Introduction
Ocean acidification and global warming represents a significant threat to the marine ecosystem across the world (Kroeker et al. 2013). The forecast of the ecological impacts of global warming and ocean acidification on marine organisms is so serious that it has now become one of the highest priorities for management, science and environmental policies. A comprehensive understanding of variable effect of global warming and subsequent ocean acidification on the marine organisms is a vast issue (Kroeker et al. 2013). According to Kroeker et al. (2013), in response of global warming and ocean acidification the effect of marine species is diverse and variable across all the marine taxonomic groups. So in order to limit the scope of the research and make it more specific, only coral reefs and its transformation arising out of ocean acidifications is being scrutinised. This is because coral reefs are crucial for the betterment of ecosystem and the nature of facilities they provide to the subtropical and maritime nations(Guest et al. 2012). However, the majority of the coral bleaching events occur as a result of disruption of the equilibrium between the scleractinian corals and their subsequent algal symbiotants (Guest et al. 2012). The bleaching of corals is not just a process of wearing ripping of the algal symbiotants from the coral counterpart, but it also lead to increase in the rate of mortality among the coral reefs of the ocean (Guest et al. 2012). Ocean acidification and global warming cast extinction threat towards the coral reef ecosystems due to the process of large-scale coral bleaching events are directly associated with the increase in the temperature of the sea along with the acidification of sea water (Kaniewska et al. 2012; Humanes et al. 2017). Thus it can be rightly concluded that coral reefs, one of the vital counterpart of the Earth’s ecosystem is facing increased threat of extinction arising out of global warming and ocean acidification (Guest et al. 2012).
The following assignment aims to perform a critical review of literature directed towards the effects of global warming and ocean acidification on the ecosystem of the marine corals. In order to perform the critical review of literature, a specific methodology for the selection of literary articles will performed Upon selection of the articles, two themes will be generated and based on which a definite picture of the affect of global warming and ocean acidification on coral reef will be draw. At the end, the review will aim to provide a compact conclusion followed by possible recommendation in order to overcome the condition.
Rationale
Coral reefs are biologically diverse and highly productive marine organisms. They are crucial for billions of coastal dwelling people across the planet whose livelihood is based on industries like tourism and fishing(Kaniewska et al. 2012). However, coral reefs are recently experiencing significant increase in global stressors elements. Ocean acidification and global warming have evolved as principal threats towards the survival of coral reefs in long term. Rapid warming of the ocean temperature is causing mass bleaching of the coral reefs while steady acidification of oceans have lead large decrease in the carbonate ion concentration and this in turn have decrease the ability of marine calcifiers to precipitate calcium carbonate(Kaniewska et al. 2012). According to Kwiatkowski et al. (2015), mass bleaching of coral can, however does not necessarily, result in the generation of extensive coral mortality. As per the climate projections, at present the conditions which are causing bleaching will occur more frequently on coming decades (Kwiatkowski et al. 2015). Coral bleaching leads to the degradation of the population of the coral reef and is the major concern for the environmental managers(Humanes et al. 2017)(please refer to figure: 1). The early history of life stages of corals is essentially susceptible to quality of the ocean water and climate pressures. Proper understanding of the additive impacts of these stress factors on the life stages of coral reefs is essentially crucial for undertaking proper management initiatives which is aimed towards abiding the resilience of coral reef communities (Humanes et al. 2017).
The following review of literature aims to highlight two most important factors associated behind disruption of the coral population in the marine life via coral bleaching. The review also aims to throw light on the possible consequence of coral bleaching and how it affects the marine diversity. At the end, the review will recommend strategies that can be under-taken in order to overcome the environmental threats coming from coral bleaching or loss of coral diversity.
Methodolog
Thematic review of literature was chosen in order to analyse the effect of global warming and ocean acidification on coral bleaching. For performing thematic analysis, literary articles were retrieved from the databases like Medline and PubMed on the basis of the broad key word search. The keywords those were used for article search includes corals, coral reefs, coral AND ocean acidification, coral AND global warming, coral bleaching AND ocean acidification, coral bleaching AND global warming. The main inclusion criteria was research articles published within last 10 years in order to get actual scenario of the coral bleaching in present day global warming scenario. Other inclusion criteria include, research articles published in English, and also include both primary and secondary literatures. The main exclusion criteria include the papers published on the interaction of coral with algae with coral and effect of coral on marine ecosystem. This is because, these two processes were out of the scope of the study. Based on the keyword search 30 articles were found relevant. Then on the basis of title scrutiny, 15 were found relevant with the scope of the review. After analysing the abstract of the study, 10 were found to be relevant. Based on these 10 articles two themes for the review was generated. Then those 10 articles were read thoroughly and out of which 2 articles were selected for critical analysis for two selected theme.
Coral Bleaching in Relation to Ocean Acidification
High level of atmospheric carbon-dioxide and subsequent increase in acidification of ocean has been cited as major factors responsible for extinction of coral reefs in present geological time. Atmospheric carbon dioxide reacts with the salty ocean water which leads to the production of carbonic acid(Kaniewska et al. 2012). The proton (H+ ions) formed as a result of dissociation of the carbonic acid (HCO3-) into bicarbonate ions (CO32-) react with the carbonate ions leading further production of carbonic acid. Increase in the amount of carbonic acid decreases the pH of the ocean, making it extra acidic while reducing the amount of free carbonate ions available for the marine organism. This acidified oceanic atmosphere leads to coral bleaching(Kaniewska et al. 2012).
According to Kaniewska et al. (2012), increase in the acidification leads to coral specific responses which result from the increase in the production of the reactive oxygen species (ROs) and/or increase in the concentration of the reactive nitrogen species (RNS). Kaniewska et al. (2012) cited disturbance of the Symbiodinium cell and host response (mitochondrial ATPase activity) as the two major factors behind the generation of ROS and RNS. The production of ROS in the coral cells has been further indicated by the up regulation of the FAD-linked oxidase, catalase and seleno-protein. This increase in the concentration of the ROS and RNS causes down-regulation of calmodulin (CaM) and this in turn results in the disruption of the calcium homeostasis. Kaniewska et al. (2012) sated that disruption of the equilibrium of the calcium homeostasis lead to increase in the internal calcium [Ca2+] ion concentration. Increase [Ca2+] causes modification of the actin cytoskeleton followed by further modification of the membrane cytoskeleton and cell receptor/adhesion protein(please refer to figure 2). Due to the modification of the cell-surface protein, the coral will start expelling the algae (zooxanthellae) residing in their surface tissues thereby influencing the coral to turn completely white, a procedure known as coral bleaching (Kaniewska et al. 2012)(please refer to figure 3).
According to the reports published by Hooidonk et al. (2014) aragonite saturate state is the major significant factor behind the bleaching of the corals. Hooidonk et al. (2014) have further highlighted that ocean acidification and its effects on coral bleaching further aggravates upon in the thermal stress and this is irrespective of the genetic backup of the coral reefs and its metabolic outcomes as proposed by Kaniewska et al. (2012). As per the reports as per reports published by Hooidonk et al. (2014), severe yearly bleaching is expected to initiated 10 to 15 years after in the high-latitude reefs in comparison to those of reefs present in lower latitudes. Within this tenure of 10 to 15 years, the aragonite concentration keeps on decreasing and therefore any benefits of the high latitude reeds on the late onset of the annual bleaching can be effectively reduced by the effects of the ocean acidification. However, Hooidonk et al. (2014) have further opined that irrespective of the genetic backup of the coral reefs, there is no long-term refugium arising out of the acidification and subsequent coral bleaching. The studies published by Tolleter et al. (2013) further proved in accordance with the studies of Kaniewska et al. (2012). According to Tolleter et al. (2013) light is not directly responsible for the formation of the reactive oxygen species. Tolleter et al. (2013) opined that there is no relation of light behind the process of coral bleaching. Thermal stress along with ocean acidification leads to the process of coral bleaching. Tolleteret al. (2013)is of the opinion that the process of coral bleaching is a significant threat to mankind as it is increasing the risk of disrupting the equilibria of the marine ecosystem.
Global Warming and its effect on Coral reefs
The impact of change in climate on coral reefs, presents interesting opportunity to study coral resilience. This is because; reefs are the first ever ecosystems that experiences chronic impact of change in the climate or more previously environmental temperature. Therefore, the lessons from the effect of global warming on the coral reefs may providesignificant insight for other marineecosystems that are yet to suffer significant impacts of the adverse consequences of global warming. Moreover, one climate-associated driver can cause numerous impacts on the marine reef system. One of the best suited examples is rising sea surface temperature (SST). According to Solomon (2007), global SST has increased 0.74°C in the 20th century. This seems to cast an adverse impact on the growth and calcification of the huge coral spp. of Great Barrier Reef and thereby leading at least 14% decrease in the process of calcification since 1990 De'ath et al. (2009).Rise in SST is also responsible for the process of coral bleaching which in turn damages the coral reef. According to Friele et al. (2013) exceptionally high SST in summer, for example only +1°C rise in ST can result in mass mortality at global to regional scales of coral reef Friele et al. (2013). The magnitude and frequency of profound bleaching is expected to rise potentially as SST continues to increase Friele et al. (2013). Thus, risein SST has both anacute and chronic impact on the overall marine reef ecosystems. The massive negative impact hampers the extension rate of the carbonate skeleton of corals, and thus influences increase in the size-dependent mortality rates along with disturbed ecological interactions and decrease population recovery rate (Humanes et al. 2017). Changes in the ecological parameters are supposed to increase the instability ecosystem system thereby potentially decreasing the size of the coral basin and expanding the live span of reefs shifting towards a substitute attractor. The influence of the global warming or change in the other ecological parameters causes sudden mortality. This pushesthe system closer or at times across an unstable equilibrium. This again reduces resilience (Humanes et al. 2017). Each impact of rise in global temperature is supposed to occur on a different span of time and is likely to differ among species. Thus, single driver (SST) emerge tremendous potential for complex dynamics. Moreover, it is also not clear that whether the duality of its effects will be synergistic, additive or antagonistic (Humanes et al. 2017).
According to, Humanes et al. (2017),the effects of global warming, nutrient enrichment and suspended sediments are most prominent on the mechanisms occurring on the earliest life history stages of the corals (fertilization followed by embryo development). As per the findings of Humanes et al. (2017), environmentally realistic yet increased levels of high temperatures or suspended sediments at nights can have significantadverse impacts on the reproductive success of Acroporatenuis. According to Friele et al. (2013), apart from global warming, unfavourable climatic conditions which hamper the gamete fertilization and embryo development of the coral include strong waves wind, currents, dredging and flooding. All these result in decreased reproductive success rate of A. tenuis populations.
Friele et al. (2013)have opined that global warming or upliftment in the temperature of the ocean results in the formation of the suspended sediments. These suspended sediments affect the coral gamete fertilization and thereby reducing the marine coral population. Humanes et al. (2017) highlighted the primary mechanism that effects the coral gamete fertilization. According to Humanes et al. (2017), suspended particles lead to the coagulation of the sperm of A. tenuis. Sperm coagulation in turn leads to lowering of the concentrationof the sperms present in the nearby vicinity of eggs. This reduces the total sperm-egg encounters and thereby reducing the success rate of fertilization. In order to study the effect of suspended sediments of gamete fertilization of A. tenuis, Humanes et al. (2017) employed silt-clay sediments. According to Mumby, Steneck and Hastings (2013) have, slit-clay sediments are extremely colloidal and cohesive due to their strong chemical and physical characteristics (small size of grains). Mumby, Steneck and Hastings (2013) have further opined slit-clay sediments readily form flocs that are likely to attract coral sperm and thereby narrating their viewed effect on success of coral gamete fertilization.
Results elucidated by Humanes et al. (2017) on the effects of high under water temperature on fertilization success of coral are in sync with the studies documented by Carricart-Ganivet et al. (2012). According to Carricart-Ganivet et al. (2012), elevated temperatures are associated with the reduced fertilization success of the coral. Carricart-Ganivet et al. (2012) highlighted reduced motility of sperm flagella as the principal reason behind which would decrease the number of sperm-egg interactions, disruptioninthe equilibrium of the biochemical processes that are involved in early cell division of coral embryo. Carricart-Ganivet et al. (2012) opined that there is no significant effect on the nutrient enrichment overthe fertilization success of marine coral under high temperature. However the study undertaken by Humanes et al. (2017) showed that survivorship of larvae was the single biological process that remains unaffected by the exposure nutrient enrichment treatments, when performed before the development of developmentof cilia. However, Humanes et al. (2017) reported that larval settlement success was reduced significantly when the fertilized embryos were exposed to suspended sediments. This revealed latent effects of the treatments of larval fertilization. Coker, Pratchett and Munday (2012) proposed a possible mechanism via which the suspended sediments react with the development of the coral embryo. According to Coker, Pratchett and Munday (2012) sediments lead to the production of mucus via early embryos of A. tenuis and this leads to decrease in success rate of gamete fertilization. However, Humanes et al. (2017) did not foundanysignificant effects on larval survivorship or settlement. The main variance between the two researchs was that Coker, Pratchett and Munday (2012) employed carbonate sediments and Humanes et al. (2017)employed silt-clay based sediments. Humanes et al. (2017) results indicate that the stressors elements that affect the energy demands, cellular function, or give rise to mechanical abrasion during the process of embryo development may have negative effect onthelarval settlement.
Conclusion
Thus from the above discussion, it can be concluded that global warming and acidification are two prime factors responsible for coral bleaching, one of the alarming reasons behind the disruption of equilibrium of the marine ecosystem. Recent events of ocean acidification and global warming have eroded crucial functional species from coral reefs and have subsequently left the community vulnerable towards alternative attractors. Coral bleaching is an acute mechanism that causes extreme mortality and the conditions becomes worse when the temperature of the ocean or the sea is exceptionally high. Global warming and subsequent ocean acidification also lead to decreased rate of coral calcification and this in turn reduces the somatic growth of the coral which ultimately leads towards destruction of the marine coral population. The process of coral bleaching is however, independent of light and has no relation with the photosynthetic ability of the algae residing on the coral.
Recommendation
According to Grottoliet al., (2014) additive impact of perennial coral bleaching could transform some species increasingly prone towards bleaching while experiencing a long-term reduction in the population. On the other hand, the phenotypically plastic coral species has been found to acclimatize with the surrounding climate and persist. Thus yearly process of coral bleaching and subsequent recovery could cumulatively contribute towards selective loss in coral diversity along with overall decline in coral reef. However, Gilmour et al. (2013) have opined that in the absence of chronic disturbance, it is possible to recover the isolated reefs from the catastrophic disturbances. The survival of the remnant colonies followed by high rate of increase in the recruitment of juvenile colonies help in coral recovery. The juvenile colonies replenish the bleached coral thus restoring back the population (Gilmour et al.2013). Yeeminet al. 2012 has opined in favour of conservation of coral reef in order to prevent extinction.
CAPS tool Analysis
Aim: the aim of this paper is to study the cumulative effects of the temperature stress and suspended organic nutrients on the early life stages of coral reefs. The aim is clearly stated at the title of the research article. The aim of the paper goes with the scope of this review and hone score high on relevancy.
Methodology: The methodology mostly involved the assessment of the embryo development and fertilizationsuccess in order to ascertain the change in the life history of coral in effect of temperature stress, suspended sediments and organic nutrients.
Proper data collection: The collection and interpretation of data is done in both diagrammatic, tabular and in written form and thisenable easy understanding of the elucidated results and thereby concluding the required findings.
There was also clear statement of findings in the discussion portion making the research valuable at the present day scenario.
The aim of the paper is clearly stated in the on the title however, the title of the paper lacks the explanation of the procedures that will be undertaken in order to draw the results for the chosen aim.
The abstract of the paper is also succinctly written. However, it lacks the breakup of the abstract via subheadings like aim, methodology, results and conclusion. Although the main findings and the of the research paper can be clearly grasped from the abstract and this is widespread change in the gene-expression of the coral reefs modulates the process of coral bleaching in relation to ocean acidification.
In order to ascertain the physiological and cellular impact the researcher has chosen cDNA microarrays, to track the pH change and ocean chemistry. This procedure is in sync with the regulations stated in Intergovernmental Panel on Climate Change (IPCC) and thus qualifying high in the ground of relevancy.
The data analysis was done in a succinct manner with detailed interpretation of data in diagrammatic, tabular and textual manner. This elaborate illustration of information made this research work easily comprehendible.
There was also clear statement of findings in the discussion portion making the research valuable at the present day scenario.
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