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White as a Ghost: Coral Bleaching

Coral reefs are among the most biologically diverse and economically important ecosystems on the planet. Despite their majesty they are severely under threat. Atmospheric CO2 levels have risen from 280ppm in preindustrial times to current levels of 400ppm. CO2 is one of the primary greenhouse gasses causing global climate change. This is causing an increase to the surface seawater temperatures as well as an acidification epidemic. These factors along with other anthropogenic issues pose a great threat to the future of coral reefs and the ecosystem services which they provide.

Reef-building corals are currently living close to their thermal maxima. When surpassed corals experience bleaching. Increased temperatures cause the photosynthetic symbionts of corals (zooxanthellae) to become super-sensitive to light. This results in the overproduction of oxygen radicals that damage the symbionts and/or their host. This leads to the expulsion of zooxanthellae from the host. Corals depend on these symbionts to supply energy needed for calcification. Bleaching weakens the coral and often leads to mortality. The frequency and scale of coral bleaching events during the past few decades have been unprecedented.

Coral reef systems depend on these symbiotic relationships to restrict the outward flow of life-supporting nutrients to the water column. Corals and their zooxanthellae live by limiting the flow of nitrogen and other essential nutrients to the nutrient desert represented by the tropical seas.

Zooxanthellae photosynthesize while residing inside their hosts and provide food for their invertebrate hosts by passing up to 95% of their photosynthetic productions to them. They provide the host with a supply of energy and essential compounds (sugars and amino acids). In return the coral plant provides zooxanthellae crucial plant nutrient’s (ammonia and phosphate) from their waste metabolism, along with a micro-habitat to call home.

Furthermore, Zooxanthellae also are the ones that provide the coral with its vibrant colours, giving the reef systems the majestic mosaic of colours that we know and love. Rising sea temperatures are making the Zooxanthellae become stressed when exposed to slight increases (1-2 degree) in water temperature. The resulting damage leads to the expulsion of these important organisms from their coral hosts. This leaves the coral a bleach white colour, hence the naming of the crisis coral bleaching.

Coral bleaching may occur at local scales or at geographic scales that may involve entire reef systems and geographic realms. Our planets biggest reef system, the Great Barrier Reef in Australia has been hit by four extreme bleaching events in 1998, 2002 but more worryingly two consecutive events in 2016 and 2017. The reef systems of our planet can be seen as a canary bird for climate change, when we lose the corals then we know we are in trouble.

The goals enshrined in the Paris climate agreement, which aims to hold global warming well below 2c and as close as possible to 1.5c, will not be sufficient to restore the Great Barrier Reef to its former glory. Even with strong action to tackle climate change, more warming is already locked in. Existing conservation approaches, such as improving water quality around reefs and imposing restrictions or bans on fishing, are not adequate enough reports suggest. This has led to a radical new plan to try and protect our coral reef systems.

Naturally some coral species can be found under conditions that far exceed the thermal tolerances of the same coral species at other locations. The critical worry is whether or not all coral species will be able to respond to warmer conditions quickly enough to keep up with increases caused by anthropogenic climate change. Unfortunately the rate of warming has never been seen on our planet and it is a doubt whether species can keep up evolutionary wise.

This had led to scientists trying to assist in the adaptation of coral species, to give them a kick start in being able to cope with these extreme changes. There are two possible interventions that show promise: assisted gene flow and assisted evolution. Both techniques use existing genetic material on the reef to breed hardier corals, and do not involve genetic engineering.

Assisted gene flow is the idea to move warm-adapted corals to cooler parts of the reef. Corals from the north of the Barrier Reef are naturally more adapted to 1c and 2c higher summer temperatures compared to corals further south. This means there is an opportunity to build resistance to future warming in corals in the south. Manually moving some of the northern corals south could help overcome physical limitation of natural north-to-south larval flow. If enough corals could be moved it could help heat-damaged reefs recover faster with a more heat-resistant coral gene stock.

While assisted gene flow may be effective for southern or recently degraded reefs, it will not be enough or feasible for all reefs or species. This is were assisted evolution could step in. Assisted evolution is essentially artificial selection ramped up. It combines multiple approaches that target the coral host and its essential microbial symbionts. Evolution of symbionts in isolation from coral hosts has been fast-tracked to resist higher levels of heat stress. When symbionts are made to reengage with the coral host, benefits to bleaching resistance are still small, but with more work it is expected that we will start to see hardier symbiosis.

Following on, experiments have created new genetic diversity of corals through hybridisation and researchers have selected these artificially for increased climate resilience. Natural hybridisation happens only occasionally on the reef, so the results of these new experiments give new options for climate hardening corals using existing genetic stocks.

Assisted gene flow and assisted evolution represent manageable risk because they use genetic material already present on the reef. The interventions speed up naturally occurring processes and do not involve genetic engineering. Seems as though humans have created the rapid change in climate it is only right that we try our best to rapidly help species survive it.

We are seeing much more severe bleaching events much more frequently. Humans have created the problem of climate change and the species of our planet are struggling to cope with such rapid changes. The coral reef systems around the world not only support the majority of our fish stocks but also hold great economic power through the amount of tourism they bring to less economically developed countries. It is great to see that steps are being made to help species deal with the problems we have created, however this is just a cure. We must try our hardest to halt the effects. By cutting down our personal greenhouse gas emissions (whether this is using electric cars or becoming vegan etc) we can slowly but surely slow down and perhaps stop climate change all together, giving the species who we share our planet the chance to thrive once more.

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