rate and also becomes much more susceptible to diseases.  As a result, we will see a reduction in the population of these species of coral. Although the individual obtains increased tolerance to bleaching events, at a molecular level, the DNA of the coral remains the same and there is no benefit to any of the offspring as it is a phenotypic change and not a genetic change. The other thing that can happen is the coral will die and become covered in algae, which isn’t all that useful for marine life.

In the last 20 years, there has been a 50.7% reduction of overall coral population caused by bleaching among the GBR. Scientists are trying to develop an understanding of coral genetics by viewing the genetic diversity of the population. In order for a species to survive and adapt selectively, there needs to be a broad range of genotypes to “select” from. So in event B, we can see natural selection in play. There’s an increased surface temperature of the sea, and one type of coral has a particular genotype, which has allowed it to survive under this stress. And when it reproduces, the population shows an increased abundance of fitter genotypes that are passed through the generations. 

Unfortunately there is a shortness of genetic diversity in the coral population and the ability for coral to adapt in an evolutionary sense is slow. It is believed that some species of coral have a genotype that has a higher tolerance level to climate change and decreased pH levels of the sea. Scientists are currently trying to sequence the genome and determine a way to help coral express this gene and increase their survival, however costs of this study is demanding and quite challenging for scientists to understand.

By Britney Lergessner

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