Long-Term+Effects+of+Coral+Bleaching

=Introduction =

= = toc Coral reefs in coastal shallow seas occupy less than 1% of the ocean floor but hold 25% of all marine life. They are the //most// biodiverse and productive ecosystems in the world. Biodiversity is the measure of how many different species reside in an ecosystem. As the number of species goes down, this indicates that the ecosystem can no longer support them and thus is declining in health 6]. As the global temperature continues to increase, many coral reef systems have been undergoing more frequent and prolonged coral bleaching. Being the most biodiverse ecological system means that because of the nature of coral bleaching and its relation to temperature, coral reefs have become a good indicator for overall global ecological health of our seas as global temperatures continue to rise 3]. Consequently, many scientists have been undertaking massive long term research to understand how the rise in global temperatures is affecting one of our most fragile of ecosystems. Specifically, scientists have studied what happens to the coral as they bleach, and what the long term consequences of the disappearing coral are. Additionally, heat adaptation and possible genetic treatments have been explored to help save some of the sea’s most important species.

=Coral Bleaching =

= = Coral is an animal that has a symbiotic, or mutually beneficial, relationship with algae. The algae live inside the coral's cells, and during the day provide the coral with energy by using the sunlight to make food, a process called photosynthesis. Although the coral has polyps (similar to an anemone's tentacles) that snatch up microscopic animals from the water at night, the algae provide the coral with most of its energy. Stress from temperature causes the coral to expel the algae that are inside their bodies, thus lightening their color (bleaching) 4]. However, with the algae gone, the coral has lost its primary source of energy. Though the coral can still survive in this state, any prolonged bleaching will kill the coral.  There are a couple of different kinds of bleaching events-- partial and full bleaching. Partial bleaching is where only part of the reef or animal has undergone bleaching, whereas full bleaching is bleaching in its entirety. If the bleaching event, or how long the coral stays bleached, is short enough, the coral can recover its algae in time and survive. However, the coral is still significantly weakened, so if it is not killed outright by the bleaching itself, it is now exposed to other secondary threats like disease and parasites 8]. Most reefs never fully recover, as was commonly seen in the mass bleaching events all around the world at various times. Mass coral bleaching events reduce the health of the reef ecosystem because with each bleaching event not only does the amount of coral decrease, but the overall diversity of coral species decreases as well. As the diversity decreases, so does the recovery rate of the reefs and the overall health of the reef ecosystem.

=Coral Adaptation to Heat =

It has been found that there are some coral that can adapt to the change in heat. As we’ve discussed before, the corals are dependent on the algae that lives within them. Well, just as there are many kinds of coral, there are also many kinds of algae. Although the vast majority of them are not tolerant of the rising temperatures, research has shown that there are a few varieties of coral-algae combinations that are resistant to higher temperatures 9]. These specific corals survive because as the animal senses the rise in temperature, its occupying algae are able to switch to a more heat tolerant variety. This, however, is the exception and not the rule, as the vast majority of coral are unable to do this. It was also found that after the thermal stress event had passed, subsequent reversal to the less tolerant coral/algae combination occured 4]. Further research is being done on how quickly and for how long the coral can adopt the more heat-resistant algae. It is also important to ascertain just how heat-resistant these algae are, and if more species of coral can adopt the new algae. However, the behavior of this adaptation is still largely not understood. It is uncertain what the outcome would be if there are successive bleaching events, or if other differing species of coral could adopt the heat resistant algae 5]. What is known is that only a handful of coral species are currently able to adapt to the continued rise in temperatures 1].

=Adaptation and Coral Biodiversity =

Heat resistance in coral also brings about another discussion of diversity. As stated before, the more diversity there is in an ecosystem the healthier and more robust it is. If there were to be a mass bleaching event that is bigger than others previously recorded, It would follow that the majority of the survivors would have some kind of heat resistance. However, now exists a reef which, instead of having hundreds of coral species, only has a handful 9]10]. In evolutionarygenetics this is known as a bottleneck event. A bottleneck event is where there is a massive change in the environment (like a sustained rise in temperature) that no longer supports the entire population of various species. During the event, many variations ofa each species as well asentier species die,  leaving only a few behind. In summary, a bottleneck event takes a very diverse population and reduces it to a few very specific survivors. Thus if the environment were to shift to another unfavorable condition, like in the case of the coral a disease outbreak or another greater rise in temperature, the few remaining species will too disappear 7]. Because the rate at which the global temperature is increasing is unnaturally fast, research has concluded that any survivors from bleaching events cannot evolutionarily adapt fast enough. Consequently long-term projections state that most, if not all, corals will disappear 2].

=Applications of Genetic Engineering =

Based on the discoveries of heat-resistance in some algae/coral combinations, a new area of research has been developed. A postulation for a genetic intervention has been made by many researchers. The idea was to increase the coral’s resistance by replacing many non-heat resistant algae with their heat resistant counterparts in order to rescue more varieties of coral 9]. Another option was to engineer more heat resistant algae, and/or coral to accept that algae 10]. Whether this is done by genetically modifying the algae or the coral is still unknown. There are logistical matters as well when considering the modification of hundreds of different species of coral. Furthermore, there could be serious ecological ramifications that are still unknown. Genetic interventions and therapies are still in their infancy of understanding. Although there is a more solid understanding of the mechanisms of genetics and the manipulation of genetic material, long term effects are not understood as well. It is just the beginning in understanding the complexity of the coral-algae relationship, let alone creating an intervention. Especially when cheaper alternatives to the problem are available, it is still important to understand the symbiotic relationship between the coral and its algae. With no knowledge, no possible intervention of any form could be made.

=Conclusion =

= = The coral reef ecosystem is a complex and diverse one. Though there are signs that there are few corals that can adapt to the rise in temperatures, it is unknown whether or not they can survive more extreme temperatures. Furthermore, if only a handful of species can adapt to the rise in temperatures, this puts the whole ecosystem at risk for if these few go, they all go 10]. With further understanding of the adaptive coral and its algae, it is feasible that there are some genetic interventions that can be made. This proposition is still far in its infancy. Not only does more research need to be done on the process and consequences of genetic engineering on one species, but also the possible long term effects on the ecosystem as a whole need to be taken into consideration.

<span style="background-color: rgba(0,0,0,0); color: #000000; font-family: Arial; font-size: 16px; text-decoration: none; vertical-align: baseline;"> Continued rises in global temperatures will cause mass bleaching events to occur in totality. Research has concluded that a permanent raise of a few degrees in ocean temperature is enough to trigger total bleaching events. With the coral gone, so to will its many inhabitants disappear 1]. Nevertheless, due to the nature of how corals proliferate if there are a couple reefs left, their spawn can repopulate the earth's shallow seas.

=<span style="background-color: rgba(0,0,0,0); color: #000000; font-family: Arial; font-size: 16px; text-decoration: none; vertical-align: baseline;">References =

= = <span style="background-color: rgba(0,0,0,0); color: #000000; font-family: Arial; font-size: 16px; text-decoration: none; vertical-align: baseline;">1] Baker AC, Glynn PW, Riegl B. 2008 Climate change and coral reef bleaching: an <span style="background-color: rgba(0,0,0,0); color: #000000; font-family: Arial; font-size: 16px; text-decoration: none; vertical-align: baseline;">ecological assessment of long-term impacts, recovery trends and future outlook. <span style="background-color: rgba(0,0,0,0); color: #000000; font-family: Arial; font-size: 16px; text-decoration: none; vertical-align: baseline;">Estuarine, Coastal and Shelf Science. 80:435-471.

<span style="background-color: rgba(0,0,0,0); color: #000000; font-family: Arial; font-size: 16px; text-decoration: none; vertical-align: baseline;">2] Hoeke R, Jokiel P, Buddemeier R, Brainard R. 2011. Projected changes to growth <span style="background-color: rgba(0,0,0,0); color: #000000; font-family: Arial; font-size: 16px; text-decoration: none; vertical-align: baseline;">and mortality of Hawaiian corals over the next 100 years. Plos One. 6:e18038.

<span style="background-color: rgba(0,0,0,0); color: #000000; font-family: Arial; font-size: 16px; text-decoration: none; vertical-align: baseline;">3] Hughes TP, Kerry JT, Álvarez-Noriega M, et al. 2017. Global warming and recurrent mass bleaching of corals. Nature. 543:373.

<span style="background-color: rgba(0,0,0,0); color: #000000; font-family: Arial; font-size: 16px; text-decoration: none; vertical-align: baseline;">4] Jones RJ. 2008. Coral bleaching, bleaching-induced mortality, and the adaptive significance of the bleaching response. Marine Biology. 154:65-80.

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<span style="background-color: rgba(0,0,0,0); color: #000000; font-family: Arial; font-size: 16px; text-decoration: none; vertical-align: baseline;">9] Rowan R. 2004. Coral bleaching: thermal adaptation in reef coral symbionts. Nature. <span style="background-color: rgba(0,0,0,0); color: #000000; font-family: Arial; font-size: 16px; text-decoration: none; vertical-align: baseline;">430:742-742.

<span style="background-color: rgba(0,0,0,0); color: #000000; font-family: Arial; font-size: 16px; text-decoration: none; vertical-align: baseline;">10] Sampayo EM, Ridgway T, Bongaerts P, Hoegh-Guldberg O. 2008. Bleaching susceptibility and mortality of corals are determined by fine-scale differences in symbiont type. Proceedings of the National Academy of Sciences of the United States of America. 105:10444-10449.