Coral reefs under threatFor those of you who watched David Attenborough’s Blue Planet 2 series on TV at the end of last year, the demise of our coral reefs will not be new news. It was a timely reminder that yet another of the world’s ecosystems is under threat.Coral reefs make up less than one per cent of the Earth’s undersea ecosystems, but we should not underestimate their importance. Often referred to as the ‘rainforests of the sea’, coral reefs are vital to ocean health and balance – providing a unique and diverse ecosystem that provides food and shelter for an estimated 25 per cent of all fish, crustaceans, and other sea life. They also provide food, medicine and shoreline protection for the human population, and their contribution to tourism is a highly important economic factor.These reefs are, however, now under increasing threat according to new research published in the journal Science showing an escalating rate of bleaching events over the last four decades. To illustrate this, the leading author of the article, Professor Terry Hughes, explained that the Great Barrier Reef has now been bleached four times since 1998 – including, for the first time, back-to-back events in 2016 and 2017, causing unprecedented damage. Statistics show that the world has lost roughly half its coral reefs in the last
30 years. What is going wrong?Reef-building corals – most of which grow at one inch or less each year – are found in the shallower and warmer waters of the subtropical and tropical oceans. Corals are dependant for their survival upon a symbiotic relationship with single-celled flagellate protozoans, 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. Poor environmental conditions will limit the coral’s ability to provide for the zooxanthellae’s needs; so in such situations, in order to ensure its short-term survival, the coral-polyp will expel the zooxanthellae. This results in the coral having a lighter or completely white appearance – hence the term ‘bleached’. In addition, because the zooxanthellae provide so much of the coral’s energy needs through photosynthesis, their removal results in the coral beginning
to starve.Although there are other factors that can affect the survival of the coral, by far the most significant damaging element is elevated sea water temperature caused by greenhouse gas emissions. Compounding this is ocean acidification as the sea water captures increasing levels of carbon dioxide from the atmosphere. Although these two factors have different impacts on the coral – the former reducing food availability and the latter inhibiting the building of the coral skeleton – both are contributing to the coral’s destruction.Coral reefs can survive short-term environmental disturbances; but if negative conditions persist, the reef’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. If the coral polyps die of starvation after bleaching, this cannot happen and they will decay causing the reef structure
to collapse.Some hope Not every reef that experiences a stressful event is destroyed – some, called ‘resilient reefs’, are capable of bouncing back. Research is also being carried out on aiding reef regeneration and researchers have now successfully accelerated the formation of new coral colonies on small areas in the Great Barrier Reef by using ‘baby corals’ conceived and successfully settled directly on the Reef through a pioneering pilot project funded by the Great Barrier Reef Foundation. The technique, called ‘larval reseeding’, involves capturing coral eggs and sperm during spawning, rearing the resulting coral larvae, and then delivering the healthy ‘baby’ corals on to small areas
of reefs.Although reef regeneration in this way marks great progress, it does not address the basic problem of global warming and this is something for which everyone has a responsibility. It’s easy to rely on research and researchers to solve environmental problems, but unless the general public acts to curb carbon emissions and pollution, the fate of the coral reefs – and much much more – is in the balance.ReferencesHughes et al (2018). Spacial temporal patterns of mass bleaching of corals in the Anthropocene. Science 359(6371): 80-83.