Bleached corals have lowered defenses against disease, so often will suffer further damage and death as disease moves in after a bleaching event. In this photo, healthy brown coral gives way to the frontlines of disease. It can be hard for coral to recover for a bleaching event. The best bet is the some healthy tissue still remains deep in the skeleton and, if conditions improve, this coral can grow and recover, spreading to the rest of the skeleton.
The corals pictured here are still in recovery after a mass bleaching in Panama in the summer of You can see some bleaching on the tops, but the sides are looking good. The common name, Zooxanthellae, actually represents a group of single-celled algae that are symbiotic with corals and other reef organisms.
Once there they provide sugars, glycerol, and amino acids to the coral and in return the coral provides carbon dioxide, phosphates, and nitrogen compounds to the zooxanthellae. For most of the corals in the lagoon and on the reef of Tetiaroa the zooxanthellae produce a golden-brown color, but there are also species with a light blue color. Zooxanthellae also have symbiotic deals with other organisms besides coral. For instance, sea anenomes get their color from hosting zooxanthellae, as do jellyfish, and the brilliantly colored giant clams, Tridacna maxima.
Most coral species have become highly dependent on the contribution that Zooxanthellae make to their nutrition. Zooxanthellae typically spend their entire life on the organism to which they are attached.
The exception is when coral bleaching occurs, and the Zooxanthellae are expelled from the coral. Zooxanthellae often suffer from bacterial infections that attack corals. Many bacterium interfere with the photosynthetic processes of these organisms.
Zooxanthellae can help host coral harvest light. This helps the host meet its carbon and energy needs. In addition, Zooxanthellae give host corals their color. The research of Levy et. Coral bleachings are caused by a disruption in these relationships. Symbiotic relationships with corals and other organisms are common in tropical waters with a low abundance of nutrients. These relationships are significantly less common in temperate waters.
The Adaptive Bleaching Hypothesis ABH suggests that if the loss of Zooxanthellae occurs due to environmental change, the host organism forms a new symbiotic relationship with a different type of Zooxanthellae. These new endosymbionts are blelieved to be better adapted to the new environment. Other research on the adaptations of coral and Zooxanthellae suggest that corals that have been damaged due to high temperatures contain an abundance of Zooxanthellae that are thermally tolerant Baker et.
The symbiont changes during the stress period. It is suggested that these corals will be resistant to future thermal stress because they now have an endosymbiont that will better help them manage these environmental conditions.
Rowan also shows that corals adapt to high temperatures by hosting Zooxanthellae that are specifically adapted to such conditions. In addition to living in coral, Zooxanthellae can inhabit clams, nudibranches, flatworms, octocorals, sea anenomes, hydrocorals, mollusks, zoanthids, sponges, Foraminifera, and jellyfish.
For example, the Montastrae species, which causes Yellow Band Disease, affects the zooxanthellae directly rather than the coral 7. Besides the nutrient shuffling, there seems to be another level to the zooxanthellae-coral symbiotic relationship. Scientists found that a coral, Acropora, lacked an enzyme needed for cysteine biosynthesis. It thus needed Symbiodinium for the production of this amino acid.
The genome size for the zooxanthellae algae is about 1, Mbp while the coral is approximately Mbp: the coral most likely rely on the algae for more than just the enzyme needed for cysteine biosynthesis 9. Sure enough, other studies have shown phosphate-linked relationships between these two species.
Zooxanthellae extracted from the Acropora coral had two acid phosphatases P-1 and P The activity of these enzymes shows that perhaps their role is involved in the mobilization of a phosphate storage compound. The exact role of these enzymes is unknown, but it seems that the symbiotic relationship between coral and zooxanthellae is phosphate limited But together, the coral and zooxanthellae can synthesize twenty amino acids 17 Figure 6.
There is also a relationship between the amount of time the tentacles of the coral spend expanded or contracted and the amount of zooxanthellae present on the coral. In general, there was lower photosynthetic efficiency in the zooxanthellae coral species that has their tentacles expanded only at night than the species with their tentacles constantly expanded. Also, the zooxanthellae density was higher in the continuously expanded tentacle species.
These differences were found only in the light however, because when the species were placed in the dark no differences were found. Thus the light has a relationship with the coral and zooxanthellae, which was assumed because zooxanthellae are photosynthetic organisms.
Conclusively, the species with continuously expanded tentacles have dense populations or small tentacles. The findings suggest that small tentacles do not shade the zooxanthellae, thus they are all visible to the light, and that dense populations are necessary to harvest the light.
So the species with these proactive properties expand continuously to collect all the light, while the species with few zooxanthellae only expand at night Another study related the exposure of the coral to oxygen as a means for oxygen radical accumulation in its tissues The O2 concentrations were found to increase by a pH of about 1.
Thus causes an increase of oxygen radicals in the coral tissues from the molecular oxygen, and the radicals can destroy cells. This study found that the anemones with higher chlorophyll, and thus higher Symbiodinium , actually adjusted their protein expression so the fluctuating oxygen concentrations would not be destructive.
This is just another example of how the coral changes its innate reactions to adjust for its symbiotic algae Figure 7. Furthermore, it was found that the temperate symbiotic sea anemone, Anthropluera balli, incorporates a maternal inheritance of the zooxanthellae because the anemone live in locations of low zooxanthellae algae.
It was found that the spawned ova consistently contained zooxanthellae, and were released into the ocean water to become fertilized and grow. The zooxanthellae was clearly integrated into the life cycle of this particular sea anemone, and was found to localize at one end of the embryo to become integrated within the endoderm, which as mentioned above is where the zooxanthellae live within coral This study brings arise the question of how zooxanthellae disperse among the coral.
Another study discovered that the zooxanthellae can be released by the host in ways such as predation, extrusion, spontaneously, osmotically, or as we know, due to temperature or stress.
This particular study proposes another way for zooxanthellae to disperse, through the feces of their predators. Interestingly, photosynthetic rates from the unharmed species were very similar to the rates from the fecal zooxanthellae that made their way through a digestive tract. Furthermore, the zooxanthellae reinfected sea anemones after their travel through the digestive tract of their predator.
This finding showed that predation is an important means by which the zooxanthellae are dispersed among a coral reef The relationship between Symbiodinium and coral has been known for about fifty years.
One of the first studies found that certain dinoflagellates fixed labeled carbon from CO2 and moved it to their host sea anemone after forty-eight hours.
This study also showed that Symbiodinium produced higher amounts of carbohydrates when living inside a host rather than free living After this symbiotic relationship was discovered, other studies delved further into how the algae and coral used the nutrients they acquired from the other. One study found specifically that the algae fixed the carbon primarily as glycerol, which was then taken up by the coral tissue as proteins and lipids It was also discovered that the other organic acids produced by the Symbiodinium were different biochemically, even though they looked the same This information was the beginning of other scientists discovering the increasingly wide variety in the taxon of dinoflagellates.
It is not entirely sure how the coral does this, but some studies have hypothesized. Grant et.
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