Zooxanthellae and Coral- a Love Story

by AN

The ocean is home to many fascinating creatures that have created their own magical undersea world. An important role in this world is played by the dinoflagellate Zooxanthellae. Zooxanthellae are single cell marine organisms that rose to fame by their association with coral reefs. The two organisms have a mutually beneficial relationship that- like most relationships- can get complicated under stressful conditions. Coral provide zooxanthellae with carbon and a cozy, protected habitat with plenty of sunshine. In turn, the zooxanthellae will use the carbon to undergo photosynthesis. The coral will be awarded about 90% of the sugars, lipids, and oxygen produced by their tenants for growth and respiration. Furthermore, the coral can use the photosynthesis byproducts to strengthen and lengthen their calcium carbonate skeleton (1). This is an inspiring process of recycling that results in almost no wasted energy or nutrients. Unfortunately, this relationship is traveling down a bumpy path and coral reefs are suffering greatly. Zooxanthellae are beginning to move out of their coral homes due to conditions caused by global climate change. This results in the process of coral bleaching, where corals lose their defining color and structure and have trouble engulfing enough nutrients.

Coral reefs are home to a wide range of marine

diversity, including this beautiful parrot fish. 

Zooxanthellae can exist as free-living cells or in symbiosis with marine creatures. In the free-living state, they have two flagella that they lose during colonization of a host. They are sensitive to any stressors in the environment, and prefer to reside in temperatures colder than 33 degrees Celsius. Even a slight increase in temperature can have dire consequences. Global warming is a huge concern for these microscopic eukaryotes, as the ocean temperature has already risen 0.85 degrees Celsius in the last century (2). When zooxanthellae undergo changes in temperatures, they are no longer able to provide sufficient nutrients for their coral, and are expelled from their homes. The coral then lose their color slowly starve to death (2). Another preferred condition of zooxanthellae is clear ocean waters, to soak in maximum light energy from the sun. In turbid waters, they have more trouble photosynthesizing (3). If they cannot produce enough food for themselves or their coral, they will die, leaving their coral bleached and devoid of nutrients. Bleached coral are also more susceptible to disease, which suggests that zooxanthellae are able to outcompete other microorganisms for colonization (1). When the zooxanthellae no longer colonize coral, they risk invasion of deadly disease and illness. Pollution of the oceans and global climate change are enemies of zooxanthellae; when their environment becomes turbid and warmer, the coral symbionts will seek a new home.

An important genus of zooxanthellae is Symbiodinium, which form obligate mutualisms with stony corals. The Symbiodinium genus is the most well studied zooxanthellae, and a common symbiont of coral. Symbiodinium displays extensive genetic diversity. They are spherical shaped cells and contain a multilobed chloroplast. Their genome contains twenty-six chromosomes, guarded inside of a relatively large nucleus (4). Due to the diversity observed within this genus, it is classified into different groups, or clades, based on genetic similarities. Within the eight different clades of this genus, there are multiple subclades, or strains. Different strains contain different DNA and display different phenotypic behaviors. A coral rarely inherits its zooxanthellae, and must acquire them in open waters. However, occasionally the Symbiodinium will be inherited vertically and transmitted from parent to offspring. The environmental conditions in the location of the host encourage specific pairings between mutualistic partners (5). One coral can host multiple strains of Symbiodinium, and even display selective behavior in their affiliation with certain strains. Some strains are able to combat higher temperatures, and therefore seasonal shuffling of Symbiodinium strain is not uncommon for a stony coral (6). By adapting this behavior of selectively hosting heat tolerant symbiotic zooxanthellae, coral reefs may find a way to thwart global warming (7). In fact, it has been shown coral reefs that are most severely affected by climate change have adopted Symbiodinium that are resistant to warmer temperatures (7). If ocean temperatures continue to increase, however, zooxanthellae may not evolve fast enough to save its hosts and even the most heat tolerant strains will have trouble surviving. Environmental impacts imposed by humans have already devastated 30% of the world’s coral, indicating the Symbiodinium may not survive the drastic changes (1).

From here.

However, there is a beacon of hope for tropical coral reefs: Symbiodinium clade D. It will persevere higher temperatures and provide coral with stress resistance, possibly saving future generations (1). Genetic biomarkers are used to distinguish clade D from the other eight clades, but categorizing species and subclades has been a persisting dilemma. Clade D Symbiodinium are considered generalists, and occupy a wide variety of hosts. Its low host specificity could either be attributed to properties that allow the dinoflagellates to survive diverse environments, or that it is less likely to be evicted by its host because of thermal resistance.  However, corals that adopt Symbiodinium clade D in times of stress will allow their original strains of zooxanthellae to repopulate within 3 years (1). Further investigation of this phenomenon should involve close monitoring of coral symbionts before, during and after exposure to an increase in water temperature. It is possible that more common clades, such as B or C, can out compete clade D in certain environments. Coral that harbor clade D grow more slowly than those who are colonized by clade B or C, so once the coral no longer perceives thermal stress as a threat, they could prioritize faster growth over stress protection (1). The fitness trade-offs of clade B or C versus clade D explain coral shuffling of the Symbiodinium. There has been disparity of heart resistance within clade C, suggesting that the clades continue to evolve and adapt to climate change or that horizontal transfer occurs between clades.

Coral reefs give life to an incredible amount of marine biodiversity. The ecosystem they provide can be almost entirely attributed to the dinoflagellate mutualists residing inside of them. If global warming succeeds in killing marine zooxanthellae, it will endanger an abundance of marine life that relies on coral reefs for protection and housing. With rising temperatures of the ocean comes coral bleaching, which is highly correlated with coral reef mortality. It is the responsibility of microbiologists and marine biologists of the world to commit to saving these beautiful eukaryotic microbes that give the ocean so much life. Further investigation of the genetic diversity and stress adaption of Symbiodinium species will pave the way to providing coral reefs with the necessary nutrient to rebuild its community.

References:

1.     Stat, M and Gates, R. Clade D Symbiodinium in scleractinian corals: A “nugget of hop, a selfish opportunist, an ominous sign or all of the above. Journal of Marine Biology. 2011.

2.     University Corporation for Atmospheric Research. How much has the global temperature risen in the last 100 years. https://www2.ucar.edu/climate/faq/how-much-has-global-temperature-risen-last-100-years

3.     Douglas, A.E. Coral bleaching-how and why. Marine Pollution Bulletin. 46(4); 385-392. April 2003.

4.     Noaa ocean service education. March 2008. http://oceanservice.noaa.gov/education/kits/corals/media/supp_coral02bc.html

5.     Blnk, R.J. Cell architecture of the dinoflagellate Symbiodinium sp. Inhabiting the Hawaiian stony coral Montipora verrucosa. Marine Biology. 94(1); 143-155. February 1987.

6.     Coffroth, M and Santos, S. Genetic diversity of symbiotic dinoflagellates in the genus Symbiodinium. Protist. 156(1); 19-34. June 2005.

7.     Mieog, J et al. Quantification of algal endosymbiont (Symbiodinium) in coral tissue using real-time PCR. Molecular Ecology Resources. 9(1).January 2009.

8.     Baker, A et al. Coral reefs: Coral’s adaptive response to climate change. Nature. 430(741). August 2004.