Coral reefs are the home of approximately one-fourth of all known marine species on earth, making it the most diverse ecosystem on the planet, all while occupying less than 1% of the ocean floor. Coral Termseefs are an ecosystem with many species, however, they are vulnerable to even the slightest environmental impact. Human activities are damaging the coral reefs on the Earth. Roughly one-quarter of all coral reefs across the globe are considered to be damaged beyond repair, and two-thirds of them are under serious threat. More than 500 million people worldwide rely on coral reefs for food, income, and protection against extreme weather effects. A combination of global warming, ocean acidity, pollution, and other environmental disturbances stress corals, resulting in death and irreversible damage.
Coral reefs are now undergoing a record-breaking, global bleaching event. Corals are made up of thousands of individual organisms known as polyps. The specific types of corals that build reefs, the colonies of polyps work together to create a calcium carbonate skeleton. Reef-building corals get a majority of their energy and nutrients from single-celled algae inside of the coral polyps, called zooxanthellae. Zooxanthellae undergo photosynthesis, which is the reason why all coral reefs appear near the surface of the water. These algae are also the reason algae tend to have a green to brown color. What scientists think is happening, is that the increase in temperature due to global warming and increases greenhouse gas emissions are damaging the photosystem in the chloroplast. The symbiotic relationship is gone, so the coral expels the algae, exposing the white color of the coral. The corals are not dead, but because they get 90% of their energy from the algae, if the temperature of the water remains high the coral will eventually die. What happens next is determined by the severeness and how lasting the high temperatures are. The zooxanthellae can return to the coral within weeks if the water cools down fast enough. If not, the corals die of starvation or disease. The reefs then get taken over by an overgrowth of seaweed.
Ocean acidification is related to the carbon dioxide being released into the atmosphere and reacting with seawater. The concentration of carbon dioxide in the Earth’s atmosphere now exceeds 380 ppm, which is more than 80 ppm above the maximum values of the past 740,000 years. Carbon dioxide reacts with water to form a weak acid known as carbonic acid, H2CO3, which results in acid levels increasing in the oceans of the world. Then, when they dissociate, losing one hydrogen ion, it turns into bicarbonate. If it dissociates again, you are left with carbonate. The lowering pH of the ocean weakens animal shells and corals. Ocean acidification is a current key problem that all marine animals have to face, especially coral reefs. As previously mentioned that corals get organic nutrients from a mutualistic symbiont – zooxanthellae to grow. These algae are sensitive to acid rising in the seawater.
In recent years, as a result of human interference on coral reefs, coral mortality is higher than ever before. When this occurs, the algae immediately take this opportunity for growth. In this situation, macroalgae continue to overgrow and negatively affects the growth of corals.
During the past 30 years, more and more scientists were on to the problems of sedimentation and nutrient enrichment and chemical pollution and oil spills. Basically, the scientists wanted to understand how did sedimentation and nutrient enrichment and chemical pollution and oil spills impact on the corals? There are various mechanisms by which these factors can harm coral reefs. First of all, suspended sediment makes water turbid which allows less sunlight to penetrate the water. With less sunlight, zooxanthellae which live within the coral tissue can not photosynthesize to produce organic nutrients that support the corals to thrive. In addition, it is known that zooxanthellae are sensitive to chemical changes. In industrial wastewater and farming runoff, there are always chemical compounds which can make zooxanthellae toxic to corals as well as in the case of oil spills. After a period in this situation, corals will die. Furthermore, farming near the coast results in large quantities of sediment and soil going into the sea and onto coral reefs. This dirt, silt or sand can make water muddy, smothering the corals. In addition, the use of fertilizers for farming is another problem resulting in increased nutrient flow into the ocean. The discharge human sewage leads to the nutrient enrichment in certain areas, especially in the estuary, which may result in rapid growth of algae which as mentioned before they will outcompete corals, cutting off the supply of light.
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