Corals and Symbiodium: The Relationship Explained

I read and summarized the journal article, “The engine of the reef: Photobiology of the coral–algal symbiosis” written and researched by Melissa S. Roth. Ms. Roth’s report on the previous and present research on Scleractinia and Symbiodinium symbiosis includes discussion on light, photobiology of reef-building corals and their green algae, and the nature of their relationship.

[Check out the slideshow for more.]


Untitled, 2014; my painting

The author introduces Scleractinia coral and Symbiodinium (photosynthetic dinoflagellates), by telling us that coral reefs, which are a huge part of the ecosystem, are at risk of depreciation. She briefly describes both eukaryotes’ taxonomy. The Symbiodinium (genus) is categorized by the kingdom Chromalveolata, division Pyrrhophyta, and the class Dinophyceae. Reef-building coral taxonomy as the phylum Cnidaria, class Anthozoa, and the order Scleractinia.

Roth demonstrates how light works in oceans. The electromagnetic spectrum is a model of wavelengths of visible light ranging from to 400 – 700 (Reece 190). The full spectrum touches the surface of the ocean. Because blue wavelength extends the farthest into the ocean (400 – 500) and ultraviolet and red wavelength declines the quickest, blue reaches the lower depths than ultraviolet (200 – 400) and red light (620 – 740). So, unlike blue light, these wavelengths don’t get absorbed in some of the corals more than thirty meters away from the water’s surface.

The author extrapolates the photobiology of corals. She explains that most coral live less than thirty meters from the water’s surface so they can get the full spectrum of light, but other corals live more than thirty meters such as the Montastraea cavernosa (3 to 91 m) and the Leptoseris hawaiiensis (60 m). These corals tend to use blue light and are heterotrophic.

To make sure they get as much as light as possible, coral extracellular skeleton deflect lost wavelengths to force them to enter Symbiodinium. Because of this, under blue light, Symbiodinium can work just as well as if they were consuming the full spectrum.

Roth summarizes the fluorescent proteins in coral. Because of their fluorescent proteins, corals are equipped for high-levels light, heat, and pH. Light can affect coral by regulating its FP protein-making genes. In corals, there are mostly the green FPs, but they also have cyan, red, and chromoproteins. Chromoproteins soak in light, but don’t fluoresce. To tolerate heat and pH changes, these kinds of proteins have an eleven-stranded beta-pleated sheet and a lone alpha helix with three amino acid color compounds. For efficiency, Roth discusses the fluorescent proteins (FPs) in corals that take in extreme light energy and radiate lower light energy.

In response to extreme light, Roth mentions that at daytime, corals have also been known withdraw their polyps back, but at night, corals stretch out their tentacles to eat their kill. Coral that live closer to the water’s surface can draw back completely during low tides hiding them from light. In response to low light, corals can expand the surface area to volume ratio to pump as much CO2 and O2 as possible in and out. When coral have enlarged, Roth concludes, it allows for more light to affect coral.

Roth focuses on the photobiology of Symbiodinium. These green algae change inorganic carbon such as CO2 into organic carbon. Roth describes the photosynthetic process of Symbiodinium. In Symbiodinium, chlorophyll a, chlorophyll c2, and peridinin are pigments made depending on which wavelength. Chlorophyll a absorbs wavelengths between 435 to 400, chlorophyll c2 soaks 450 to 460 nm of light, and peridinin uses light of the 478 to 500 nm wavelength.

Too much sunlight can lead to excess light, heat, and reactive oxygen. Excess light causes Symbiodinium to make a large mass of oxygen that turns into reactive oxygen such as singlet oxygen, (¹O2), and superoxide, (O2-). Reactive oxygen destroys lipids, proteins, and DNA in both Scleractinia and dinoflagellate.

Light stress can melt Symbiodinium thylakoid. While heat stress make the thylakoid rot. To combat stressors such as light, heat, and reactive oxygen, Symbiodinium use a photosynthetic apparatus where light can get transferred into different pathways as either fluorescence, heat, and photosynthetic product. The Symbiodinium have other ways of combating stress such as superoxide dismutase that catalyzes superoxide into water and oxygen. Catalase that turns hydrogen peroxide into water and oxygen. The collection of hydrogen peroxide is what kills Symbiodinium off.

Roth reports on the nature of the coral and Symbiodinium relationship writing that scientists found different kinds of this green algae and categorized them into nine clades A through I. It was once believed that algal-coral symbiosis was mutualistic, but in fact, there are some Symbiodinium that are parasitic such as the A and D clades. She also mentions that Symbiodinium rely on corals for them to regulate their metabolism. Researchers grew Symbiodinium in petri dishes to find that they have a slower metabolism compared to living inside the coral.

Green algae, that live in the in between the polyp and the coral flesh, have adapted to light changes too. At noontime, in some corals that live close to the surface of the water, Symbiodinium expel 80% of the light.

Reef-building coral can have up to two green algae cells in one of their endoderm cell, but the Symbiodinium population may change to a new light environment especially when there are considerable amounts of reactive oxygen. This can lead to coral bleaching because as Symbiodinium die off, the rest are left to deal with the high amounts of reactive oxygen.

The researcher elucidates how thickness of the outer-layers of coral influence Symbiodinium photosynthesis. The amount of thickness dictates how levels are absorbed. Wavelengths between 400 – 700 nm tend to decline. However, absorption of seven-hundred to eight-hundred nanometers of wavelength is constant throughout the coral’s extracellular surface. Corals that live in hot climates such as the Caribbean, in the summer, the coral exterior is thinner than the winter session.

While adjusting to a new light environment Symbiodinium change their quantity of proteins in the chlorophyll and the thylakoid. Like how coral withdraw themselves from high-levels of light, Symbiodinium can decrease their chlorophyll per season. If it’s winter, chlorophyll become abundant. If it’s summer, chlorophyll decline. The loss of green algae pigment leads to coral bleaching.

To protect Symbiodinium from any further light damage. Apart from chlorophyll, there are accessory pigments in Symbiodinium such as the carotenoids. There are two categories of these kinds of pigments. One is a hydrocarbon (carotenes) and the other is hydrocarbons with oxygen (xanthophylls). When extreme light stress the algae, xanthophyll turn the pigment diadinoxanthin (C40H54O3) into diatoxanthin (C40H54O2). When low heat encounters Symbiodinium, the opposite occurs. When faced with a different light environment, Symbiodinium tend to increase their levels of these carotenoids in fifteen days. While under high heat, in five days.

“The engine of the reef: Photobiology of the coral–algal symbiosis” by Melissa S. Roth has a lot of connections to other biological topics. Additional eukaryotic animals have endosymbiotic relationships with green algae such as the C. roscoffensis. Like the coral, green algae live inside them providing the worm with sugar. C. roscoffensis would do a remarkable thing. In the intertidal zone, when the tide is out, it would pop out of the sand twice a day (in the morning and then, at the evening) to catch the rays of the sun so that their algae can photosynthesize (Carsons).

Corals are animals that behave like plants, but what most people don’t realize is that plants can behave like animals. Plants can move as if they were animals via gravitropism and thigmomorphogenesis. Gravitropism refers to how a plant would grow to adjust itself to gravity. Shoots show negative gravitropism and roots show positive gravitropism (Reece 855; 856). Thigmomorphogenesis, because of an obstacle, is a deviation from a plant’s path (Reece 856). While coral don’t move at all. These animals are happy to share their body with another eukaryote that provides them with sustenance when prey is not around. With their algae, they survive.

Green algae and plants have a lot in common. They both photosynthesize, blue light affects them, and heat stress causes harm.

Symbiodinium and plants both use the chlorophyll organelle to make sugar and oxygen during the photosynthetic process. So, the steps of photosynthesis, apart from the unique characteristics of the green algae such as the photosynthetic apparatus, are almost the same. For plants, chlorophyll a and b (accessory pigments) are used. Chlorophyll b is activated by 400-500 and 600-700 (Reece 191-192).

Photons integrate into photosystem II and electrons flow through the electron transport chain photosystem I to make NADPH. For Symbiodinium, cytochrome b6f passes the electron to oxygen and creates ATP. For plants, cytochrome a3 is used (Reece 173). For Symbiodinium and plants, NADPH and ATP power carbon fixation and the Calvin-Benson cycle.

Not only does blue light influence green algae, but it affects plants too. Plants have cryptochromes. In plants, cryptochromes are pigments that get activated by blue light to open stomata to let CO2 in and water vapor out. When cryptochromes are stimulated by this specific wavelength, proton pumps change the action potential within the plasma membrane. This allows water and potassium to flow through the membrane channels such as aquaporins into the guard cells and open them so they can release water vapor from the xylem and accept CO2 molecules (Simons).

While light can activate vital molecules in green algae and plants, alternative environmental aspects such as light and heat stress can damage plants and Symbiodinium. While Symbiodinium manage oxidative and heat stress by using a special photosynthetic apparatus, plants deal with heat (3-10° C) by closing their stomata so that water stays in the xylem. Unfortunately, the evaporation of water is a way for plants to keep cool so heat stress can lead to dehydration (Simons; Reece 857). However, some plants that live in hot climates have adapted. To shield proteins from extreme heat, these plants use heat-shock proteins (Reece 858).

I loved reading “The engine of the reef: Photobiology of the coral–algal symbiosis” by Melissa S. Roth. So far, it was one of the best journal articles I’ve read. The author explains the research behind how photobiology influence the coral and green algae relationship well, which is unique for journal article.

This article was also frustrating. I got this article to answer some of my questions about coral and green algae, but it only lead me to more questions!


References:

Topic article:

Supporting references:

  • Holmes-Farley, R. (n.d.). Aquarium Chemistry: The Chemical and Biochemical Mechanisms of Calcification. Retrieved November 29, 2016, from http://www.advancedaquarist.com/2002/4/chemistry
  • Carsons, Rachel. “Tides” (1951). The Sea Around Us. Oxford University Press, 1951. Retrieved August 31, 2016. PDF
  • Reece, Jane B.; Urry, Lisa A.; Cain, Michael L.; Wasserman, Steven A.; Minorsky, Peter V.; Jackson, Robert B. (2013-10-18). Campbell Biology. Pearson Education. Kindle Edition.
  • Simons, Rachel (2016). Functional Biology: Plant resource aquisition and transport, Plant responses and signalling. Lecture.

 

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