Coral bleaching

Coral bleaching occurs when coral polyps expel algae that live inside their tissues. Normally, coral polyps live in an endosymbiotic relationship with this algae crucial for the health of the coral and the reef. The algae provides up to 90 percent of the coral's energy. Bleached corals continueto live but begin to starve after bleaching. Some corals recover. Coral bleaching occurs when coral polyps expel algae that live inside their tissues. Normally, coral polyps live in an endosymbiotic relationship with this algae crucial for the health of the coral and the reef. The algae provides up to 90 percent of the coral's energy. Bleached corals continueto live but begin to starve after bleaching. Some corals recover. Above-average sea water temperatures caused by global warming is the leading cause of coral bleaching. According to the United Nations Environment Programme, between 2014 and 2016 the longest recorded global bleaching events killed coral on an unprecedented scale. In 2016, bleaching of coral on the Great Barrier Reef killed between 29 and 50 percent of the reef's coral. In 2017, the bleaching extended into the central region of the reef. The average interval between bleaching events has halved between 1980 and 2016. The corals that form the great reef ecosystems of tropical seas depend upon a symbiotic relationship with algae-like single-celled flagellate protozoa called zooxanthellae that live within their tissues and give the coral its coloration. The zooxanthellae provide the coral with nutrients through photosynthesis, a crucial factor in the clear and nutrient-poor tropical waters. In exchange, the coral provide the zooxanthellae with the carbon dioxide and ammonium needed for photosynthesis. Negative environmental conditions thwart the coral's ability to provide for the zooxanthellae's needs. To ensure short-term survival, the coral-polyp then expels the zooxanthellae. This leads to a lighter or completely white appearance, hence the term 'bleached'. As the zooxanthellae provide up to 90 percent of the coral's energy needs through products of photosynthesis, after expelling, the coral may begin to starve. Coral can survive short-term disturbances, but if the conditions that lead to the expulsion of the zooxanthellae persist, the coral's chances of survival diminish. In order to recover from bleaching, the zooxanthellae have to re-enter the tissues of the coral polyps and restart photosynthesis to sustain the coral as a whole and the ecosystem that depends on it.If the coral polyps die of starvation after bleaching, they will decay. The hard coral species will then leave behind their calcium carbonate skeletons, which will be taken over by algae, effectively blocking coral re-growth. Eventually, the coral skeletons will erode, causing the reef structure to collapse. Coral bleaching may be caused by a number of factors. While localized triggers lead to localized bleaching, the large scale coral bleaching events of the recent years have been triggered by global warming. Under increased carbon dioxide concentration expected in the 21st century, corals are expected to becoming increasingly rare on reef systems. Coral reefs located in warm, shallow water with low water flow have been more affected than reefs located in areas with higher water flow. Elevated sea water temperatures are the main cause of mass bleaching events. Sixty major episodes of coral bleaching have occurred between 1979 and 1990, with the associated coral mortality affecting reefs in every part of the world. In 2016, the longest coral bleaching event was recorded. The longest and most destructive coral bleaching event was because of the El Niño that occurred from 2014–2017. During this time, over 70 percent of the coral reefs around the world have become damaged. Factors that influence the outcome of a bleaching event include stress-resistance which reduces bleaching, tolerance to the absence of zooxanthellae, and how quickly new coral grows to replace the dead. Due to the patchy nature of bleaching, local climatic conditions such as shade or a stream of cooler water can reduce bleaching incidence. Coral and zooxanthellae health and genetics also influence bleaching. Large coral colonies such as Porites are able to withstand extreme temperature shocks, while fragile branching corals such Acropora are far more susceptible to stress following a temperature change. Corals consistently exposed to low stress levels may be more resistant to bleaching. Scientists believe that the oldest known bleaching was that of the Late Devonian (Frasnian/Famennian), also triggered by the rise of sea surface temperatures. It resulted in the demise of the largest coral reefs in the Earth's history.