The Great Barrier Reef is unquestionably a wonder of the world. The largest coral system on Earth, it consists of more than 2,900 individual reefs and stretches over 2,300km (1,400 miles). Famously visible from space, it represents one of the most biodiverse known habitats and is of huge importance to the Australian economy, both because of tourism and because it supports fisheries. Moreover, the reef is of huge cultural significance for many Pacific communities. And it is in trouble.
The biggest threat to the reef is coral bleaching. This is caused by rising sea temperatures, and as a result of the complex nature of coral. Within the tissues of the millions of living creatures that comprise coral are microscopic, plant-like organisms called zooxanthellae, which capture sunlight, convert it to energy, and provide nutrients to the coral.
However, if sea temperatures rise, the coral expels the zooxanthellae and loses its colour. This doesn’t kill the coral straight away, but bleached coral is effectively starving and, if conditions do not return to those hospitable to zooxanthellae, it will die. Researchers in Australia are now trying to find ways to help corals in the Great Barrier Reef resist higher temperatures without bleaching, which they hope will preserve this unique environment.
Although it’s a world away from the factories, medicines and synthetic materials that characterise engineering in much of the world, this effort is nonetheless engineering of an important kind.
Coral bleaching is a natural event, and research indicates that bleaching has occurred many times during the reef’s existence. But extreme heat waves in 2016 and 2017 affected up to two thirds of the reef, and current extreme temperatures are likely to have similar consequences. However, other reefs can withstand conditions in warmer waters: the Red Sea is consistently warmer than the seas around the Great Barrier Reef, for example.
Dutch researcher Madeleine van Oppen of the Australian Institute of Marine Sciences in Townsville, Queensland, visited London last year to talk about her work in engineering hardier coral. Van Oppen’s work focuses on two techniques: assisted gene flow and assisted evolution. The first of these works by moving warmth-adapted corals to cooler parts of the reef; the northern extreme of the reef is routinely 1°C to 2°C warmer than the southern portion in summer. Corals are mobile in their larval form but, under normal conditions, larvae from the north do not travel south because the main ocean current that flows across the Pacific splits off the coast of northern Queensland, and flows are not favourable to north-south transfer. The researchers are experimenting with manually moving some of the northern corals south. If enough corals were moved, it could help heat-damaged reefs recover faster.
Assisted evolution is a somewhat more complex technique, which van Oppen described in a paper published in the Proceedings of the National Academy of Sciences in 2015 and in Nature Ecology and Evolution in 2017. “It’s artificial selection on steroids,” she said. Targeting both the coral host and its symbiotic zooxanthellae, it takes several different tacks to improving their resistance to stress, in this case from heat.
One way to do this is by a technique called stress conditioning. This involves exposing coral to heat levels that approach those that will cause bleaching, and to investigate, firstly, whether the coral can adapt to this and, secondly, whether it can pass those adaptations to further generations. Evidence for this exists in some plants and animals, but it is not yet known whether coral can be stress-conditioned. Van Oppen and her colleagues are looking at this technique in the National Sea Simulator (SeaSim), a marine research aquarium in Townsville, which can store more than 3.5 million litres of seawater and can carry out spawning experiments on many reef organisms simultaneously and over several generations.
Another approach is more typical to genetic engineering, involving creating hybrids by bringing together compatible eggs and sperm from different coral species. This is known to occur naturally in some types of coral, increasing genetic diversity and producing novel genetic combinations that may be useful in selective breeding. “It’s quite rare in nature, but not difficult to do under laboratory conditions,” van Oppen said. Working at SeaSim, the researchers are looking to hybridise multiple pairs of coral species during their annual spawning (a major and predictable event) and grow their young under controlled conditions to select for climate resilience, then crossbreeding strains to produce desired results exactly the same way that conventional husbandry has worked for many centuries in agriculture. Hardy specimens could then be transferred to the reef itself.
Yet another approach is one that is sometimes used in humans to give health benefits: probiotics. These are live organisms, generally bacteria, which can confer beneficial effects if they establish colonies inside their hosts. Coral contains several potential habitats where probiotic colonies could be established, including the layer of mucus that coats its surface, digestive systems and even its mineral skeletons. Van Oppen and colleagues, including Katarina Damjanovic, are trying to develop probiotics that could either help coral tolerate the heat better, or help it recover faster from bleaching events by creating a more hospitable environment for the essential zooxanthellae. “One thing that probiotics could do is mop up oxygen radicals that occur in water and are damaging to the living coral tissue,” van Oppen said. “One big advantage of this approach is that we could administer the probiotic anywhere on the reef.”
Van Oppen is open about the potential for these techniques. “Our big hope is that it can buy us enough time to tackle the warming without the reef dying in the meantime. She also admits that, even if the technique was successful, it would change the Great Barrier Reef significantly. “At the moment we have a very diverse reef, with many different coral species,” she said. “We are not going to be able to create successful strategies for all of those species, so even if successful, we will have a much less diverse reef. We simply don’t know what the consequences of that for other life might be.
“If the reef rejects the results of our experiments, the effort will be wasted.” However, she adds: “The risk of doing nothing is just far too great.”