The beach, a dream vacation destination for many. Vacationers imagine themselves lying out in the sun enjoying a refreshing beverage and looking out at the mystical ocean. Shortly offshore of your tropical vacation lies an intricate ecosystem capable of sustaining a level of biological diversity matched by few others. An explosion of color greets the eye of any observer and at the heart of it all, is the coral. Coral is made up of a large number of species that belong to a larger group of organisms, the phylum Cnidaria. Cnidaria are the organisms responsible for creating the large limestone skeleton that is the most commonly recognized feature of corals. However the visible Cnidaria are only the beginning of the coral reef’s biological story.
Currently the coral reef ecosystems are in danger. Despite what many elected officials like to think, the Earth is currently undergoing a gradual increase in overall temperature, also known as global warming (1). The Cnidaria that make up coral reefs are very sensitive to changes in the environment and the warming of ocean waters are a major contributor to the increased death of Cnidaria in recent decades (2). However there is more than meets the eye when it comes to Cnidaria, as they cannot survive without a microscopic organism called Symbiodinium. Without living Cnidaria the coral reef ecosystem quickly collapses, revealing the white limestone skeleton beneath (Fig. 1) (2). Thus, the effect of global warming on coral reefs revolves around the disruption of this key relationship.
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| Figure 1 from bajoelmar.weebly.com. Image depicting the dramatic effect of coral bleaching. On the left is a healthy coral reef that is supporting a diverse marine community. On the right is a bleached section of the same reef 5 years after the healthy image was taken. The characteristic white of the coral’s limestone skeleton can be seen and the reef is unable to support a thriving marine community. Obtained from: http://bajoelmar.weebly.com/uploads/2/6/6/6/26663401/322183761.jpg?539 |
Many Cnidarian species form important mutually beneficial relationships, or symbioses, with a group of microscopic organisms called Symbiodinium. Symbiodinium are eukaryotic, nucleus containing, single celled organisms that can use the energy from light to create sugar from carbon dioxide (CO2) through the process of photosynthesis. A portion of the Symbiodinium life cycle involves a free-living stage where the Symbiodinium cells are not associated with Cnidaria. During the free-living stage Symbiodinium are able to swim using a whip-like tail called a flagella. The symbiosis between Cnidaria and Symbiodinium begins when a Symbiodinium cell is “eaten” by a Cnidarian cell. The mechanism by which Cnidaria internalize Symbiodinium is called endocytosis and is essentially the same process used by Cnidarian cells to ingest food, except the Symbiodinium are kept alive instead of being digested. Exactly how the Symbiodinium avoid digestion is not currently known, but it is a key step in the establishment of the symbiotic relationship (3). Once endocytosed by a Cnidarian cell, the Symbiodinium lives within the Cnidarian cell itself. The Cnidaria host provides protection from predators and a steady supply of nitrogen and phosphorous nutrients to the Symbiodinium. In return Symbiodinium performs photosynthesis to create sugars that can be used as a source of energy by the Cnidaria (3).
In 2015 a species of Symbiodinium had its entire genome sequenced (4). The genome sequence revealed some interesting findings with regards to Symbiodinium’s symbiosis with Cnidaria. Due to the intimate association between the two organisms, Symbiodinium and Cnidaria have evolved over time together. As a result of this coevolution, evidence of Cnidarian genes being present in the Symbiodinium genome were found, implying that at some point in history a Cnidarian species transferred a portion of its genetic material to Symbiodinium (4). The transfer of genetic material across broad species differences is termed horizontal gene transfer. The finding of horizontally transferred genes in Symbiodinium is important because horizontal gene transfer is not heavily documented between two eukaryotic species (5). In addition to evidence of horizontal gene transfer, Symbiodinium’s genome sequence showed genes encoding proteins important in nutrient transport that are complementary to Cnidarian nutrient transport genes (5). The complementing transporter genes further support the nutrient sharing hypotheses and provides insight into potential ways that Symbiodinium and Cnidaria exchange nutrients.
In addition to the sharing of nutrients, the limestone skeleton that Cnidaria grow on is thought to play a role in Cnidaria’s symbiosis with Symbiodinium. Limestone is a mineral primarily composed of calcium carbonate (CaCO3) and is thought to be produced by the Cnidaria themselves through a process called calcification. Calcification is a similar process to how bones are made in humans (6). The white limestone reflects light and helps Symbiodinium capture the light’s energy. By increasing the capturing of light, Symbiodinium is able to produce more sugars and the overall energy available to the Cnidaria increases (2, 3). However when exposed to increases in temperature, as results from global warming, the organelle in Symbiodinium where photosynthesis takes place, chloroplasts, become damaged in a way that increases the production of reactive oxygen species (2). Reactive oxygen species, as their name implies, react with a large number of molecules important for the function of an organism. Of specific importance is the ability of reactive oxygen species to damage DNA. Photosynthesis and other metabolic reactions involving oxygen normally create low levels of reactive oxygen species as byproducts, and thus Cnidaria and Symbiodinium have proteins that protect against the damaging effects by converting reactive oxygen species into a harmless form. However the protective proteins can become overwhelmed when too many reactive oxygen species are produced. The overwhelming of the protective proteins can then lead to the death of either the Cnidarian cell, the Symbiodinium cell, or both. Stress and death induced by global warming and mediated by reactive oxygen species is thought to play a major role in coral bleaching due to the disruption of the symbiosis between Cnidaria and Symbiodinium (2).
Despite Cnidaria being thought to primarily produce the limestone skeleton it grows on, it has been found that rates of calcification increases during times of high photosynthesis by the Symbiodinium symbiont (7). The simultaneous increase in both calcification and photosynthetic processes predicts a potential link between the two processes. The link could be simply attributed to an overall increase in metabolism since Cnidaria would have more sugar available from photosynthesis, or the link could be attributed to the Symbiodinium cells contributing directly to the increase in calcification. A paper published in 2015 by Frommlet et al. (8) shows evidence that the means by which calcification occurs may not be as simple as originally thought, and that it may even involve a third level of symbiosis.
Frommlet et al. were growing a species of Symbiodinium in culture and found that one culture had calcium carbonate, limestone, deposits in it (Fig. 2). The researchers found this to be quite odd and further examination of the culture found it was contaminated with bacteria (Fig. 2G). The bacteria were isolated and grown without any Symbiodinium and no limestone deposits formed. The contaminated Symbiodinium were treated with antibiotics to kill the bacteria, and again no limestone deposits formed. Since neither the bacteria nor the Symbiodinium cells alone could produce limestone, but when combined were able to form limestone structures, it was concluded that the interaction between the Symbiodinium and the bacteria was creating the limestone. The association of limestone formation and light was also examined for the Symbiodinium interaction with bacteria. The researchers also found that the limestone formed mostly during times when light was present and photosynthesis could thereby occur. So not only was the interaction creating limestone deposits, but it is driven by photosynthesis, like what is seen in the Cnidaria symbiosis. The evidence provided by the Frommlet et al. (8) paper suggests that bacteria may be a third player in coral reef symbiosis by participating in skeleton formation.
The coral reef ecosystem is a mystically beautiful place, and at the heart of it all are the Cnidaria and their friends the Symbiodinium. The delicate symbiosis between these two organisms is being threatened by increasing ocean temperatures. Without the Cnidaria-Symbiodinium symbiosis, coral reefs die and take the diversity of life that the coral reef ecosystem sustains with it. The symbiosis may also contain bacterial members further emphasizing how co-dependent organisms in the coral reef ecosystem are upon each other. Thus the threat of global warming is imminent and coral reefs are among the first on its list of destruction.
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| Figure 2 from Frommlet et al. 2015 (8) A through H are microscopic pictures of the Symbiodinium forming limestone structures when interacting with bacteria. G uses a stain that shows the bacteria in blue and the Symbiodinium in red. I through K are scanning electron micrograph pictures of the limestone structure formed by Symbiodinium and the bacteria. L is a chemical analysis of the structure that showed it to be made up of calcium carbonate. Obtained from: http://www.pnas.org/content/112/19/6158/F1.expansion.html |
References:
1. Cook J, Nuccitelli D, Green SA, Richardson M, Winkler B, Painting R, Way R, Jacobs P, Skuce A. 2013. Quantifying the consensus on anthropogenic global warming in the scientific literature. Environ Res Lett 8:024024.
2. Weis VM. 2008. Cellular mechanisms of Cnidarian bleaching: stress causes the collapse of symbiosis. J Exp Biol 211:3059–3066.
3. Davy SK, Allemand D, Weis VM. 2012. Cell Biology of Cnidarian-Dinoflagellate Symbiosis. Microbiol Mol Biol Rev 76:229–261.
4. Lin S, Cheng S, Song B, Zhong X, Lin X, Li W, Li L, Zhang Y, Zhang H, Ji Z, Cai M, Zhuang Y, Shi X, Lin L, Wang L, Wang Z, Liu X, Yu S, Zeng P, Hao H, Zou Q, Chen C, Li Y, Wang Y, Xu C, Meng S, Xu X, Wang J, Yang H, Campbell DA, Sturm NR, Dagenais-Bellefeuille S, Morse D. 2015. The Symbiodinium kawagutii genome illuminates dinoflagellate gene expression and coral symbiosis. Science (80- ) 350:691–694.
5. Boto L. 2010. Horizontal gene transfer in evolution: facts and challenges. Proc Biol Sci 277:819–827.
6. Sapir-Koren R, Livshits G. 2014. Bone mineralization is regulated by signaling cross talk between molecular factors of local and systemic origin: The role of fibroblast growth factor 23. BioFactors 1–14.
7. Al-Horani F a, Al-Moghrabi SM, De Beer D. 2003. The mechanism of calcification and its relation to photosynthesis and respiration in the scleractinian coral Galaxea fascicularis. Mar Biol 142:419–426.
8. Frommlet JC, Sousa ML, Alves A, Vieira SI, Suggett DJ, Serôdio J. 2015. Coral symbiotic algae calcify ex hospite in partnership with bacteria. Proc Natl Acad Sci 112:6158–6163.
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