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Assisting Coral Reef Survival in the Face of Climate Change

Periodic Reporting for period 2 - CORALASSIST (Assisting Coral Reef Survival in the Face of Climate Change)

Reporting period: 2019-02-01 to 2020-07-31

Coral reefs are critically important for the services they provide to human society. Reef building corals produce a skeleton of calcium carbonate, creating vast, complex geological structures which can be seen from space. Reefs cover only a small area of the world’s oceans, but they are one of the most diverse and productive marine ecosystems. They provide critical services such as food from fishing and tourism revenue from activities such as scuba diving. Critically, they protect shorelines and coastal communities by acting as a natural defence against storms and waves. Corals are very sensitive to even small increases in seawater temperature and can “bleach” and die when temperatures rise just one degree Celsius above normal summer levels. Major coral bleaching events have been happening more frequently and with more intensity since the 1980sand this has led to recent catastrophic levels of coral mortality. Like trees in a forest, corals create much of the complex habitat structure needed to support the remarkable levels of biodiversity and productivity found on reefs. When corals are lost due to bleaching they can become overgrown by fleshy seaweeds that prevent recovery. Once corals are lost, many fish and other organisms that associate with live coral are also lost. Eventually, the coral skeleton that provides the physical structure on reefs will erode. This leads to less habitat for fish and loss of their ability to protect shorelines. Corals have the capacity to adapt to change, but it is not known if they will be able to continue to adapt to the current unprecedented rapid pace of warming. This fact has led to a complete re-evaluation of current management practices of coral reefs, because even carefully managed and protected areas have succumbed to bleaching. If we want future generations to experience coral reefs, we may need to develop and adopt innovative techniques for conservation. Various techniques have been proposed, including assisted gene flow (AGF), involving the movement of better adapted individuals within and between populations, and selective breeding for heat tolerance. If successful, these would involve seeding reefs with more heat tolerant corals. So far, these approaches have only been tested at small scales and mostly in the laboratory. We already know that there is a lot of variation in heat tolerance from one coral to another, even within a single species on the same reef, but we don’t know if this variation can be harnessed for conservation of reefs. Our research aims to find out if these methods will work in the real world and what risks are involved. For example, we may find that if we selectively breed more heat tolerant corals, they actually grow more slowly or have fewer offspring. We also do not know whether corals pass on heat tolerance to their offspring and whether this persists throughout their lives. Finally, we need to develop cost-effective ways of delivering selectively bred corals to reefs at meaningful scales. It is critical that we do this research now to establish whether these techniques can help some coral reefs survive into the future. CORALASSIST addresses the considerable remaining knowledge gaps that need to be filled if we are to be able to practically implement selective breeding and AGF and control for the inherent risks involved in using these approaches.

CORALASSIST is an ambitious, ground breaking, research project that aims to provide to a) investigate the feasibility of implementing AGF and selective breeding in coral reef ecosystems, b) increase our understanding of coral’s natural ability to adapt to global warming, and c) provide guidance for coastal managers about new and innovative ways of helping corals adapt to global change. The four main questions that CORALASSIST aims to answer are as follows:

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 increased numbers of heat tolerance corals in populations?
CORALASSIST has carried out four expeditions to Palau in Micronesia since October 2017. Our first aim was to carry out tests of heat-tolerance on a large number of corals from three species (Acropora digitifera, Goniastrea retiformis and Echinopora lamellosa). These experiments involve tests on small fragments of coral (“nubbins”) in flow-through sea-water aquarium tanks where we increase temperatures gradually over time to mimic a natural coral bleaching event. We assess the average health status (i.e. whether coral nubbins bleach and die) to compare performance of each coral colony over time. To test whether the health status data are reliable (they are based on visual scoring of stressed corals), we are using machine learning approaches to assess colour change of nubbins from photographic images. The experiments so far have shown us that even within a single population of corals, there is a great deal of variation in the way individuals respond to temperature stress and we have call this relative heat tolerance (RHT). By doing repeated temperature stress experiments tests on fragments from several adult parent colonies we have also shown consistency in the classification of the RHT categories from colonies tested in consecutive years.

The temperature stress tests have allowed us to select 14 parents for breeding from two of our study species (Acropora digitifera and Goniastrea retiformis) with relatively high and low heat tolerance. We were successful in spawning and selectively breeding these corals in 2018 and 2019 and to produce multiple unique crosses from parents with varying RHT. We were also able to identify relatively high and low tolerant colonies of a third species (Echinopora lamellosa) but were not able to collect sufficient amounts of gametes to be able to carry out selective crosses for this species. To date we have produced 1 and 2- year old individual F1 corals (close to 1600 individuals) and over 600 of these have been outplanted to the reef to assess their survivorship and growth rates. We have found that survivorship varies considerably, with our data showing that rearing corals for longer periods in nurseries leads to much higher survivorship rates. To overcome these observed early mortality bottlenecks, we are testing an innovative technique for transplanting juvenile corals that will reduce nursery times and increase early survivorship. This involves a ceramic substrate that provides protection for newly settled corals from fish grazing and can be attached quickly and easily to the reef for seeding reefs with sexually propagated corals. The remainder of our approximately 1000 F1 corals from both species are in an in situ ocean nursery. Many of these will be used for further experiments on heritability of heat stress tolerance during our next field season. To test for heritability of heat stress tolerance, we have conducted heat tolerance assays on embryos, larvae and juvenile F1 corals from all crosses of A. digitifera and in each case showed evidence of heritability, with crosses from parents with high relative heat tolerance having higher tolerance than those from parents with low relative heat tolerance.

To assess whether more heat tolerant corals tend to produce fewer eggs or have slower growth we collected samples to assess reproductive outputs and have carried out 3D imaging of all individual tagged coral colonies at two or three time points. High-resolution 3D models (a non-invasive method) allow us to measure coral growth over time with millimetre precision as well as capturing external structural change of complex shapes. These images and samples are currently being analysed to gain insights into potential trade-offs. To understand the role of the coral associated endosymbiotic dinoflagellate and bacterial microbiome communities on the corals’ heat tolerance we are sequencing ~250 coral samples using next generation techniques. This makes it possible to track even small-scale changes in the algal and microbial communities, for example between generations or different genotypes that may be related to heat tolerance.

A major goal of CORALASSIST is to understand the role that different proteins play in protecting certain corals from heat stress. To do this, we first needed to optimise the sample preservation and processing protocols which will allow us to work with a range of samples from different life stages (including sperm, eggs, larvae, young juveniles, adult animals and secretome) in corals and anemones (often used as model organisms). We are using Liquid Chromatography-Mass Spectrometry, a cutting-edge technique to assess the complete range of proteins produced by corals showing different levels of heat tolerance, to explore the underlying mechanisms. We have already established a robust library of a comprehensive set of proteins produced by one of our species (A. digitifera), a vital first step in validating our experimental approach. So far these experiments include assessing the overall protein expression profiles for different coral life stages; comparing the baseline protein abundance differences between coral colonies (and their offspring) with different heat tolerance levels and; exploring usefulness of coral secretome for the assessment of the coral condition (i.e. health status) and potentially in identification of more tolerant colonies. This latter approach is being tested as a non-destructive approach to measuring protein expression in stressed corals.
CORALASSIST has examined in depth the level of intra-population variation in coral heat tolerance over a realistic time period in an experimental set-up for three species with contrasting life histories. We have carried out one of the first studies to selectively breed corals with known differences in heat tolerance and one the first to test for heritability in their offspring at multiple life stages. CORALASIST has also carried one of the first studies to assess the success of coral selective breeding in a field setting by outplanting and monitoring selectively bred corals to their home reef. Our longer term goals are to examine the underlying causes of variation in heat toleranace using a multi-disciplinary approach (i.e. using physiological, immunological and proteomic approaches). Furthermore, we aim to spawn and breed an F2 generation from our current F1 generation in 2021 with the aim of studying trans-generational heritability and conducting QTL mapping of genes associated with heat tolerance. Finally, we hope to assess narrow sense heritability by comparing the phenotype of offspring with the mid-phenotype of parents by carrying out heat tolerance tests of adult F1 corals in 2021.