didiertomas2385
22.04.2021 13:14:11
Liz is a marine biologist, environmental regulation specialist, and science writer. She has previously studied Antarctic fish, seaweed, and marine coastal ecology.
See full version: What Causes Coral Bleaching, and Can Our Reefs Recover
Liz is a marine biologist, environmental regulation specialist, and science writer. She has previously studied Antarctic fish, seaweed, and marine coastal ecology.
The devastating effects of coral bleaching extend to humans, too, since coral reefs are considered major food sources. Tourism linked to coral reefs makes up an estimated $36 billion-dollar industry upon which many economies are built. The complex, 3D structure created by corals also protects adjacent shorelines by dampening the impact of incoming waves. When coral reefs bleach, these benefits are greatly diminished. A bleached reef has fewer fish available for human consumption. Similarly, a reef lacking its world-famous colors and diverse marine life provides a blow to the tourism industry. [links]
These stressful situations are thought to cause serious damage to the coral's zooxanthellae, preventing the algae from photosynthesizing properly. Normally, the coral digests damaged zooxanthellae as part of the animal's natural maintenance process, but when large swaths of zooxanthellae are damaged all at once, the coral cannot keep up. The build-up of non-functional zooxanthellae can cause damage to the coral itself, leading a coral to forcefully release its algal inhabitants in a desperate attempt at self-preservation. here
Ocean acidification represents a key threat to coral reefs by reducing the calcification rate of framework builders. In addition, acidification is likely to affect the relationship between corals and their symbiotic dinoflagellates and the productivity of this association. However, little is known about how acidification impacts on the physiology of reef builders and how acidification interacts with warming. Here, we report on an 8-week study that compared bleaching, productivity, and calcification responses of crustose coralline algae (CCA) and branching (Acropora) and massive (Porites) coral species in response to acidification and warming. Using a 30-tank experimental system, we manipulated CO(2) levels to simulate doubling and three- to fourfold increases [Intergovernmental Panel on Climate Change (IPCC) projection categories IV and VI] relative to present-day levels under cool and warm scenarios. Results indicated that high CO(2) is a bleaching agent for corals and CCA under high irradiance, acting synergistically with warming to lower thermal bleaching thresholds. We propose that CO(2) induces bleaching via its impact on photoprotective mechanisms of the photosystems. Overall, acidification impacted more strongly on bleaching and productivity than on calcification. Interestingly, the intermediate, warm CO(2) scenario led to a 30% increase in productivity in Acropora, whereas high CO(2) lead to zero productivity in both corals. CCA were most sensitive to acidification, with high CO(2) leading to negative productivity and high rates of net dissolution. Our findings suggest that sensitive reef-building species such as CCA may be pushed beyond their thresholds for growth and survival within the next few decades whereas corals will show delayed and mixed responses.
Effects of experimental ocean acidification… [links]
The authors declare no conflict of interest. here
These organisms exist in isolated patches at depths down to 2000 meters. In these conditions, the corals grow considerably slower than shallow-water reefs. They feed on zooplankton and in some cases use chemicals coming out of the sea floor for a source of nutrition. Like their shallow-water counterparts, deep coral reefs provide a habitat for a diverse array of creatures. However, at the same time, it is clear that deep corals are in a particularly precarious situation, as we will see later on. more
Experiments are one of the best ways to forecast the future, but they have significant limitations. In particular, it is impossible to replicate natural growing conditions in the lab, and further, experiments are conducted at intervals that are significantly shorter than the changes that are occurring in nature. more
The recent decrease in CO3 2- has also begun to lower calcification rates of the coralline algae. These species are composed of high Mg calcite, which is the most soluble form of CaCO3 (more so than low Mg calcite and aragonite), so they are particularly prone to ocean acidification. Experimental work confirms that calcification of the coralline algae is particularly sensitive to CO2 levels with growth rates slowing significantly, and actually, dissolution beginning at moderately high CO2 contents. [links]
So, we are left with a lot of questions: will the rates of saturation decrease and temperature rise be too rapid for modern species to adapt? Will algae take over the niche of shallow-water corals and dominate the low pH oceans of the future? Or will a few species of coral and possibly coralline algae develop the ability to calcify rapidly enough to survive the current threats and take over the niche of species that do not? Will Zooxanthellae themselves be able to adapt and assist corals in calcifying? Finally, when will feedbacks, largely through weathering, come into play and make conditions more favorable for calcifying organisms? For some of these questions, it’s a matter of wait and see. However, ongoing research should shed light on others. For example, the genetics of coral populations are currently being explored to understand the ability of corals to adapt to environmental change.
The increase in acidification of the ocean occurs when the ocean waters absorb large volumes of carbon dioxide released into the atmosphere through the burning of fossil fuels. This condition creates an acidic medium in the waters that in turn lowers the pH. The weak carbonic acid formed reacts with and destroys the calcium carbonate which is the primary shelter for the corals. [links]
Almost 25% of marine organisms rely on the coral reefs for food, shelter, and as a breeding ground. They form a primary habitat for more than 4,000 species of fish, 700 different species of corals, and thousands of other species of flora and fauna. Due to their significance to marine life, these reefs are sometimes called the rainforests of the sea. Coral reefs also protect the coastlines from storms and erosion and act as medicine. Additionally, corals are used as souvenirs for home decoration and in making jewelry.
Cyanide fishing, blast fishing, or overfishing using trawlers can be destructive to huge numbers of coral reefs. The Great Barrier Reef which is home to thousands of different species of fish, sea urchins, turtles, snakes, whales, dolphins, mollusks and many more is under threat of coral bleaching due to destructive fishing practices.
However, this perfect mutual relationship between the coral reefs and the algae may be short-lived if the corals detect stress in their environment. The stress can come as result of change in conditions such as elevated temperatures due to global warming, run-offs, and pollution, overexposure to sunlight, oceanic acidification or lack of nutrients. To survive under these conditions, the reefs will expel the photosynthetic zooxanthellae living in their tissues causing them to turn completely white. In this state, the corals are vulnerable to diseases. more
Coral and algae have a symbiotic relationship. The microscopic algae called zooxanthellae live embedded in the tissues of the coral. Like most plants, the zooxanthellae can undergo photosynthesis. This process entails the conversion of simple inorganic substances such as water and carbon dioxide in the presence of sunlight to glucose and oxygen. The glucose produced forms the coral’s primary food which provides energy to it. As a result, the reefs grow much faster and healthier than if they relied on planktons for food. It is the presence of the algae that give the corals their characteristic brown color. In turn, the corals supply the algae with chemical components such as ammonia and phosphates that are very crucial for their survival.
Coral reefs are large underwater structures composed of skeletons of marine invertebrates called corals. The most extensive coral reefs are found in clear shallow waters in the tropics and subtropics with the largest being the Great Barrier Reef in Australia which is 2,400 kilometers long. Coral bleaching is a major environmental concern, a process that can lead to the death of corals present in the reefs. However, bleaching is not a completely irreversible process. The article strives to understand the process of bleaching and its harmful effects and also mentions how the process can be reversed.