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Bleaching resistant Kimberley corals?


 
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Dr Verena Schoepf, Research Associate with the University of Western Australia’s ARC Centre of Excellence for Coral Reef Studies, explores the causes and effects of coral bleaching in one of the most stress-resistant corals of the world. The Kimberley region of north-west Australia reefs have essentially one of the harshest environments for coral reefs in the world, making the coral there special. While it isn’t the hottest reef environment in the world (the Persian Gulf is), water temperatures vary by up to 7 degrees daily and reach up to 37 degrees for short periods of time. 

 

 
This is because the Kimberley has the world’s largest tropical tides which also means that corals are often exposed for long periods to the air. High tide and low tide occur twice a day in the Kimberley. The spring tides can be 11-12 metres and the neap tides are at least 5-6 metres. The daily temperatures swings are therefore extreme, and much more so than in the Persian Gulf. Corals are very vulnerable to heat stress and so are sensitive to global climate change and ocean warming.

 
Australia is already part of the third and longest global coral bleaching event on record that started in 2014. In 2016 the Kimberley corals were affected by severe bleaching even though they are one of the most heat tolerant corals in the world. Coral bleaching is primarily caused by heat stress. Coral live in a vital symbiosis with tiny algae (zooxanthellae) that live inside the coral tissue. The heat causes the zooxanthellae to produce toxins (reactive oxygen species) that negatively affect the health of the symbiont and the coral. The symbiosis then breaks down and the coral expels most of the zooxanthellae and, since the corals get most of their brown, greenish colour from the zooxanthellae the corals then start to turn white. The zooxanthellae algae are microscopic plants while the host corals are animals (related to anemones and jellyfish) and so they are still able to get food from other sources like zooplankton when bleached. However most corals rely on their symbionts, the zooxanthellae, for most of their food and energy. 

 
When the coral is white or bleached, it is not dead. It is in a severe state of distress but it can recover. If temperatures remain high the white coral will die and its skeleton will be colonised by turf algae and it will become a grey/greenish colour. Some corals are more resistant to heat stress. The massive corals typically do better than the fast growing branching ones, and deeper corals can be less affected by bleaching than shallow corals. Even when a coral is white there are still some zooxanthellae surviving in its tissues so it is these populations of symbionts that will begin to reproduce asexually when conditions become more favourable. That is one way corals can recover. Corals can also get new zooxanthellae from the water column. 

 

 
There are different species of zooxanthellae. Some are generalists and can associate with a range of coral species. Others are very specific to either particular coral species or particular regions. To complicate matters further some zooxanthellae are more heat resistant than others. 

 
Most corals cannot increase feeding rates dramatically when bleached to make up for the loss of energy due to bleaching. At least one coral species in Hawaii, however, can get most of its food energy from feeding on zooplankton in the water when bleached.This makes them much more resistant to heat stress and Dr Schoepf is starting to look at whether Kimberley corals may have the same capacity. Most corals grow in nutrient poor waters (which is why the water is so clear). However this is not the case for the Kimberley reefs. There the turbid water means that the corals have potentially a greater opportunity for feeding from the water column. 

 

Dr Verena Schoepf was interviewed for A Question of Balance by Ruby Vincent, July 2016. Summary text by Victor Barry. Images provided by Dr Schoepf are listed in order below:

Healthy green and brown Kimberley corals increasingly exposed as the tide drains out (Verena Schoepf)
Low tide and exposed corals showing patches of white bleaching 
(Chris Cornwall)
Verena working in ultraclean laboratory environment 
(Verena Schoepf)
Widespread bleaching of coral (Morane Le Nohaic)
Kimberley bleaching of massive coral (Chris Cornwall) 


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A very fine Port Kimberley corals part 2

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