While summer is synonymous with relaxation – especially to school-going children – in the Western hemisphere, the situation is the exact opposite for corals (young or old). During summer, water temperatures routinely hit a scorching 34 oC in the Red Sea. Relative to the other oceans in the world, these conditions are considered extremely harsh for corals: however, the Red Sea still teems with life. It is home to some of the most pristine coral reefs on the planet, 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 is the reason corals can construct their majestic structures in spite of the oligotrophic nature of the sea. These structures form the foundation of the coral 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. Despite being terrestrial, humans do 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 obvious ecosystem services such as slowing down the erosion of shorelines.
Ironically, the very same symbiotic relationship that underlies the success of reef-building corals is also their Achilles heel: this sensitive relationship is prone to breaking down. These corals are notoriously sensitive to many environmental stressors such as temperature, high UV intensities, and pollution, among others. When coral reefs are stressed, coral bleaching occurs – the symbionts are progressively expelled from the host tissue for reasons that are not fully understood. The extensive loss of symbionts turns the host tissue translucent, which causes them to appear brightly white due to the underlying skeleton shining through. In this state, corals can only survive for a limited time and die if the unfavorable conditions persist. In extreme cases, like in the case of the 1998 worldwide bleaching event, large reef areas and even entire coral reefs can be affected, causing massive bleaching events and large-scale mortality. It was estimated that about one-sixth of all coral colonies worldwide were lost throughout the 1997–1998 El Niño event.
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. Current projections of 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, a fear that is further nurtured by observing increased losses of coral cover and the progressive deterioration of coral reef health worldwide.
Currently, in terms of temperature, the Red Sea has an environment that is close to the projected scenarios for the other oceans of the world; yet, strangely, corals in the Red Sea manage to thrive under conditions that are lethal to 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 (K-Pg extinction event) that led to the demise of dinosaurs, 65 million years ago. The question is, how?
Genetic adaptation is a slow process that is dependent on the random nature of mutations, with advantageous alleles spreading to fixation 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 how much, a gene will be expressed. This 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, similar efforts in corals remain in its infancy. 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 of time. Our research currently centers on understanding the exact ways these mechanisms work and to what extent they allow corals to adapt.
It is tempting to believe that the 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, with neither the potential reduction of genetic diversity caused by artificially selecting corals, nor require the monumental effort required to restore the reef through continuous transplantation of single colonies. Also, 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 iconic ecosystems for future generations – diving trips during their summer holidays would not just be a pipe dream of yore!
Co-Author: Yi Jin Liew, Molecular Diagnostics Solutions group in CSIRO.