23 September 2020
Epigenetic adaptation of corals: A new hope?
Published online 15 October 2015
The study of epigenetic mechanisms in regional corals is essential to securing the future of reefs everywhere.
While most people yearn for summer, it is a fraught season for corals of the Red Sea with water temperatures routinely reaching a scorching 34°C.
Despite these searing temperatures, the Red Sea teems with life and is home to some of the most pristine coral reefs on the planet. Their survival in these conditions is a testament to the resilience and adaptability of reef-building corals.
Their success is based on a unique relationship with an intracellular dinoflagellate symbiont, known as Symbiodinium sp. In return for the protection provided by the coral, Symbiodinium provides the host with nutrients generated through photosynthesis.
The photosynthetic pigments in the symbionts also contribute to the vivid hues of corals. This symbiotic relationship makes the majestic structures of corals possible, despite the oligotrophic nature of the sea.
These structures form the foundation of the reef ecosystem – one of the most productive biomes on the planet – and they are often likened to being the “rainforests of the oceans” due to the sheer amount of biodiversity supported by the reefs.
Humans also benefit greatly from a healthy reef system. Coral reefs are an important source of revenue in the form of ecotourism and fisheries; but also provide less tangible services such as slowing the erosion of shorelines.
Ironically, the same symbiotic relationship that underlies the success of reef-building corals also makes them vulnerable. This delicate balance is prone to breaking down.
These corals are notoriously sensitive to many environmental stressors such as temperature, high UV intensities and pollution. When coral reefs are stressed, the symbionts are progressively expelled from the host tissue for reasons that are not fully understood. This causes coral bleaching, with the coral’s tissue turning translucent and appearing brightly white; this is the underlying skeleton shining through.
Despite the doom and gloom, corals have survived several mass extinction events.
In this state, corals can only survive for a limited time and die if the unfavorable conditions persist. The 1997-1998 El Niño event is estimated to have wiped out about one-sixth of all coral colonies worldwide.
Despite their economic and ecological importance, coral reefs are among the most endangered ecosystems on this planet, with 19% of the coral reefs worldwide already lost since 1950 and a further 35% of the remaining reefs being threatened. Projected temperature and CO2 increases over this century paints a bleak scenario where climate change might outpace the ability of corals to adapt to the predicted changes.
When it comes to temperature, the Red Sea environment is close to the projected scenarios for the other oceans of the world. Yet strangely, Red Sea corals thrive under conditions that would not be tolerated by their relatives in other oceans.
What are the adaptations that allow the corals from the Red Sea to live under these extreme conditions? Do other coral species also have a similar potential to adapt to future ocean environments and can they do so in light of the rapidly changing environment?
Despite the doom and gloom, corals have survived several mass extinction events, including the one that led to the demise of dinosaurs, 65 million years ago. But how?
Genetic adaptation is a slow process that is dependent on the random nature of mutations, with advantageous alleles – variants of genes - spreading across the population via natural selection. This process occurs over very long time scales, especially so in species with long generation times such as corals.
However, many organisms possess non-genetic mechanisms that allow them to quickly adapt to changes in a more limited way by “stretching” physiological capacities that are predefined by their genomes. These mechanisms, collectively termed epigenetic mechanisms, allow organisms to modify the “context” of their genome to adapt and respond to stresses.
Epigenetic modifications do not change the actual genetic code, but alter how it is used – such as when, and to what degree, a gene will be expressed.
This process allows organisms to optimize their physiological responses in the face of long-term stress.
These evolutionarily-advantageous responses are then imprinted on the genome using “molecular tags”, such as DNA methylation and histone modifications, which allows the organism to recall these specific responses in the future in order to provide protection under similar environmental conditions. Interestingly, there are also studies in some organisms, such as mice, showing that such modifications can be passed on to the next generation to provide a “head start” for their offspring.
Although epigenetics is currently under intense study in many model organisms, investigation into corals are still fledgling.
Studies that periodically subjected corals to a specific stress, such as high temperatures, show that they indeed retain a higher resilience towards similar future stresses. These observations led many to believe that corals are likely to possess epigenetic mechanisms that aid in adapting to changing environmental conditions – a hypothesis that is supported by our work on the Red Sea coral Stylophora pistillata.
We discovered extensive DNA methylation in the genome of S. pistillata and noticed that specific patterns of epigenetic marks emerge when corals were stressed for prolonged periods. Our research currently focuses on understanding the exact ways these mechanisms work and to what extent they allow corals to adapt.
Current reef restoration approaches are already overwhelmed.
Getting clear understanding of these mechanisms in corals might provide a key to maintaining and protecting coral reefs in the future by generating corals that are epigenetically adapted to the warming environment. Current reef restoration approaches are already overwhelmed due to the sheer scale of the areas that need to be conserved and restored, while the increasing rate of reef degradation suggests that the problem will be further exacerbated in the future.
The ability to generate pre-adapted coral colonies and larvae via epigenetic conditioning will allow the creation of seeding populations that repopulate the reefs naturally, without the potential reduction of genetic diversity caused by artificially selecting corals, or the monumental effort required to restore the reef through continuous transplantation of single colonies.
Due to the long-term nature of epigenetic markers, they might serve as a good indicator about the current state of a reef by providing a record of the stresses endured over the past weeks, months and possibly years, analogous to the rings of a tree bark, albeit over a shorter timescale.
The study of epigenetic mechanisms in corals is an exciting new field that has great potential for the preservation of these unique ecosystems for future generations.