Coral Reef Bleaching: Human Threats to Coral Reefs

Coral reefs are often regarded as the most complex and diverse assemblages of fauna and flora in the oceans. They are important for numerous reasons; they provide a livelihood to millions of people, are a major source of food to humans, and they have a significant role in the survival of the rest of the ecosystem. However, despite occupying the oceans for 500 million years, their position is being threatened largely by a host of human-influenced activities. Coral bleaching is the main threat to coral reefs, caused when the symbiotic zooxanthellae living within the corals become expelled. Climate change is regarded as being largely to blame for the increase in bleaching, as it is thought to lead to warmer oceans, and more extreme environmental conditions. Other human factors such as overfishing, pollution, sedimentation and weapons testing are also thought to be having detrimental effects on the reefs. If steps are not made to protect the reefs, their distribution and the diversity of coral organisms will be severely reduced. I suggest further studies into coral resilience involving different species of zooxanthellae with varying resistances to environmental stresses, in order to determine if recolonisation of bleached corals can readily occur. Other suggested studies will look into changes in management practices to aid in the preservation of coral reefs.


Keywords: coral reef; bleaching, anthropogenic; zooxanthellae; resilience; management

Coral-Zooxanthellae Symbiosis - Costs

Despite the clear advantages obtained by the symbiosis, there are several costs induced from it. The coral sustains costs due to investment of resources in a system to expel algae if numbers exceed the optimal level or if environmental conditions are not favourable. Also the excretion of perialgal vesicles containing nutrients destined for the host cells uses resources. For the zooxanthellae, regulation of growth rate occurs as a result of the symbiosis as growth rate is slower when residing within coral than when in the coccoid stage. Expulsion from the host is also costly, as well as the costs involved in transport of carbon to the host, limiting its own usage. In order for the symbiosis to be a mutually beneficial one, the benefits must outweigh the costs for both members. If this is not the case, the symbiosis is likely to collapse, with the zooxanthellae being expelled or leaving the coral. A host of environmental factors act on members of the symbiosis, with extreme conditions potentially leading to an unbeneficial symbiosis and loss of zooxanthellae from the corals.