Warm water corals live in association with algal partners (zooanthellae), if these algae leave the consequences for the coral are stark – death or years of vulnerability. Globally such events, known as coral bleaching (because the corals go transparent after algae loss) are increasing in frequency and severity. It’s even been mentioned on Neighbours – why do corals lose their algae?..
The way in which corals and zooanthellae live together, sharing resources, is known as a symbiosis. The algae live inside the coral, sheltered from predators in the water, gaining phosphorus, nitrogen waste and carbon dioxide from its partner whilst giving its host the product of photosynthesis – energy. As such both gain from each other, primarily acquiring increased rates of growth and regeneration – a reason for its evolution. However this relationship is fragile, the algae live life in the fast lane whereas the coral are slow movers, growing over decades and centuries – or in scientific terms, algae have high metabolic rates (the speed of energy use) and coral have low ones. A way in which the host corals get round this is to increase its own uptake of energy and also try and decrease the photosynthetic output of its zooanthellae helpers. Nevertheless there remains, for all these symbiotic interactions an upper and lower limit within they can occur.
The brown colouration is the zooanthellae inside the coral polyp.
Coral bleaching is a sign that metabolic rates have been changed – and the triggers for this are many: increased/decreased sea surface temperatures, changes in salinity and nitrogen levels, disease, increased sun radiation and others. Most have been caused by a combination of increased water temperatures and sun radiation – during periods of extreme warmth. The last major global bleaching episode was during the El Nino of 1997-1998, a period of unrivalled sea warmth. Since then these events are becoming all too often; this year 90% of Thailand corals have bleached and an alarming 20% have died. Whilst the El Nino of 13 years ago was a freak the rise in sea surface temperatures of the last decade can, in large part, be placed on global warming (be it man-made or a cycle). Corals and the amazing abundance of life that survives nearby is under stress because of this.
To be clear, there is no coherent explanation of why the symbiosis breaks down during warmth. They have always bleached on seasonal variations – but mass bleaching events of late are undoubtedly anthropogenic (man-made) in origin. The triggers are well known but the mechanism of algal expulsion and why the algae relocate is the debate of many a coral reef scientist. One of the most coherent explanations is that it’s a mechanism in which to reduce stress – it is also, the last line of defence against extreme temperatures, usually occurring at 1-2 degrees Celsius above the ambient conditions. Before this coral and algae reduce mortality using fluorescent proteins to scatter light, proteins that absorb ultraviolet radiation, heat shock proteins, a system of oxidant removal (as photosynthesis rates increase oxidants do to, leading to cell damage) and lastly the coral start sustaining themselves through filter feeding as best they can. This happens seasonally, and doesn’t cause long term damage but periods of extreme, or extraordinary, warmth causes a major shift in metabolic rates and a communication breakdown between the partners – zooanthellae are expelled rapidly by the coral. The limits in which a symbiosis can occur has been broken – photosynthesis occurs too rapidly at high temperatures, the host coral cannot cope with the abundance of energy; loss of zooanthellae reduces stress. A large loss of zooanthellae leads to the discolouration of the host and a terminal decline as its own feeding cannot sustain it.
Ghostly white corals litter the sea.
Other scientists think that the bleaching process is actually a way in which to facilitate adaptation to climate change. Ejecting algae that survive best in less warm seas so that new algae can take their place, these ones able survive in extreme warmth. This “Adaptive Bleaching Hypothesis” has gained favour amongst scientists but relies on 5 major assumptions – the main one being that many types of zooanthellae actually exist in one area of coral.
It is important to discover the true reason for loss of algae in corals, although it certainly appears the trigger is increased sea temperatures. The reefs that warm water corals build are extremely important and we must learn more about them – we must protect them.
