Coral Bleaching

By AV

When many of us think of coral reefs, we think of the Great Barrier Reef in Australia. This reef is a famous destination for tourists and SCUBA enthusiasts. Coral reefs, such as the Great Barrier Reef, are hot spots for diverse sea life. These reefs host a variety species such as sharks, fish, whales, dolphins, turtles and many more (1). The diversity of life and color in the Great Barrier Reef can be seen in figure 1. While this makes coral reefs fun to visit, they are actually a very important component of ocean ecosystems. In general the ocean is pretty inhospitable; it can be difficult for sea life to find nutrients in some areas. However, coral reefs are exceptionally rich in nutrients (2). Many types of sea life cannot survive in inhospitable areas of the ocean, but the coral reefs are cities of life. Unfortunately, these cities are in danger. Global warming is causing ocean temperatures to rise, which is killing the coral (2). The richness of nutrients in the coral reefs is maintained by a close-knit relationship between coral and a dinoflagellate microalgae called zooxanthellae. These zooxanthellae live inside of the coral and give the coral its vibrant colors (4). A specific dinoflagellate genus, called Symbiodinium, is the main inhabitant of coral (3). One specific species, S. trenchi, may be able to save the coral during temperature increases (6). This relationship, or symbiosis, between coral and Symbiodinium is full of give and take as the zooxanthellae provide energy and nutrients, such as sugar, to the coral and the coral provides inorganic nutrients, such as nitrogen, to the zooxanthellae. (2). The nutrient cycling in this relationship keeps the high density of nutrients close to the coral reefs, which is what allows other animals and sea life to thrive in these areas (2).

Figure 1. Diversity of sea life in a coral reef.

The ability of zooxanthellae to conduct photosynthesis and turn light energy into nutrients is one of the main reasons that coral reefs are so abundant in diverse life (2). Unfortunately, the friendship between zooxanthellae and coral is a tenuous one, and zooxanthellae are prone to leave when under stress.  Rising ocean temperatures are stressful to the zooxanthellae, and they can no longer utilize light to create nutrients as efficiently (4). Not only are zooxanthellae less efficient, they actually begin to produce toxic compounds (5). These toxic compounds are harmful to the coral, and the coral either eats or ejects the zooxanthellae back into the ocean in response (5). Coral and zooxanthellae are typically resilient to small variations in temperature, but global warming is causing changes in magnitudes of degrees (5). While a few degrees may not seem like a lot, it is in fact quite drastic for sea creatures accustomed to a stable temperature. A group of researchers showed that there are different methods of bleaching in response to what they termed “moderate temperature stress” as increasing three degrees and “harsh temperature stress” as increasing five degrees from normal (5). The coral is more likely to eat and digest the zooxanthellae under moderate stress conditions but eject live zooxanthellae under harsh stress (5).

Figure 2. Image of coral reefs before (left) and after (right) a bleaching event.    

Since the zooxanthellae are what gives coral color, the color of the coral to turns to white when the zooxanthellae are eaten or ejected. An example of the stark difference between a healthy coral and a coral after bleaching can be seen in figure 2. This is why the process is known as coral bleaching (2). Coral can die after bleaching, and this can have major impacts on ocean ecosystems (2). Since the coral reefs are the nutrient sources for many other types of animals, as well as create protection from environmental hazards such as storms, losing these reefs would be devastating. One specific consequence would be a decline in fish populations, which would affect the food chain. This is not only a problem for the animals who prey on the fish such as birds, but also for the human population (2). Back in 2005 there was an increase in temperature in the Caribbean Sea and massive coral bleaching resulted (7). It was extremely worrisome that the sea life would be in danger. However, a specific species of zooxanthellae called Symbiodinium trenchi was resistant to the negative effects of temperature increases. Symbiodinium trenchi was able to efficiently provide the coral with nutrients and help the coral survive until the ocean temperature returned to normal when the regular symbionts could take up residence in the coral once again. Surprisingly, S. trenchi is rarely found in the Caribbean (6). The ability of S. trenchi to live within the coral in the Caribbean Sea saved those coral from dying during the extreme coral bleaching (6). In this way, S. trenchi also saved the ocean ecosystems surrounding the coral. Strangely these zooxanthellae species do not seem to maintain a relationship with coral once temperatures return to normal (6). It seems that the species were fleeting symbionts, which makes them all the more interesting.

After the coral bleaching event, and saving by S. trenchi, scientists wanted to know what made this zooxanthella species resistant to temperature changes and why it could not maintain a relationship with coral in the Caribbean. The ability of S. trenchi to resist damage from temperature changes is likely due to the structure of the thylakoids’ membrane. Thylakoids are the light harvesting complexes within chloroplasts. If a thylakoid membrane has a high concentration in a certain type of fatty acid, the zooxanthellae owner is much more likely to be temperature change resistant. Specifically, zooxanthellae with high concentrations of this type of fatty acid are much more resilient to damage due to toxic compounds generated from photosynthesis during stress (3,5). One group of researchers hypothesized that S. trenchi may not be able to continue a relationship with coral when in competition with zooxanthellae that are better suited for the normal temperatures of the Caribbean Sea (6). This would be due to what they deemed “physical trade-offs” (6), or rather S. trenchi may be a weak competitor because it exerts more resources towards temperature resistance. While S. trenchi is able to provide heat tolerance to the coral, an extended relationship can actually be detrimental to the coral and surrounding ecosystem (8). So, it is probably a good thing that S. trenchi cannot become a permanent resident within coral.

So although S. trenchi was a hero in the Caribbean Sea coral bleaching, this does not mean that we can count on S. trenchi or other temperature-stable zooxanthellae to save the day in another mass bleaching event. We must take matters into our own hands and work together to stop global warming and subsequent ocean temperature increases. Through this we might have hopes of reducing coral bleaching and saving ocean ecosystems.

Resources

1. Great Barrier Reef Marine Authority. Facts about the Great Barrier Reef. Australian Government.

2. Hoegh-Guldberg, Ove. 1999. Climate change, coral bleaching and the future of the world’s coral reefs. Marine Freshwater Research 50:839-66.

3. Berkelmans, R and M van Oppen. 2006. The role of zooxanthellae in the thermal tolerance of corals: a ‘nugget of hope’ for coral reefs in an era of climate change. Proceedings of Biological Science 273(1599): 2305–2312

4. Brown, B. 1997. Coral Bleaching: Causes and Consequences. Coral Reefs 16, Suppl.: S129—S138.

5. Fujise, L, et al. 2014. Moderate Thermal Stress Causes Active and Immediate Expulsion of Photosynthetically Damaged Zooxanthellae (Symbiodinium) from Corals. PLOS One 9(12): e114321

6. LaJeunesse, T, Smith, R, Finney, J, and H Oxenford. Outbreak and persistence of opportunistic symbiotic dinoflagellates during the 2005 Caribbean mass coral ‘bleaching’ event. Proceedings of the Royal Society 276(1676) DOI: 10.1098/rspb.2009.1405

7. Penn State. 2009. Global Warming Causes Outbreak Of Rare Algae Associated With Corals, Study Finds. ScienceDaily.

8. Stat, M and R Gates. 2010. Clade D Symbiodinium in Scleractinian Corals: A “Nugget” of Hope, a Selfish Opportunist, an Ominous Sign, or All of the Above? Journal of Marine Biology 2011 doi:10.1155/2011/730715