Everyone knows that ocean warming is a huge concern when it comes to the ability of corals to survive in our reality of climate change. Corals thrive within a very narrow temperature range, and pushing the upper limit of this range by just 1° C above average can cause widespread coral bleaching and death (to understand the fascinating basics of coral biology, head over to our Coral Biology online course).
The number of coral restoration projects has exploded over the past few years, and rightly so; we need a lot of active effort to keep up with the rate of coral loss from bleaching and disease. But if the conditions that cause the decline remain or get worse, is restoration attainable? Corals do show potential to adapt and evolve to changing environmental conditions. However, atmospheric CO2 levels have rapidly increased over the past 3 centuries since the onset of the industrial revolution, with an increase of more than 80% expected by the middle of this century. Corals, and the oceans in general, have not experienced this kind of rapid environmental change in such a short period of time in any of the Earth's history and it has become clear that the rate of environmental change is exceeding the rate at which corals can naturally evolve.
"We anticipate that conventional management approaches will be insufficient to protect coral reefs, even if global warming is limited to 1.5°C. Emerging technologies are needed to stem the decline of these natural assets."
This unfortunate fact is the driver behind the emergent field of development and application of novel intervention strategies. For several years now, the coral science and restoration community have discussed the idea of using "super corals" in restoration efforts that are adapted to higher extreme temperatures. These super corals could be naturally adapted, living in more extreme environments such as hot, shallow lagoon areas or countries that have higher average sea temperatures than the site of restoration effort. The creation of heat tolerant super corals through assisted evolution is also being explored, through methods such as selective breeding, preconditioning, or manipulation of the coral microbiome.
"Assisted evolution is the acceleration of natural evolutionary processes to enhance certain traits"
As humans, we have the ability to identify problems, experiment, and come up with solutions to those problems. However, biology and ecology are so complex that it can often be impossible to ascertain every single outcome of an action. The desire to take action to prevent widespread extinction of coral species and to help them adapt to current and future environmental conditions is strong. However, the science of assisted evolution in corals is still very new, and it's important to balance this desire with treading carefully in to the unknown. This need for caution is beautifully examined in a new research paper published in Nature by Scucchia et al. titled The role and risks of selective adaptation in extreme coral habitats. The authors investigated the intraspecific differences in colonies of Porites lutea living in reef habitat and mangrove lagoon habitat. The mangrove lagoon water is characterised by warmer temperatures, lower pH (i.e. more acidic) and lower oxygen conditions than the nearby reefs. In the world of a coral, this translates to a more stressful environment. So, if the corals in this stressful environment are thriving with the increased temperatures and increased acidification that the reef site is predicted to experience in coming years, could these more tolerant corals not be used in restoration efforts on the reefs, thus making the reef more resilient? Well, it turns out it's not that simple... biology is complex, remember? Through the use of molecular and morphological techniques, Scucchia et al. showed differences in gene expression, energetics and morphometrics between the reef and lagoon corals. The lagoon corals were able to survive in their extreme habitat, but this came with trade-offs. One of the key differences was a down-regulation of genes coding for skeletal organic matrix proteins in the lagoon corals, leading to a higher porosity and lower density in their calcium carbonate skeletons. Extreme environments generate stress responses which are energetically demanding, and in this case come at the cost of reduced calcification. Further findings include an overall reduction in genetic heterogeneity as well as individual gene expression in the lagoon P. lutea compared to the reef P. lutea. As the authors state, harsh environments elicit strong selective pressures that decrease intra-population genetic diversity. So what would happen if these super heat resistant corals were transplanted to a reef site in an attempt to resist future environmental conditions? They may survive the heat and the increased acidity, but the reduction in calcification may lead to more colony breakage in the higher energetic (wave action) reef environment. The reduced genetic diversity and gene expression in these highly adapted corals could also leave them at risk to other stressors such as disease, which are also predicted to increase.
"These findings demonstrate the need for caution when utilizing stress-tolerant corals in human interventions, as current survival in extremes may compromise future competitive fitness."
The paper by Scucchia et al. is a really interesting, easy read that I encourage all to take a look at. Included in the paper are electron microscopy images that clearly show the aforementioned differences in skeletal morphology and are quite fascinating to look at. Their results are a solid reminder that there is much we still have to learn about the complex biology of corals and the coral holobiont. Not only must we move cautiously ahead with the application of novel intervention strategies such as assisted evolution, but we must also carefully consider the consequences and trade-offs of using naturally highly adapted corals in direct transplantation projects.