by Tamaki Bieri
Coral bleaching in the news
Currently, the third massive worldwide coral reef bleaching event is unfolding. Corals from Hawaii to the Indian Ocean are affected, and recently, the worst bleaching ever witnessed in the Great Barrier Reef was reported. Once rare, coral bleaching events have increased both in frequency and severity over the past decades. Similar to the event in 1998, the current bleaching is caused by a combination of global warming and a strong El Niño. Whereas the last two global bleaching events (in 1998 and 2010) lasted one year each, the current event is spanning two years.
Why do corals matter?
Coral reefs are among the world’s most complex ecosystems and are of immense ecological, economical, and aesthetic value. They provide habitat for a unique community of organisms including fish, turtles, sharks, crabs, shrimps, urchins, sponges, and microbes. For coastal communities, they provide food, coastal protection, and recreational possibilities.
Despite their importance, the health and extent of coral reefs around the world has declined rapidly over the past several decades due to human influences. These include pollution, overfishing (fish remove macro-algae that compete with corals for living space), and increasing atmospheric CO2 concentration that causes the oceans to become warmer and more acidic. Reef-building corals live near their upper thermal tolerance limits, meaning that even small increases in seawater temperature can cause stress, and consequently, bleaching. Furthermore, acidic seawater harms corals because it dissolves their skeletons, which are made of calcium carbonate.
Unfortunately, this trend of warmer and more acidic oceans is likely to continue, as global average ocean surface temperature has increased by 0.7°C, and pH has decreased by 0.1 units to 8.0 during the 20th century. Average sea-surface temperature (top 700 m) is projected to increase by another ~2.6°C, and pH is projected to decrease to ~7.7 by the year 2100 if the worst-case IPCC (Intergovernmental Panel on Climate Change) scenario is applied.
What exactly is coral bleaching?
Corals are animals that live in a mutually beneficial relationship with single-celled algae (zooxanthellae) of the genus Symbiodinium. Most of the energy used by corals for growth and reef formation comes from these single-celled algae, which live in specialized compartments (“symbiosomes”) within the cells of corals. The algae perform photosynthesis and provide part of the generated carbohydrates to the coral in exchange for a protected environment and compounds needed for photosynthesis.
Under stress, the algae disappear from the coral tissue and the white coral skeleton can be seen through the transparent tissue: thus coral bleaching. If prolonged, bleaching can lead to death of the coral because there are no longer any algae left to provide energy.
However, despite the biological interest and practical importance of bleaching, how the algae disappear from the coral during a bleaching event has remained very poorly understood and is the subject of our investigation.
How do the algae disappear from the coral cells?
Four mechanisms that could explain the disappearance of algae from the coral tissue have been reported. First, algal cells can be degraded within the coral cells (Fig. 1b). Second, intact or degraded algae can be expelled from the coral cells (Fig. 1c). Third, coral cells containing algae can detach from the rest of the animal (Fig. 1d). Fourth, coral cells containing algae can die in place (Fig. 1e).
On the one hand, it is possible that all four mechanisms are occurring in parallel, contributing to overall bleaching at different degrees. On the other hand, differences reported in past studies could simply stem from the fact that different species, stress conditions and experimental designs have been used to investigate coral bleaching.
To our knowledge, none of the previous studies has attempted to quantitatively evaluate the relative contributions of the various bleaching mechanisms to overall bleaching. Also, none of the studies has tested a variety of stress conditions on genetically identical organisms.
Using the sea anemone Aiptasia to study corals
Corals are not ideal experimental organisms: they are protected by law, difficult and slow to grow, and have a skeleton made of calcium carbonate that poses many challenges for performing quantitative cell-biological studies.
Instead, in our recently published research, we have used the small sea anemone Aiptasia, which harbors the same type of algae as those found in corals. Aiptasia is extremely hardy. It also grows and multiplies quickly by leaving small pieces of tissue behind, out of which baby anemones form. This generates a large population of genetically identical individuals.
We subjected these anemones to a combination of temperature and light stress conditions, and assessed the relative contributions of the four possible bleaching mechanisms in parallel.
Which one of the four mechanisms is responsible for most of the bleaching?
When compared to control condition, neither the percentage of detached anemone cells containing algae (Fig. 1d), nor the percentage of algae being degraded within the anemones (Fig. 1b) increased under stress. Death of anemone cells containing algae (Fig. 1e) increased shortly after the animals were exposed to stress but returned to normal levels within one day, long before bleaching became apparent (in addition, preventing cell death with chemicals did not stop bleaching). In contrast, under stress, the number of algae released from the anemones increased significantly, and the majority of those algae were not surrounded by anemone cells. Therefore, we concluded that expulsion of algae seems to be responsible for most of the disappearance of algae in bleaching anemones and corals (Fig. 1c).
Global warming and corals
Our study has shown that the majority of algal cells disappear from the coral tissue due to expulsion. What remains unclear is which molecular mechanisms trigger the release of algae, and whether it is the coral or the alga that initiates the expulsion that leads to bleaching.
Understanding the basic biology of the relationship between corals and the single-celled algae is of paramount importance. However, this information alone will not alleviate the effect of global warming on coral reefs. If we keep our current carbon dioxide emission rates, coral reefs are likely to become rare or extinct within the next century. It is in our interest to protect corals, which form the foundation of a valuable ecosystem upon which not only a large number of animals but also we humans depend on for survival.
The full research article published in PLOS One can be accessed here.
Tamaki Bieri is a 2010 fellow of the Fulbright Science and Technology Award, from Switzerland, and completed her PhD in Biology at Stanford University in 2015.