Heat resistant coral

A way to spot corals resistant to the phenomenon of coral bleaching - which is threatening reefs around the world - has been published by researchers in Australia. The aim is to find corals from among the existing population that are naturally better able to tolerate the higher water temperatures that are driving the problem, and use those to understand the basis of that tolerance and possibly seed threatened reefs with those more robust varieties. But first you’ve got to find them, and that’s what the Minderoo Foundation’s Kate Quigley has been able to do by combining good old fashioned expeditions to The Great Barrier Reef with some machine learning techniques…

Kate - Reefs around the world and in particular, The Great Barrier Reef have suffered quite a lot of bleaching and mortalities due to global climate change and specifically warming due to global climate change.

Chris - And when coral bleaches in that way, what's actually happening?

Kate - It's a disruption in a relationship. Even though a coral looks like a slimy rock, it's actually an anima. Inside of that coral lives little algae, which are very similar to little plants. These algae provide most of the coral's food. They use the sun to go through some chemical processes and then they give that sugar to the coral. The problem arises when water temperatures become too hot and they stay hot for long. This disrupts that relationship, causing the coral itself to go white and if those conditions persist, the coral animal can eventually die.

Chris - But not all corals are equivalently vulnerable. Is that what you are postulating? If you're saying 'I'm going hunting for corals that are more resistant to that', then it's not a level playing field.

Kate - Some corals are intrinsically more tough and can survive increased temperatures. Other corals just seem to be less tough. We're really looking for these tough corals.

Chris - And is it the corals that are tough, or is it the algae that are tough, or is it both?

Kate - It's both. The coral animal is quite complex and some of this tolerance comes from the underlying DNA of the coral animal, and some of the toughness comes from the symbiote and together they create the response that we see on the reef.

Chris - So how did you actually go hunting then in this Italy sized hay sack?

Kate - Some of the first locations were just 'let's look for hot water'. I looked at hours upon hours upon hours of satellite data stretching over many years and had some best case scenarios in terms of where we could potentially find these heat tolerant corals. Once I had these particular areas, we went on an expedition and we collected corals that were essentially reproductively ready. We wait for a few days after the full moon, till the corals are ready to spawn. Once they spawn, we collect eggs and sperm and we mix them in specific combinations to create new combinations of coral babies. We put those coral babies through a series of different temperature experiments, where we can say, 'okay, these coral babies survive better under high heat stress, and then they survive less well.'

Chris - And what about the algae? The symbiotes that are part of the equation as well. How do they get added?

Kate - We gave them specific cocktails of symbiotes that we were hoping would further increase their heat tolerance. That means at the end of all of our experiments, we could essentially identify which genetic combinations of corals were really tough and which algae combinations were really tough.

Chris - When you do that, what sort of enhanced tolerance can you endow these corals with?

Kate - By doing this form of selective breeding, we were able to increase heat tolerance by 3 degrees Celsius.

Chris - Is that enough?

Kate - 3 degrees is a good start, but given the rate of warming, as well as the magnitude of warming that we're projected to see as soon as 2050, we're first going to need very strong action on climate change, even though these results are very promising.

Chris - Is there any way to spot places where there might be even more resistant corals and even more resistant algae that could be brought together in the way you have and get an even higher temperature tolerance?

Kate - Yes. This was the second part of our paper. We employed machine learning. You can just stick all this data into an algorithm and what it allowed for us to find was places around The Great Barrier Reef that fit the conditions that we had seen in our models. It told us that we needed to go to reefs that had high yearly temperatures. We also needed to reefs that had experienced a particular level of heat stress in previous years, so we could get a value around that. We also needed to go to places that had lots of variability, so in any one given day, the temperature fluctuation was quite high. With those 3 ingredients combined that was essentially the perfect cauldron for producing heat tolerant corals. We were able to find locations on The Great Barrier Reef that had this particular cocktail of scenarios and that allowed us to find that about 7.5% of the reef area on The Great Barrier Reef had these kinds of refugia.