Many theories have been proposed as to what causes coral bleaching and how serious the threat is to coral ecosystems. While literature holds many theories on coral bleaching, the continuing theme for this review is to what extent is coral bleaching a threat to coral reefs around the world and what is the main cause of coral bleaching. This review will explore the coral and algae symbiont relationship, its role in coral bleaching as well as reviewing past bleaching events and exploring potential adaptation solutions. The literature around these topics will be critically reviewed and analysed. Coral is home to many marine tropical and subtropical species, which rely heavily on the resources provided by the reefs - over 4000 species of molluscs …show more content…
and over 15000 fish species found in Australian coral reefs alone (Johnson & Marshall, 2007). This illustrates coral bleaching importance in marine biology, which is why the processes and possible solutions in preventing coral bleaching will be explored. Coral has an important symbiotic relationship with zooxanthellae, which is a Dinoflagellate protist (division Pyrrhophyta, class Dinophycaea, genus Symbiodinium) (Bayer et al., 2012).
The genus Symbiodinium has 8 divergent lineages from clades A to H, with C as most predominant in corals (Stat, Morris, & Gates, 2008). A close endosymbiotic mutualistic association is formed between the zooxanthellae and the host as the zooxanthellae cells of Symbiodinium live within the vacuoles of the coral host cells (Johnson & Marshall, 2007). Approximately 95% of the photosynthetic products that zooxanthellae produce are given to the host coral (Littman, Willis, & Bourne, 2011). Some sources vary on this percentage however reliable sources such as (Bhagooli & Hidaka, 2004), (Littman, Willis, & Bourne, 2011) and (Hoegh-Guldberg, 1999) state that it is a 95% transfer of photosynthetic products to the coral. Coral provide inorganic waste metabolites such as inorganic nitrogen and phosphorus as well as shelter to the zooxanthellae in exchange for organic nutrients such as amino acids and sugars, and this is well supported by all literature reviewed in this section (Stat, Morris, & Gates, …show more content…
2008). Coral bleaching is the process where corals are pushed beyond their tolerances (typically thermal) inducing stress and the relationship between the coral and their symbiont zooxanthellae starts to break down. This may occur if zooxanthellae starts to experience photoinhibition - photoinhibition is damage to zooxanthellae’s photosynthetic capacity, sensitivity to photoinhibition can vary between Symbiodinium (Hoegh-Guldberg, 1999). Loss in the symbiont causes the coral to turn white as it looses the pigmentation that zooxanthellae provide. Coral can recover from coral bleaching if conditions return to normal before the coral dies, however the stress put on the coral makes it still susceptible to disease (Johnson & Marshall, 2007). If environmental factors which have caused the bleaching do not return to normal within a couple of weeks then the coral will die and algae will start taking over and from this point no new colonies will form. Should sea surface temperatures and factors that cause coral bleaching continue to increase, then it is unlikely corals will continue to survive bleaching events. Some of the factors that cause coral bleaching include sea surface temperature, salinity and poor water quality from sediment and pollution run off (Johnson & Marshall, 2007). Corals exist where salinity is 32 to 40 parts per thousand, however flourish with salinity at 35 ppt, should this decrease due to heavy rain and flooding the corals will die (Hoegh-Guldberg, 1999). Anthropogenic activities such as land clearing increase sedimentation and this presence of suspended particles can stress the coral causing a coral bleaching response. Sediments interfering and disrupting the photosynthesis can also cause a bleaching response (Meehan & Ostrander, 1997). Coral can remove the sediment in various ways including using their tentacles to move it or trapping it with mucus however the amount of energy the coral has to use in order to do this can cause a lot of stress upon the coral which can also cause the coral to bleach (Meehan & Ostrander, 1997). While these stresses are threats to coral reefs, sea surface temperature has become the most predominant cause of coral bleaching. Recently in 2016 the Great Barrier Reef endured a mass bleaching event due to sea surface temperatures increasing with reports of 93% of the Great Barrier Reef becoming bleached ("Great Barrier Reef Marine Park Authority - Reef in Brief - Edition 51", 2016). 60%-100% of coral has been severely bleached on 316 reefs covering the northern half of the reef, reports Professor Terry Hughes from ARC Centre of Excellence for Coral Reef Studies. To the north of Port Douglas there are already measures of a 50% mortality rate of coral, with some reefs likely to reach a 90% mortality rate. Sea surface temperatures are currently 3oC warmer than the average for this time of year and the coral reef continues to experience severe heat stress ("Great Barrier Reef Marine Park Authority - Reef in Brief - Edition 51", 2016). While peer reviewed journal articles are still yet to further investigate and report this bleaching event, many articles that have been recently published corroborate these results. This in not the first mass bleaching event and it is unlikely to be the last.
Bleaching events are occurring all over the world and in the Great Barrier Reef alone there have been mass bleaching events in previous years including 1997–1998, 2001–2002, 2005–2006, 2008-2011 and now 2016. The 1997-1998 bleaching event showed that 74% of inshore and 21% of offshore reefs had moderate to high levels of bleaching with most coral recovering, except in severe areas such as Palm Island where 70% of corals died ("Bleaching Events - AIMS", 2016). In 2001-2002 54% of corals out of 641 reefs were bleached and around 70% of corals died in the Bowen area ("Bleaching Events - AIMS", 2016). 2005-2006 mass bleaching resulted in 98% of corals bleached and a mortality rate of 39 per cent on the reef flats and 32 per cent on the reef slopes ("Bleaching Events - AIMS", 2016). From 2008-2011, freshwater bleaching occurred with the Queensland floods and subsequent run off ("Bleaching Events - AIMS", 2016). The 1998 and the 2002 bleaching events were the worst on record for the Great Barrier Reef (Johnson & Marshall, 2007). While the Australian Institute of Marine Science is not a peer review journal article it is created from the work of many AIMS scientists whom are investigating coral bleaching. This source can be deemed reliable as many other sources of literature support these dates of Great Barrier Reef bleaching and the extent to at which the reef bleached. AIMS scientists return from the field and then present their findings to the institute; this is then reviewed, considered and published by AIMS, further illustrating the credibility of the source. With clear support and corroboration from various other sources it is therefore important to acknowledge and investigate this literature. The more intense the bleaching the greater the mortality it appears, and the intensity is typically determined by how high the temperatures are (above mean summer temperatures), and how long they remain at those temperatures. While many of the corals recovered from these bleaching events there were still high mortality rates and therefore if this further increases then mortality is likely to overtake reproduction rates and then coral reefs will diminish. Authorities have predicted that coral will disappear within 20-50 years should sea surface temperature increase due to global warming (Baird, Bhagooli, Ralph, & Takahashi, 2009). Not only that but Hoegh-Guldberg, 1999 also states that between 50-70% of coral reefs are under direct threat due to various human activities, thus indicating the threat humans have put upon this species. Many species will loose their home and source of food if coral bleaching kills the coral and this was evident after one of the largest global coral bleaching events in 1998 in the western Indian Ocean (GRAHAM et al., 2007). The 1998 bleaching event triggered by the El Niño that year was the first major global coral bleaching event and is said to have killed 16% of the corals on reefs around the world (Marshall & Schuttenberg, 2004). There was a correlation between the bleached coral and decline in abundance of certain fish species in studies undertaken in the --- area. The second global bleaching event occurred in 2010 and then now the third in 2016. The coral reefs are clearly in danger and if the high level of coral bleaching continues or increases dramatically then coral reefs may be lost forever. The phenomenon of coral bleaching and the threat it poses to coral reefs is well supported by most peer reviewed literature on the topic, particularly in the literature explored in this review. While the various literature explored in this review consider coral bleaching an issue to coral reefs, not all believe that it will be the end to coral reefs entirely, many provide potential solutions for coral reef survival – thus indicating diversity within the literature. Many projections and models of the danger that coral bleaching pose to the reefs excludes corals ability to adapt (Logan, Dunne, Eakin, & Donner, 2013). It has been found that some species of coral are greater affected and more susceptible to bleaching than others. Genera, such as Stylophora, Pocillopora and Acropora, have been found to be highly susceptible to coral bleaching Cyphastrea, Goniopora, Galaxea and Pavona have proven to be highly resilient in bleaching events (Baird, Bhagooli, Ralph, & Takahashi, 2009). This may be due to animal morphology and tissue thickness which could be an adaptation that coral species could embrace in order to minimise the severity of coral bleaching, this is supported by (Bhagooli & Hidaka, 2004) and (Baird, Bhagooli, Ralph, & Takahashi, 2009). The variance in susceptibility may be reflected in genotypic and phenotypic differentiation, thus indicating natural evolutionary change (Obura, 2009). A corals ability to resist stress indicates their ability to adapt to environmental changes and this could be crucial in coral reef survival should the temperatures continue to increase (Obura, 2009). The adaptive bleaching hypothesis (ABH), was first explored by Buddemeier and Fautin in 1993. The ABH theory has since been explored by various scientific papers which have looked into more specific detail however the basis on the ABH theory is that the bleaching process occurs and this allows the coral to change symbionts to a more thermally adapted symbiont. However this makes several assumptions that have no yet been proven to occur between the coral and zooxanthellae. Stat, Morris, & Gates, 2008 presented the possibility of corals adapting to predominantly harbor clade D Symbiodinium rather than clade C as clade D has been shown to have a higher tolerance to thermal stress (Stat, Morris, & Gates, 2008). However an issue which presents itself is that in the studies conducted, juvenile corals with clade C have been seen to grow up to two and three times faster than those harboring clade D (Stat, Morris, & Gates, 2008). This could mean that if corals were to change clade then coral growth will decline and subsequently reproduction (Stat, Morris, & Gates, 2008). A study between clade C and A also took place and it was found that corals which had clade A were significantly less healthy than those with clade C, this could indicate that clade A does not provide enough nutrients to the coral host thus potentially making the coral very susceptible to disease (Stat, Morris, & Gates, 2008). The main theories that are currently being investigated are symbiont shuffling, physiological acclimatisation (of both coral and zooxanthellae), natural selection and community shifts (Logan, Dunne, Eakin, & Donner, 2013). The common adaptation solution presented is for the species to adapt to a more heat-tolerant genotype or taxa or if not to become further physiologically acclimatised (Logan, Dunne, Eakin, & Donner, 2013). Logan, Dunne, Eakin, & Donner, 2013 suggest that an over prediction of current day bleaching events has occurred when using 1900–1919 climatology with the no adaptive responses model (Logan, Dunne, Eakin, & Donner, 2013). This could indicate that due to global warming the coral has already had to adapt in the past and could possibly do so once again. In saying this corals maximum adaptation to increasing sea surface temperatures is unknown and the time in which it may take corals to adapt is also unknown and therefore in the process of adaptation many coral species may already start to become extinct. Therefore further investigations into coral adaptations under continuing thermal stress is crucial. Coral bleaching is a topic that has created a great diversity of literature with a variety of studies and models. After thorough examination of the literature it is clear that this issue is of great importance and has therefore being studied extensively, this topic has been well answered by the literature reviewed to the extent at which it can be at this point in time. While many studies have been conducted there is still a lot of progress needed in order to prevent extinction on the heavily relied upon reefs. Both arguments of ‘to what extent is coral bleaching a threat to coral reefs’ provide sounds arguments which is why an alternate hypothesis is proposed – Coral bleaching is currently a threat to coral reefs globally however there are various adaptation processes of which coral and zooxanthellae may be able to undertake in prevention of future coral reef extinction. In saying this, it is clear that human impact has had a part in coral bleaching and therefore selective breeding or assisted evolution may need to occur should coral not be able adapt fast enough naturally. Further studies and investigation into coral and Dinoflagellate adaptation is imperative to coral reef survival and should be continued in the coming years.
The outer layer of a reef consists of living animals, or polyps, of coral. Single-celled algae called zooxanthellae live within the coral polyps, and a skeleton containing filamentous green algae surrounds them. The photosynthetic zooxanthellae and green algae transfer food energy directly to the coral polyps, while acquiring scarce nutrients from the coral. The numerous micro habitats of coral reefs and the high biological productivity support a great diversity of other life.
This website gives a large historical overlook and possible conclusion to the issue of coral reef bleaching. Since the 1980’s episodes of coral reef bleaching and death have occurred almost every year in one or more of the world's tropical or subtropical seas. Bleaching happens in episodes, with the most severe typically accompanying coupled ocean–atmosphere phenomena. Bleaching episodes have resulted in loss of coral
Coral Reefs are said to be the “tropical rainforest” of the sea. They are home to over 25% of all marine life (http://coralreefalliance.org.stories/storyReader$77). Over the past few decades they have been subjected to destructive anthropogenic practices. Some of the major threats to coral reefs include sedimentation, water pollution, harmful recreational activities, and global warming. All of these things cause stress on corals and can potentially cause mortality. Corals are made up of two parts, a polyp and zooxanthellae. A polyp is a calcerous body that grows from a hard part of the ocean floor. Zooxanthellae is a photosynthetic algae which lives in the polyp and provide energy for themselves and the coral. In many cases, corals undergo “bleaching,” which is a process where corals lose the zooxantheallae or chlorophyll pigment, and turn white (Wilkinson et all). After bleaching corals can survive for several months. It is possible for corals to recover by hosting more zooxantheallae, but it can take between 5 and 50 years for them to recover completely (Wilkinson et all, 1999).
Coral bleaching is when the tiny algae that lives in the coral which causes it to be bright and colourful, gets so stressed out when the water temperature exceeds the limit that it can handle, which then makes the algae
Leading scientists advise climate change will cause increases to the frequency and intensity of extreme weather events. Rising sea levels pose a significant risk to coastal communities, while the world’s oceans could become too acidic to support coral reefs and other calcifying marine organisms. Coral reefs contain only six per cent of the area of the Great Barrier Reef, yet they provide critical habitat and food for numerous species in the ecosystem. However, climate change has already impacted coral reefs in the Great Barrier Reef as corals are very helpless against its potential impacts. Eight mass coral bleaching events has occurred since 1979, triggered by unusually high water temperatures. And because of this, zooxanthellae (photosynthetic algae) leave their tissues and corals will have no more colours hence ‘bleaching’. Without the zooxanthellae, the corals that remain gradually starve to death. Once the coral dies, fish and a multitude of other marine species are soon affected. Rising sea levels and more frequent and intense storm surges will see more erosion of Australia’s coastline, causing community and residential
Imagine a lush underwater place. Beautiful structures colorful animals and places that would never have been thought of before. In reality that's not what it really looks like, really the ocean is a desolate place that is barren like a desert. Many marine species have been lost by as much as 49%.(seeker) A big part of that reason is because of coral bleaching which causes as stated above a decline in species that depend on the coral as there home. Coral bleaching is a serious problem that could cause problems for everyone if we don’t fix it.
Bleaching is when the ocean water become too warm, “corals will expel the algae (zooxanthellae) living in their tissues causing the coral to turn completely white. This is called coral bleaching. When a coral bleaches, it is not dead. Corals can survive a bleaching event, but they are under more stress and are subject to mortality.” (Cave and Gillis). When a coral bleaches, it is more of like a self defense mode, which makes it turn white. The coral loses its food source as well. The downfall of this is that without the cooler waters, the corals could die of starvation or disease. Cooler waters from below can help a bleached coral bounce back to its natural state. However, if the water stays too warm for an extended period of time, “the corals don't just bleach, they cook and they die very quickly,.” (Westcott). Throughout time, much of the corals near Cairns and northward have lost the most corals. Hughes states that, “near Cairns, the Great Barrier Reef has lost 47 percent to 83 percent of their coral and as you go northward, the Great Barrier Reef has lost 11percent to 35 percent of its corals.” (Cave and Gillis). In another report, “a study last year found the largest die off of corals ever recorded with about 67 per cent of shallow water coral found dead in a survey of a 700km stretch.” (Johnston). In 2016 and 2017, there were back to back bleaching events. According to CNN reports, “back to back bleaching events in 2016 and 2017 have devastated a 1,500 km (900 miles) stretch of the UNESCO World Heritage Site.” (Westcott). Before the 2016 bleaching event of the Great Barrier Reef, there have only been two bleaching events, 1998 and 2002. According the Hughes, “only 9 percent of the reef has avoided bleaching since 1998.” (Cave and Gillis). That means that over 90 percent of the Great Barrier Reef has been affected by coral bleaching at least once in the past 19 years.
In absence of calcium carbonate, coral and shellfish can’t survive. The ecosystem of ocean depends on these organisms because they offer a large source of food at the bottom of the food chain. They need to be abundant in order t...
...of coral. Zooxanthellae provides corals with their pigmentation as well as glucose and amino acids. Loss of zooxanthellae leads to "bleaching", a loss of coloration and stamina within the coral that lends the organism prone to disease.
Over the last century, [it is evident that] the current levels are beginning to impact organisms that make their shells out of the minerals aragonite and calcite…” (Bralower) “Coral reef[s] are highly vulnerable to ocean acidific...
Soft corals are grouped in the phylum Cnidaria, class Anthozoa, and order Alcyonacea. Their distinguishing characteristic is that their polyps always bear eight tentacles which are on both edges fringed by rows of pinnules (Fabricius and De’ath, 2002). The common name “soft coral” comes from the fact that they have no massive external skeleton as compared to the more commonly studied hard corals.
Rates of coral calcification on the Great Barrier Reef and many other reef systems around the world have declined by 15-20% since 1990 due to increasing thermal stress (De’ath 2012). It has already been mentioned that the oceans absorb almost all of the excess heat from climate change. This affects coral reef systems in what is known as coral bleaching. The term bleaching refers to the loss of corals’ colors. A coral’s color is the result of the zooxanthellae present.
Coral reefs are well known for their colorful array of marvelous sights including a parade of exotic flora and fauna. They are said to be the foundation for a quarter of marine species, and are a crucial support for human life as well. The coral reef ecosystem is a diverse collection of species (ranging from microscopic to larger-than-life in size) that interact with each other and their physical environment. If any piece of a coral reef is harmed or removed the entire community can be seriously affected, even to the point of collapse. Unfortunately, human impact has resulted in long-term stresses that, unlike the short-term stress of natural disasters, coral reefs are not as capable to recover from. “Approximately half of the world’s coral reef ecosystem resources are considered by scientists to be in 'poor' or 'fair' condition and have declined over time due to several anthropogenic threats” (“Status of Corals” par. 2). Through poorly administered commercial practices, carelessness and ignorance among the common people, and human aggravated natural processes coral reef destruction has become a serious issue afflicting the world’s biodiversity and it’s intricate biosphere mechanics (“Human Impact on the Great Barrier Reef” 1) (“What Are Coral Reefs?” 1).
Soft corals such as sea fingers and sea whips, are soft and bendable and often resemble plants or trees. These corals do not have stony skeletons, but instead grow wood-like cores for suppor...
Coral reefs need to be preserved for many reasons. In this paper, I will discuss a few of them. First of all, they house a collection of diverse organisms, and contribute to fisheries which provide food items such as fish, crustaceans, and mollusks. Furthermore, coral skeletons are being used as bone substitutes in reconstructive bone surgery and may be able to provide important medicine, including anti-cancer drugs and a compound that blocks ultra-violet rays, they even help reduce global warming by taking carbon dioxide out of the air. These reefs provide a habitat for many species.