Climate change and coral bleaching - Curious
Coral polyps, which are animals, and zooxanthellae, the plant cells that live within them, have a mutualistic relationship. Click the image to see an animation. Rising ocean temperatures may push coral reefs to their limits. of the coral- zooxanthellae relationship, and if warm for long enough, will lead. Diagram of coral and zooxanthellae relationship. Key for above graphic. Coral polyps, which are animals, and zooxanthellae, the plant cells that live within them , have a This animation shows how the products created by the algal polyp and .
Millions of polyps working together in a cooperative colony generation after generation create the limestone skeletons that form the framework of the beautiful coral reef. Corals begin life in tropical waters as free-floating larvae.
After a relatively short period of time, the larva eventually attaches itself to a hard surface and becomes a polyp. Polyps divide asexually and form colonies.
Coral colonies reproduce both sexually and asexually. In sexual reproduction, the coral polyps release both eggs and sperm into the water. This is also known as coral spawning. One type of asexual reproduction occurs when fragments of coral are broken off as a result of storm action. The broken pieces of corals usually survive and continue to grow and produce a new colony. What Do Corals Eat? A coral polyp consists primarily of tentacles, a mouth and a gut think upside down jellyfish. Many corals are passive feeders on plankton.
Most corals also get nutrition from microscopic algae zooxanthellae living within their tissue. Coral polyps are generally nocturnal feeders and are provided sugars made by their photosynthetic zooxanthellae during the day. Corals extract calcium and carbonate from seawater to build an inner skeleton that is external to the coral. This external skeleton lies underneath a thin layer of tissue. Over the years millions of coral polyps in colonies create the framework of the coral reef.
Coral reefs grow very slowly. It may take up to a hundred years for a coral reef to grow one meter around three feet. Hard corals, also called reef-building corals, produce a rock-like skeleton made of the same material as classroom chalk calcium carbonate. These skeletons and the various shapes of different colonies form the familiar structure of the reef. Hard corals rely on symbiotic algae zooxanthellae living within their tissues for nutrition and energy to build their skeleton.
Climate change and coral bleaching
They must therefore live in shallow clear water to allow sunlight to reach the algae. Soft corals look like colorful plants or graceful trees and are not reef-building since they do not produce the hard calcified skeleton of many reef-building corals. However, soft corals do produce smaller amounts of calcium carbonate that help them keep their shape. Soft corals can be distinguished from hard corals by the fact that soft coral polyps always have eight tentacles, while hard coral polyps have multiples of six tentacles.
Symbiosis is defined as the close association between two or more interacting organisms, usually of different species. The relationship is usually classified as belonging to one of three types: Changes in the physical environment such as the amount of sunlight or salinity, or the temperature, and in the biological community, such as the presence or absence of other organisms and how they interact with the symbiotic pair, may change the nature of the symbiotic relationship from one type to another.
Like organisms, symbiotic relationships are responsive to the environment and can change over time. This enormous reef is over kilometers miles long and covers more thansquare kilometers aboutmiles. Coral Bleaching Figure 3. The coral becomes bleached because it expels the zooxanthellae, leaving 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 sudden high water temperature or uncomfortable environmental conditions will expel the algae in the open water Figure 3.
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 effects 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 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 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 A study in 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. Global Warming Figure 4. It is expected that if the ocean warms just one to two degrees, the locations that are between twenty and thirty degrees North will then fall within the range of lethality for most coral species.
Some may be able to adapt, but typically the photosynthesis pathways are hindered at temperatures rising above thirty degrees Celsius.
Thus, temperature shocks resulting from global warming results in zooxanthellae adhesion dysfunction, so they detach and are expelled from the coral 5. In a study fromit was shown that the Symbiodinium density significantly decreased after twenty-seven days of heat stress In other words, different zooxanthellae are sensitive to different temperatures, and coral can expel the old algae in hopes that the less sensitive algae will have survived and become a new symbiont.
This is an idea among scientists because zooxanthellae species diversity is very widely spread Figure 5. Horizontal gene transfer and many genetic lineages make up the Symbiodinium species, causing disparity among the clades.
So although there are many Symbiodinium-like species, this idea of clade shuffling seems slightly implausible, because it usually is a matter of Another study focused on the classification of zooxanthellae They isolated compounds that were later identified as toxins that were unique from other dinoflagellates. The discovery and research into these compounds also supported that the molecules were from the algae and not a result of the host, but it seemed that variation to the host and environment caused the production of different algal metabolites.
Many other toxins and compounds were isolated in this study and added significantly to the fact that the metabolism and taxon of zooxanthellae are extremely diverse. Furthermore, it has been shown that specific Symbiodinium are more tolerant to heat and stress, and perhaps corals adopting these specific algae will be able to survive the temperature changes from global warming and natural disasters Another study found that following bleaching, corals had clade shuffled from C2 to D, because D has a higher densities and photochemical efficiency, resulting in higher thermal tolerance The coral polyps do cellular respiration, thus producing carbon dioxide and water as byproducts.
The zooxanthellae then take up these byproducts to carry out photosynthesis. The products of photosynthesis include sugars, lipids, and oxygen, which the coral polyps thus uptake for growth and cellular respiration, and the cycle continues. The photosynthesis byproducts are more specifically used to make proteins and carbohydrates in order to produce calcium carbonate for the coral to grow.
Furthermore, the oxygen is used by the coral to help remove wastes. This recycling of nutrients in between these symbionts is extremely efficient, resulting in the ability to live in nutrient poor waters. About ninety percent of the material produced by photosynthesis is thought to be used by the coral 6. In terms of disease, the zooxanthellae is commonly the point of attack, rather than the coral itself. 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.
NOAA's National Ocean Service: Corals
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