Are corals adapting to keep up with changes in ocean temperature?

Potential and limits for rapid genetic adaptation to warming in a Great Barrier Reef coral

Mikhail V. Matz, Eric A. Treml, Galina V. Aglyamova, Line K. Bay
Summarized by Time Scavenger collaborator, Maggie Limbeck

What data were used?

Researchers looked at genetic data for Acropora millepora (coral common in the Great Barrier Reef) to model (simulate) how corals will adapt to increasing temperatures, establish a direction of coral migration, and measure genetic diversity. These data were then used to predict the future survival of A. millepora in the Great Barrier Reef.

Methods

The corals used in this study were previously described in van Oppen et al. (2011) and several samples were collected from Orpheus and Keppel Islands. The coral samples were then genotyped (the genetic material was sequenced so that researchers could examine it) and that data was used to model all of the other experiments that were conducted. The coral genomes were used to look at divergence between populations (how genetically different are the populations that were sampled) and what are the demographics among populations. A biophysical model was used to examine the migration patterns between known coral habitats and the broader region surrounding the Great Barrier Reef. This model required data describing the seascape environment as well as coral-specific data relating to adult density (how many adults), reproductive output and larval spawning time, as well as how far do the larvae travel or disperse.

Results

Figure 1. A. A map of the coast of Australia and the locations along the Great Barrier Reef that coral samples were taken from. A temperature gradient is also plotted on the map, with warmer colors indicating warmer temperatures and cooler colors indicating cooler temperatures. B. A plot of the different water conditions that were measured for each study site and where each study site plots in relation to those water conditions. C. A plot of how similar each coral population was to one another. The separation of the purple dots indicates that it is more genetically separated from the other coral populations that were sampled. D. This plot further shows that the population at Keppel is more genetically distinct from the other groups as the proportion of blue to yellow is drastically increased.

The results of this study indicate that the populations examined are demographically different from one another and that overall migration of these corals is moving in a southward direction (higher latitudes). The migration southwards is still largely driven by ocean currents, rather than preferential survival of warm-adapted corals migrating to cooler locations. It was also determined through the model that those corals that were pre-adapted to a warmer climate, were able to survive gradual warming for 20-50 generations which equates to 100-250 years. However, as the temperature increased, the overall fitness (the ability of a species to reproduce and survive) of these populations began to fluctuate with random thermal anomalies (e.g. El Nino Oscillations) and these fluctuations in fitness continue to increase as warming progressed, independent of the severity of the thermal anomalies. The good news in all of this is that much of the variation in the trait associated with the ability to adapt to warmer temperatures is due to the type of algal symbionts (algae that helps the coral to survive and reproduce) in the area. This means that coral larvae have very plastic (easily changed) phenotypes (genes that are visibly expressed) and can easily adapt to whatever algal symbionts are locally available.

Why is this study important?

This study is important because it has been projected that the global temperature is going to rise 0.1°C per decade for a total of 1°C in the next 100 years and as scientists we want to know how that global temperature change is going to affect organisms. Corals function as a “canary in the coal mine” because they and their algal symbionts are incredibly sensitive to temperature and light changes in the ocean. If we know how corals are going to respond to these changes in temperature, researchers and conservationists will have a better understanding of how to better protect the coral’s environment. This study has shown that corals are able to adapt to the changes in temperature and are migrating southward, but also demonstrated that the ability of mature corals to reproduce in rising temperatures is declining. To combat this, because of this study, conservationists know and may be able to release larval and juvenile corals that have been raised in labs into new environments to perpetuate the species.

The big picture

The big picture here is that climate change is very real and we can use evolution and models of evolution to understand how organisms are going to and are reacting to increasing temperatures. This research indicates that even with low levels of mutation, corals are able to adapt to warming oceans and can associate with different, local algal symbionts as they migrate. However, mature adult corals have increasingly less fitness as ocean temperatures rise which means that they are reproducing less, leading to overall decreased coral populations. There is hope for this particular coral though, if researchers and conservationists can find a way to successfully raise coral larvae and release them into their current and future habitats.

Citation:
Matz, M. V., E. A. Treml, G.V. Aglyamova, L. K. Bay, 2018. Potential and limits for rapid genetic adaptation to warming in a Great Barrier Reef coral. PLOS Genetics, 14:4:1-19, doi: 10.1371/journal.pgen.1007220

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