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With the temperature of the world’s oceans rising, concern is growing about the ability of coral reefs to adapt to climate change. We spoke to Dr James Guest and Dr Adriana Humanes about the work of the CoralAssist project in investigating whether selective breeding and the assisted gene flow (AGF) technique can help some coral reefs survive in the face of climate change.

The world’s coral reefs have been degraded by human activities, such as overfishing, over a long period, but this has accelerated over recent decades and rates of coral mortality have increased significantly over the last three decades. The largestscale coral mortalities that have occurred in recent years have been caused by rising sea surface temperatures, as Doctor James Guest explains. “If the sea temperature rises above what corals normally experience, and that temperature remains high for even just a week or two, then the corals will start to appear very pale. That’s because they’ve lost the symbiotic algae that occur inside their tissues. Those algae are part of a symbiotic partnership with the coral animal host, which is critical to the coral’s health,” he outlines. Some corals are better able to withstand heat stress than others, a topic of great interest to Dr Guest. “We have seen that if we put corals from a genetically diverse population under heat stress, we find some individuals which are more resistant than others,” he says. “What interests me is whether the attributes that make some corals more resistant can be passed on to their offspring.”

CoralAssist project This is a central part of the research agenda in the CoralAssist project, an ERC-funded initiative which brings together scientists from several different disciplines to investigate whether the Assisted Gene Flow (AGF) technique could be used in coral reef conservation. AGF involves the deliberate movement of individuals within or between habitats for the express purpose of increasing the frequency of better adapted individuals to a desired condition in a population. A key step here is to identify the corals that are more resistant to heat stress. “We have tagged around 400 coral colonies on the reef, which we’ve been following for two years,” explains Dr Adriana Humanes, a post-doctoral researcher in the project. The aim is to identify which coral colonies have the highest tolerance to heat stress. “We take small fragments from these corals and bring them to the laboratory. Then we put them in tanks where we increase the temperature over a period similar to what they have experienced on reefs where there have been bleaching events,” combining a range of different techniques, including physiology and proteomic analysis of the different corals, Dr Humanes and her colleagues aim to build a deeper understanding of the factors that affect heat-tolerance. “We’re looking at different characteristics of these coral colonies, to try and understand what gives them this higher heat-tolerance,” she explains. While a lot of research in this area has previously centered on the genes that these corals express, the project is looking at the proteins they produce rather than the genes. “These proteins actually control the traits that are expressed,” says

We have seen that if we put corals from a genetically diverse population under heat stress, we find some individuals which are much more resistant than others. What interests me is whether the attributes that make some corals more resistant than others can be passed on to their offspring.

continues Dr Humanes. “So, we perform heat stress in a similar way to that seen in the natural environment, then record which fragments bleach and die and which ones survive.”

Once it has been established which corals have a higher tolerance to heat stress, they can be bred together to find out if these traits are passed on to their offspring. Baby corals from these selected crosses are reared until they are adults to see if this heat tolerance persists through time - it may even be possible to see if it passes on to second or third generations. The underlying mechanisms behind these differing levels of heat-tolerance among corals are not fully understood, but this is a major topic of interest for CoralAssist. By

Dr Guest. “We’re working with proteomics specialists and protein chemists who have a lot of expertise in using mass spectrometers. That’s quite a novel part of our research.”

This is a complex area, and there are many considerations to take into account when investigating coral health responses to heat stress. Alongside the coral animal itself, algae on the coral, the microbiome, and various other factors may affect heat-tolerance. “There are many areas of interest at the same time. Previous studies have suggested that in some cases, characteristics of the host coral are involved in heat-tolerance, but in other cases components of the microbiome, or the type of symbiotic algae inside the tissue may be more

Two-year old selectively bred corals being reared within an ocean nursery in Palau. Photograph by Till Roethig.

A typical healthy, vibrant coral reef in Palau. Photograph by James Guest.

important,” outlines Dr Humanes. However, while heat-tolerance may seem like an entirely positive trait in terms of ensuring the survival of corals, there may also be costs in other areas. “If the coral is allocating more energy to heat-tolerance, it could be that other traits are compromised, such as its capacity for growth,” acknowledges Dr Humanes. “There could be trade-offs, and we still don’t know how they work and how they are related to each other. That’s something we are investigating – we aim to build more knowledge of heat-tolerance and the implications.”

Climate change The project’s work is part of the wider agenda of protecting coral reefs in the face of ongoing concern about the impact of climate change. Researchers also aim to develop a cost-effective methodology to restore coral reefs after they have been damaged, using sexual reproduction. This has advantages over commonly used asexual techniques as it increases the diversity of the gene pool of restored reefs. “Genetically diverse populations have a much better chance of surviving disturbances,” points out Dr Guest. By using selective breeding scientists have the added advantage of being able to decide which corals to breed together. Corals with high heat-tolerance reproduce normally in nature; however, Dr Guest says that it’s possible to greatly increase the fertilisation and survival rates through laboratory propagation. “It may be that the eggs and sperm of resistant corals will meet in nature and they will reproduce, but many of those will die naturally before they become adults, so there may be only one or two individuals that actually continue to the next generation,” he explains. “By bringing them into the laboratory, we can ensure that almost all of the eggs are fertilised and survive. We’re essentially increasing the proportion of what we think will be more resistant individuals.”

A lot of attention is currently focused on the possibility of outplanting these types of corals onto reefs as a way of enhancing resilience. However, Dr Guest says that there are also other factors to consider beyond the heat-tolerance of the coral itself. “It’s not enough to produce a coral that has a higher heat-tolerance than if we just allowed those corals to reproduce randomly. We’ve got to be able to put that coral out onto the reef and to show that it has, on average, equal or better survival prospects than corals on the reef, which recruit naturally,” he stresses. It’s also important to develop a method of cost-effectively outplanting corals in large numbers, without having adverse consequences. “If there are trade-offs, if an outplanted coral grows more slowly for example, then potentially it may cause more problems than it solves,” points out Dr Guest. “It’s a complex area. The problems are very significant however, so we really need to do the research now to understand what will work well.”

This holds clear importance in the context of concern about the impact of climate change on coral reefs and their ability to provide ecosystem services like coastal protection, food security and job opportunities. Coral reefs are home to a quarter of all marine species and support more than 500 million people worldwide, so Dr Guest says it’s important to consider their long-term future. “It may well be worth investing in efforts to get reefs to recover quickly from heat stress and boost their resistance to future climate change,” he says. The project will make an important contribution in this respect by building a science-evidence base to inform future decision-making. “By understanding the trade-offs and benefits, we can advise the authorities on what techniques would be the most appropriate in terms of giving us the best chance of having reefs that will survive for the next few decades,” continues Dr Guest.

CoralAssist Assisting Coral Reef Survival in the Face of Climate Change Project Objectives Coralassist spans multiple disciplines and aims to answer four broad questions: 1) do trade-offs exist between heat tolerance and other fitness traits? 2) Which physiological and proteomic traits correlate with heat tolerance? 3) Is heat tolerance heritable? 4) Can AGF and selective breeding lead to shifts in heat tolerance in coral populations? Project Funding Coralassist is funded by a 5-year European Research Council Consolidator Grant awarded to James Guest. Project Partners Coralassist works closely with the Palau International Coral Reef Centre, The University of Derby, The Horniman Museum and Gardens, the Australian Institute of Marine Sciences, The University of Sydney and Secore International. Contact Details Principle Investigator, Dr James R. Guest, PhD, ERC Research Fellow School of Natural and Environmental Sciences 4th floor, Ridley Building 2 Newcastle University Newcastle upon Tyne NE1 7RU United Kingdom T: +44 (0)191 208 3619 E: jrguest@gmail.com W: https://www.coralassistlab.org/

Dr James Guest

Dr Adriana Humanes

Dr James Guest (Principle Investigator) is a coral reef ecologist based at Newcastle University in the United Kingdom examining the feasibility of assisting coral reef adaptation via assisted gene flow and selective breeding. His current research goals are to assess the role that these interventions can play in maintaining ecosystem services in the face of climate change. Dr Adriana Humanes (Postdoctoral Research Associate) is a marine ecologist interested in the impacts of climate change on coral reef ecosystems, with special emphasis on corals and their reproductive biology.