Zooxanthellae and their Symbiotic Relationship with Marine Corals: Difference between revisions
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Corals are usually colonies of polyps. Polyps are live coral tissue extensions that cover the calcium carbonate structure, and are usually only a few millimeters thick. The tissue has two layers, the epidermis and the gastrodermis where the zooxanthellae live (36). Zooxanthellae are unicellular and spherical with two flagella that fall off once they are incorporated within a host. Zooxanthellae undergo asexual reproduction by division, and most of their energy comes from performing photosynthesis using the byproducts of cellular respiration produced from the host coral. | Corals are usually colonies of polyps. Polyps are live coral tissue extensions that cover the calcium carbonate structure, and are usually only a few millimeters thick. The tissue has two layers, the epidermis and the gastrodermis where the zooxanthellae live (36). Zooxanthellae are unicellular and spherical with two flagella that fall off once they are incorporated within a host. Zooxanthellae undergo asexual reproduction by division, and most of their energy comes from performing photosynthesis using the byproducts of cellular respiration produced from the host coral. | ||
==Life Cycle== | |||
Their life cycle is in two stages: the coccoid stage and the motile mastigote phase. In the motile phase, the zooxanthellae retain their flagella and are free-living. In the coccoid stage, which is more common, the zooxanthellae are intracellular symbionts within the coral and do not keep their flagella. During reproduction, the chromosome and nuclear division occurs in darkness, while the cellular division into two flagellated cells (cytokinesis) occurs in exposure to light. This division in log phase is about every one to three days, and, in culture, division slows during stationary phase, and fewer motile cells are produced. Mitosis occurs on the coccoid cells as well, which is surrounded by a cell wall of glycoprotein’s and proteins, and only one species of zooxanthellae is known to have surface projections (13). The zooxanthellae’s chloroplast has three membranes, and the thylakoid membranes differ between species. | |||
==Genome== | |||
The Symbiodinium genome was very recently sequenced. It was found that the genome contains unidirectionally aligned genes and these genes thus form a cluster-like arrangement. There is an estimated 1,500 Mbp in the genome of the species Symbiodinium minutum and approximately 42,000 protein-encoding genes. There are also genes to regulate chromosome condensation proteins, and about two-thirds of these genes were obtained through bacterial horizontal transfer, while the other one-third most likely have eukaryotic orthologs. There are unique donor and acceptor splice sites (4). | |||
==Coral Bleaching== | |||
What exactly is coral bleaching? Well the zooxanthellae are prominent on the coral; there is approximately 1-5*10^6 zooxanthellae per cm^2 and each zooxanthella has about 2-10pg of chlorophyll. When coral bleaching occurs, they lose about 60-70% of their zooxanthellae, which in turn lose 50-80% of their photosynthetic pigments (5). The coral becomes bleached because it expels the zooxanthellae, leaving itself a bare skeleton of calcium carbonate because the algae is what gave the coral its color. The most common reasoning behind why the zooxanthellae leaves the coral is the idea that a sudden high water temperature or uncomfortable environmental conditions will expel the algae in the open water. When the algae leaves the coral, the coral begins to starve, but if the optimal conditions return soon, there is hope that the zooxanthellae will come back. If the algae do not come back because the stress is still present, however, then the coral will die. In a paper discussing the affects Hurricane Flora had on coral reefs in Jamaica, it was found that some zooxanthellae did in fact reinhabit the coral after some time, thus making part of the reef salvageable after the natural disaster (37). Interestingly, however, it was found that perhaps the differentiation of lipids in the Symbiodinium could cause varying sensitivity to thermal stress. In one study it was found that more disorganized stacking in the thylakoid membrane resulted from the Symbiodinium being exposed to high temperatures. This showed that the composition of the lipids might be important to understanding the temperature range of the algae (24). | |||
Besides the direct loss of zooxanthellae, coral bleaching can occur in other ways. UV and visible light have both been shown to have a role in coral bleaching, along with subaerial exposure, which causes an inconsistent environment for the coral. Furthermore, sedimentation has been thought to induce coral bleaching, along with dilution of waters or an influx of inorganic ingredients into the ecosystem. Also, pollution and pathogens are understandably a cause for coral bleaching to occur (5). | |||
Some of the symbiotic organisms do have a defense against the UV light, however. Mycosporine-like amino acids (MAAs) can uptake the UV light and do not require extra reactions to do so. The MAAs can also uptake radicals, but are not found in every clade of Symbiodinium (29). This study in 2000 showed that two of the three clades observed did not produce these MAAs, and the one clade that did had an increase of them during the middle of the day. This implies that some species of the Symbiodinium have adapted to the UV radiation, while some still have not, and perhaps in the future the algae with the ability to survive will attach to the majority of the coral so UV radiation will no longer be a threat to reefs. | |||
Revision as of 22:32, 23 April 2014
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Introduce the topic of your paper. What microorganisms are of interest? Habitat? Applications for medicine and/or environment?
Introduction
Zooxanthellae is the common name of the broader Symbiodinium genus, but specifically is the brown to yellow algae that lives in coral’s gastrodermis (3). Zooxanthellae is a term for any dinoflagellate that participates in symbiosis with sponges, coral, clams, mollusks, flatworms, jellyfish, etc (1,2). It is an algal protist that is best known for its symbiotic relationship with marine coral. Zooxanthellae usually occur in extremely high densities on their host, enhancing the constant exchange of nutrients between them and their host.
Shape
Corals are usually colonies of polyps. Polyps are live coral tissue extensions that cover the calcium carbonate structure, and are usually only a few millimeters thick. The tissue has two layers, the epidermis and the gastrodermis where the zooxanthellae live (36). Zooxanthellae are unicellular and spherical with two flagella that fall off once they are incorporated within a host. Zooxanthellae undergo asexual reproduction by division, and most of their energy comes from performing photosynthesis using the byproducts of cellular respiration produced from the host coral.
Life Cycle
Their life cycle is in two stages: the coccoid stage and the motile mastigote phase. In the motile phase, the zooxanthellae retain their flagella and are free-living. In the coccoid stage, which is more common, the zooxanthellae are intracellular symbionts within the coral and do not keep their flagella. During reproduction, the chromosome and nuclear division occurs in darkness, while the cellular division into two flagellated cells (cytokinesis) occurs in exposure to light. This division in log phase is about every one to three days, and, in culture, division slows during stationary phase, and fewer motile cells are produced. Mitosis occurs on the coccoid cells as well, which is surrounded by a cell wall of glycoprotein’s and proteins, and only one species of zooxanthellae is known to have surface projections (13). The zooxanthellae’s chloroplast has three membranes, and the thylakoid membranes differ between species.
Genome
The Symbiodinium genome was very recently sequenced. It was found that the genome contains unidirectionally aligned genes and these genes thus form a cluster-like arrangement. There is an estimated 1,500 Mbp in the genome of the species Symbiodinium minutum and approximately 42,000 protein-encoding genes. There are also genes to regulate chromosome condensation proteins, and about two-thirds of these genes were obtained through bacterial horizontal transfer, while the other one-third most likely have eukaryotic orthologs. There are unique donor and acceptor splice sites (4).
Coral Bleaching
What exactly is coral bleaching? Well the zooxanthellae are prominent on the coral; there is approximately 1-5*10^6 zooxanthellae per cm^2 and each zooxanthella has about 2-10pg of chlorophyll. When coral bleaching occurs, they lose about 60-70% of their zooxanthellae, which in turn lose 50-80% of their photosynthetic pigments (5). The coral becomes bleached because it expels the zooxanthellae, leaving itself a bare skeleton of calcium carbonate because the algae is what gave the coral its color. The most common reasoning behind why the zooxanthellae leaves the coral is the idea that a sudden high water temperature or uncomfortable environmental conditions will expel the algae in the open water. When the algae leaves the coral, the coral begins to starve, but if the optimal conditions return soon, there is hope that the zooxanthellae will come back. If the algae do not come back because the stress is still present, however, then the coral will die. In a paper discussing the affects Hurricane Flora had on coral reefs in Jamaica, it was found that some zooxanthellae did in fact reinhabit the coral after some time, thus making part of the reef salvageable after the natural disaster (37). Interestingly, however, it was found that perhaps the differentiation of lipids in the Symbiodinium could cause varying sensitivity to thermal stress. In one study it was found that more disorganized stacking in the thylakoid membrane resulted from the Symbiodinium being exposed to high temperatures. This showed that the composition of the lipids might be important to understanding the temperature range of the algae (24). Besides the direct loss of zooxanthellae, coral bleaching can occur in other ways. UV and visible light have both been shown to have a role in coral bleaching, along with subaerial exposure, which causes an inconsistent environment for the coral. Furthermore, sedimentation has been thought to induce coral bleaching, along with dilution of waters or an influx of inorganic ingredients into the ecosystem. Also, pollution and pathogens are understandably a cause for coral bleaching to occur (5). Some of the symbiotic organisms do have a defense against the UV light, however. Mycosporine-like amino acids (MAAs) can uptake the UV light and do not require extra reactions to do so. The MAAs can also uptake radicals, but are not found in every clade of Symbiodinium (29). This study in 2000 showed that two of the three clades observed did not produce these MAAs, and the one clade that did had an increase of them during the middle of the day. This implies that some species of the Symbiodinium have adapted to the UV radiation, while some still have not, and perhaps in the future the algae with the ability to survive will attach to the majority of the coral so UV radiation will no longer be a threat to reefs.
