The engine of the reef: photobiology of the coral–algal symbiosis
Coral reefs are built by and made up of thousands of tiny animals—coral million zooxanthellae live and produce pigments in just one square inch of coral. The survival and success of coral reef ecosystems depend on the elegant Despite the tight relationship between light and FPs (Vermeij et al., ; The dynamics of zooxanthellae populations: a long-term study in the field. . “ Reproduction, dispersal and recruitment of scleractinian corals,” in. Tiny plant cells called zooxanthellae live within most types of coral polyps. The relationship between the algae and coral polyp facilitates a tight recycling of.
This calcium carbonate bioconstruction, so extensive it is visible from outer space, is powered by the coral—algal symbiosis.
Zooxanthellae and their Symbiotic Relationship with Marine Corals
Similar to other photoautotrophs, Symbiodinium must delicately balance the sunlight absorbed and processed through photochemistry to sustain high rates of primary productivity without incurring damage.
The fixed carbon produced by Symbiodinium is translocated to fuel coral growth and calcification Goreau, ; Muscatine, Additionally, the oxygen produced as a by-product of photosynthesis may promote maximum coral calcification rates Colombo-Pallotta et al. In return, corals provide their endosymbionts with essential nutrients in a safe, sunlit habitat in nutrient-poor oceans.
This symbiosis is unique because it involves two eukaryotic organisms and the genome of the symbiont is three times larger than the genome of its host Shinzato et al. Prokaryotes and viruses are also associated with corals and Symbiodinium, but their roles are mostly uncharacterized Ainsworth et al. The tight recycling and conservation of nutrients within the coral holobiont the coral and its collective community allows coral reefs to thrive in tropical nutrient-poor oceans.
It should also be noted that there are corals without Symbiodinium and they do not require sunlight for nourishment nor build coral reefs and thus are not discussed in this review.
Click the image to see an animation. In addition to providing corals with essential nutrients, zooxanthellae are responsible for the unique and beautiful colors of many stony corals.
- Smithsonian Ocean
- The engine of the reef: photobiology of the coral–algal symbiosis
Sometimes when corals become physically stressed, the polyps expel their algal cells and the colony takes on a stark white appearance. If the polyps go for too long without zooxanthellae, coral bleaching can result in the coral's death. Because of their intimate relationship with zooxanthellae, reef-building corals respond to the environment like plants. Because their algal cells need light for photosynthesis, reef corals require clear water.
For this reason they are generally found only in waters with small amounts of suspended material, i.
Zooxanthellae and their Symbiotic Relationship with Marine Corals - microbewiki
Zooxanthellae cells provide corals with pigmentation. For example, the Montastrae species, which causes Yellow Band Disease, affects the zooxanthellae directly rather than the coral 7. 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: 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. Movement 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 History 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.
NOAA National Ocean Service Education: Corals
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.coral reef and coral bleaching
Other studies suggest that the host coral produces compounds that act as host release factors, and that these factors can control the metabolite production in the Symbiodinium Energy Storage Not only are nutrients shared between the two species, but energy and energy production is integrated as well. The Symbiodinium produced these lipids, using acetate from the coral and extra ATP, and excreted them back to their host.
These lipids are mostly wax esters and triglycerides A figure showing the decline in zooxanthellae over a starvation period http: It was further shown that the retention of this ammonium by the coral was related to the Symbiodinium because the algae uptakes most of the ammonium itself The algae were also more efficient with its use of a nitrogen source because it can use nitrite.
A study used tagged enzymes involved in the use of different forms of nitrogen, and concluded that the algae do indeed utilize nitrates. They also found that the algae densities increase with the nitrate concentration, although further details of this relationship with the coral are not known