Course:EOSC270/2022/Assisted Evolution of Coral Reefs

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Global Climate Change and Coral Reefs

What is the Problem?

Coral Reef Bleaching
Comparison of healthy and unhealthy reefs[1].

When corals are exposed to short-lived temperature increases or an overall increase of one degree celsius, they begin to bleach. When the bleaching occurs, the photosynthetic algae that lives within the cells of the coral develop a fault, ejecting the living symbiont[2]. Future mass coral bleaching events could lead to wide spread extinctions of many organisms that rely on the coral for food and shelter[3].

Coral reefs across the world’s oceans are in the midst of the longest bleaching event ever recored(from 2014 to at least 2016)[4]. Already ∼20% of the world's reefs are lost and ∼26% are under imminent threat[5]. With the increasing frequency of El Niño events, coral bleaching is happening more and more frequently. The Third Global Coral Bleaching Event (GCBE3) bleached and killed tropical reef corals throughout the world from mid-2014 through mid-2017[6]. The major El Niño event of 2015–2016 resulted in significant warming of large areas of the tropical oceans and continued bleaching of substantial areas of reef, including Australia’s Great Barrier Reef in early 2016[7].

Global and Local Reasons for Coral Reef Bleaching

Climate change is the most important reason which cause coral reef bleaching. Reefs are among the most sensitive of all ecosystems to climate change[4]. During the 20th century, due to globalization and industrialization, the amount of carbon dioxide in the atmosphere increased, leading to a greenhouse effect which changed ocean temperature and chemistry. On a global scale, anthropogenic modification of chemical and physical atmospheric dynamics that cause coral death by bleaching and newly emergent diseases due to increased heat and irradiation, as well as decline in calcification caused by ocean acidification due to increased CO2, are the most important large-scale threats[5]. On a local scale, factors that cause coral reef bleaching are overfishing and destructive fisheries, coastal construction, nutrient enrichment, increased runoff and sedimentation, and the introduction of non-indigenous invasive species[5].

Areas Where Coral Reef Bleaching is happening

Geographically speaking, coral reefs bleaching happens more often in low latitude(< 26° latitude) area[8]. However, high latitude coral reefs (>26° latitude) have been largely unaffected by thermal bleaching[8]. This might because the relatively stable seawater temperature regime in which they occur and the documented climate-induced pole ward shift (over geological and ecological temporal scales) of coral reef taxa[8]. Coral reefs face increasing pressures particularly when on the edge of their distributions[8].

How does this problem impact marine ecosystems?

When exposed to extreme temperatures, corals will bleach[9]. Populations of other organisms within these ecosystems decline as these corals decline in abundance[10], which may affect ecosystem function[10]. Many organisms are at risk of decline after coral bleaching events, some due to direct impact (such as fish and motile invertebrates which rely on the corals for food and shelter[3]). This coral loss can become the cause of local extinctions, biodiversity loss, or a change in components of these marine communities[3]. In extreme cases of bleaching, the number of fish species could decline more than 60%[10].

The reason that ocean warming can have such a devastating impact on coral reefs is due to the particular sensitivity of reef building corals to temperature rises[11][12]. Because corals are essential to both the structure and function of coral reefs[13], the vulnerability of corals reflect the vulnerability of the ecosystem[10].

What is the extent of the problem?

Over the past decade ocean temperatures have been on a steady increase due to fossil fuels trapped in the Earth's atmosphere, and the oceans have been absorbing about 90% of the remaining heat energy created by CO2[14]. Since the 1970s, the oceans have been warming an average of 0.11 degrees each decade. Even though this is a small amount, the impact it has on marine life is immense. The photic zone has been warming the quickest, absorbing a majority of the heat energy, making it difficult for marine species to survive with even the slightest change in temperature causing stress on the animals.[15]

Local solutions; Assisted Evolution

Many small-scale solutions have been proposed which may combat the effect of global warming on corals and coral reef ecosystems, including assisted evolution[16]. It has been suggested that some combination of these solutions may be the best option[16].

The above image displays corals in Hawaii following a bleaching event. The bleached corals display no visible pigmentation, whereas the survivors of the event show colour. These survivors may be deemed super-corals due to their apparent tolerance to heat.

What is Assisted evolution?

Assisted evolution uses natural selection, along with other evolutionary processes[17], to rapidly enhance specific traits in populations based on genetic material that already exists[16][17]. In the case of coral reefs, super-corals may provide this genetic material[18]. Super corals are corals that are notably resistant to change; traits such as extreme thermal tolerance may qualify a coral for this title[18]. They may hold the key for a hopeful tomorrow, because their very existence indicates that adaptation to extreme conditions can occur naturally[17]. Small populations of super corals exist (for instance, the northern region of the Gulf of Aqaba has communities of super corals[18]), and may be able to provide the genetic material needed to strengthen dying reef populations[18].

Ruth Gates and Madeleine van Oppen were both front runners in the assisted evolution of coral reefs using the following techniques, described below[17].

Preconditioning or Epigenetic Programming

In controlled environments, scientists exposed sexually matured coral colonies to fluctuating stresses related to climate change and global ocean warming[17] (such as an ambient or low pH[19]). The general goal is to induce heritable traits related to stress tolerance and fitness which may protect future generations of corals[17]. More generally speaking, preconditioning (or epigenetic programming) works to alter the heritable genetic material of adults to make them more tolerant to climate change in the hope that they may pass those traits on to their offspring.

The above image displays a coral as its microbiome is sampled. The microbiome may then be used to determine the community of symbionts that make up part of the holobiont of a coral.

Manipulation of Community Composition

The manipulation of community composition, in the case of coral reefs, relates to the microbial community associated with a coral holobiont[17]. In other words, the community that Gates and van Oppen are referring to is the microscopic community that lives in and around corals, including organisms such as Symbiodinium, prokaryotes, fungi, and viruses[17]. The intention is to interfere with this community to assist in the adaptation of corals to different conditions; different microorganisms have different effects on corals, which may be harnessed to help the coral adapt[20]. They are termed probiotics when they are used to improve the health of their hosts. In practice, researchers introduce corals to probiotics via inoculation, and the microorganisms colonize the coral host. This may increase the rate of adaptation to days or weeks[20].

Laboratory Evolution of Symbionts

This refers to the laboratory evolution of microscopic, cultured symbionts (organisms such as Symbiodinium). These photosynthetic organisms have a role in coral thermal tolerance[21], so the symbiotic relationship must adapt to the changing ocean environment[21]. To assist evolution in a controlled environment, Symbiodinium were exposed to increased temperatures and pCO2 selection[17][21]. Once the symbionts have evolved to meet the growing temperatures, coral hosts are united with the evolved symbiont cultures[17].

Selective Breeding

The selective breeding of coral populations requires specific conditions be taken into consideration; conditions related to geographical distribution, cross-fertilization abilities, and genetic markers of stress tolerance are taken into account[17]. Breeding corals from different thermal environments can then give insight into their genetics and can help identify genes that relate to heat tolerance[22]. Th is allows scientists to determine the feasibility of future genetic or evolutionary interventions[22].

How is Assisted evolution helping?

Performance of Preconditioning

Studies show that the DNA methylationadds an extra functional group to DNA which will give an organism the ability to generate new characteristics in different environments. The results vary from species to amount of time exposed to methylation. This process provides an opportunity for corals facing a rapid change in ocean temperatures to adapt and thrive. Understanding the how cells regulate the conversion of DNA to RNA will allow us to identify the signals expressed and locate the genes location[19]. The processes of modifying the gene expression has given rise to evidence that DNA Methylation is mostly reactive to a change in the environment depending on the surrounding conditions and heritability of the organism.[23]

Results of Manipulation of Community Composition

Calcification rates shown in studies suggest that certain symbionts in cauliflower (Pocillopora) corals could aid the calcification rates with respect to decline of pH, reducing the the extent of ocean acidification (OA).[10] The relationship between symbionts and their hosts, Pocillopora corals, seems to have the ability to regulate the destruction of ocean acidification on coral ecosystems. As well as having other benefits including protection from predators.[10] Stress-resistant microbes are a great way of shielding the coral communities from strains such as higher temperatures, UV radiation, and salinity.[17]

Development of Laboratory Evolution of Symbionts

Determining whether or not the selection of certain symbiodinium's could strengthen the tolerance the holobiont has to heat was done by collecting samples of three different corals and keeping them in a sea simulator for three days.[21] The symbiodinium was evolved over a timescale that was appropriate with the velocity of climate change. Findings suggested that growth rate is an important feature when selecting to increase the thermal tolerance in free-living symbiodiniums.[21] Corals are also exposed to stressors achievable in the laboratory such as high intensity lights to enhance the tolerance to other stressors like temperature and pCO2.[17]

Selective Breeding Effects

Selective breeding studies suggest that the identification and utilization of precise variants of a gene effects the genomic diversity of the offspring. Connecting specific characteristics to key traits that increase the survival of a species will assist in strengthening the resistance to stress. with the intent of these altered traits being passed down to their offspring. This strategy is on the quicker side of interventions, occurring within only a few generations.[22]

Are there any issues with Assisted evolution?

When discussing the topic of assisted evolution, the subject matter of ethics is introduced. Assisted evolution involves the modification of organism’s genes to survive conditions that humans have created. Disrupting the natural genetics of species is greatly debated; many people argue to focus more on the future, minimizing damage, and preserving the organisms we have left.[24]

It has been argued that scientists can't fully understand the risks of gene spicing. If mutated individuals interbreed, it is difficult to determine which genotypes are passed on to offspring, which could create even bigger issues. With little to no data to work off the ability to fully understand how genomes work is not enough to change DNA and be positive of the outcome. Many question how far we should go with genetic tampering and where we draw the line.[24]

What is the solution (global)?

Climate Change Threatens the Survival of Coral Reefs

Coral reef is one of the most important ecosystems in the ocean, where countless marine organisms gather and reproduce and plays an important role in maintaining the marine ecological balance. As the rapid development of industry in the 21st century, the global marine ecological environment is becoming more severe. As the rapid development of industry in the 21st century, the global marine ecological environment is becoming more severe. In the setting of global warming, the increase of carbon dioxide content will lead to ocean acidification, global warming, sea-level rise and other effects. These changes may have a negative impact on corals. At the same time, the response of corals to these changes is very complex. Over two thirds of the world's coral reefs are already seriously damaged or in danger of further degradation, and climate change is recognized as the biggest long-term threat to coral reefs[25].

Assisted Evolution in Global

Climate change is altering the marine environment, while lots of marine organisms don’t have enough resilience to such rapid change. Assisted evolution is to enhance recovery and increase the rate of naturally evolutionary processes[26]. Although coral reefs is still one of the most important techniques for coral conservation, and it could also accelerate natural recovery, but it has limits and the efficient could be low if the conservation targets are in a wide range[27]. But assisted evolution could permanently change “natural” genomes, which means that there is no requirements for management by people[28]. In order to formally implement assisted evolution on a global scale, global laws are important to regulate how the governments develop and utilize marine resources. It announced that those countries assigned the principle should have an obligation to assist in these protection efforts[29]. According to the World Heritage Convention (WHC)23 and the Convention on Wetlands of International Importance (Ramsar Convention), the coral reefs are considered into one of the marine protected areas.[29]

However, genetic engineering will always have a lot of controversy as it emphasizes human intervention. If environmentalists want to apply assisted evolution in global, a thorough risk assessment is very important. This risk assessment should consider not only its own risks of the mutation accumulation, but also broader ecological impacts. For example, the modified species may have indirectly influence in surrounded environment though competition or predation[28]. All in all, the application of genetic engineering will indeed bring new vitality and hope to environmental conservation, but the potential negative factors are also unpredictable.

Prevent Further Deterioration of Climate Change

Rather than working on the existed damage to the ecological environment, the timely implementation of protective measures is the most important. To protect the ecological integrity and biodiversity of the earth, the fundamental solution is to reduce human intervention in nature as much as possible. As lots of damage and degradation of environment are irreversible, prevention of further climate change is a sustainable and a long-term solution to protect not only coral reefs but also other natural creatures.


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