Zooxanthellae

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Zooxanthellae

Physiology

Physical characteristics

Zooxanthellae is the name given to a wide array of different algae of the genus Symbiodinium. This specific clade is primarily found in symbiotic relationships. Zooxanthellae are part of the group Dinophyta, which are single-celled mixotrophic microorganisms. [1]

A Close up of many zooxanthellae
A Close up of many zooxanthellae

This class of organisms has two flagella, one transverse and one longitudinal. The longitudinal flagella is used as a rudder for directional steering. The transverse flagella is used for propulsion. It is two to three times the length of the longitudinal flagellum and is composed of axoneme, which causes the organelle to take on a striated form (like a ribbon). The flagella possess mastigonemes (small flagellar hairs). Dinoflagellates use thecal plates as a method of defense against predators. Theca are armor-like plates on the exterior of the cell that are composed of cellulose. Note: individuals may not have thecal plates at all stages of their life cycle.

Genetic characteristics

The genetic load of a dinoflagellate is unique to members of this group. Within the nucleus, the DNA is not organized into nucleosomes, and histone proteins are absent. However, the chromosomes are always condensed. Remarkably, the genome size ranges from 3,000 to 215,000 Mbps. Comparably, the entire human genome is closer to the 3,000 Mbp mark. While the majority of the dinophyta genome is non-coding, it is thought to have a structural function. [1]


Visible Zooxanthellae within a coral polyp
Visible Zooxanthellae within a coral polyp

Behavior

These organisms are highly under-represented in the water column. With higher competition, predation, and variable nutrient availability, the zooxanthellae do not fair well in open waters. Instead, a large majority of zooxanthellae form symbiotic relationships and live within host organisms[2]. Typically, the host organism captures the algae but does not digest. The algae makes it to the epithelial layer of the organism where it can make the most of the light availability.


This symbiotic relationship has been observed in Cnidarians (Coral Polyps, Jellyfish, Sea Anemone) as well as in Nudibranchs (Sea Slugs).

Symbiotic Zooxanthellae inhabiting a sea slug
Symbiotic Zooxanthellae inhabiting a sea slug

Mutualistic Relationship with Coral Reefs

A diagram of coral symbiosis
A diagram of coral symbiosis

Corals provide a safe environment for the zooxanthellae to photosynthesize. Since the algae are within the polyp, any action to reduce predation upon the coral will also benefit the symbiont. Since the corals gain nutrients and materials through tidal action, they are able to provide the algae with the nutrients needed for photosynthesis (such as C, N, P, and Trace Elements)[2].

In return, the zooxanthellae provide food (nutrients) for the coral to use in order to grow and create their calcium carbonate skeleton. A highly efficient exchange of nutrients observed in this relationship, as the algae is found within the polyp. Several macro-nutrients are transfered from the zooxanthellae to the polyp. Sugars, carbohydrates, and lipids all serve as the coral's primary source of fuel to grow and develop. Additionally, amino acids are exported to assist in the growth of the polyp.

Zooxanthellae are also known to aid in CaCO3 formation. Through uptake of CO2, the algae raise the pH of the surrounding environment[3].


This symbiotic relationship has lasted for more than 200 million years. It is believed that lateral gene transfer has occurred between the two organisms. From this, there is a specific host/inhabitant preferance for specific varieties of zooxanthellae. This relationship is the reason for the variation in color patterns among corals.

Environmental Impact & Coral Bleaching

Bleaching occurs when environmental stress adversely affects the mutually beneficial relationship between the host organism and its zooxanthellae. Several factors can cause the zooxanthellae to be expelled from the coral polyp. The coral then loses all pigment (as the polyp is naturally clear).

Loss of zooxanthellae and pigment
Loss of zooxanthellae and pigment

Sources

  1. 1.0 1.1 Laura Barsanti, Paolo Gualtieri. "Algae: Anatomy, Biochemistry, and Biotechnology." CRC Pres. Taylor & Francis Group. 2006. Danvers, MA. Print. [cited 2013 April 12]
  2. 2.0 2.1 http://oceanservice.noaa.gov/education/kits/corals/coral02_zooxanthellae.html (accessed on 2/23/13)
  3. http://coralreef.noaa.gov/aboutcorals/coral101/symbioticalgae/ (accessed on 2/23/13)

Other Sources

Picture Sources

1. http://www.coralreefecosystems.org/

2. http://ocean.si.edu/

3. http://www2.fiu.edu/

4. http://www.seaslugforum.net/

5. http://ningalooatlas.files.wordpress.com/


The photos should be cited using the Cite method as above. These sources are too ambiguous to be useful!

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