136
Coral reefs act as natural barriers protecting the coastline
from currents, waves and storms by dissipating their force
and hence overall destructive impact (Moberg and Folke
1999). Calcite skeletons allow coral reefs to be robust structures that can withstand strong currents. However, studies
have shown that increasing temperature, along with ocean
acidification, are causing changes in skeletal growth and
robustness (Tambutté et al. 2015). To understand how, one
must first look at the physiology of corals.
Symbiodinium (i.e., dinoflagellates) residing in coral tissue provide the majority of the coral hosts’ energy demands,
allowing them to successfully thrive in oligotrophic waters
(Yellowlees et al. 2008; Baker et al. 2015). The distribution
of coral reefs is heavily limited by sea temperature, where
the sensitivity to temperature stress depends on the physiological tolerance limits of both endosymbiotic partners
(Putnam et al. 2012). When temperatures exceed tolerance
limits, the most common response is the expulsion of symbionts from the tissue of the coral hosts, an event known as
coral bleaching. The response to the loss of symbionts and
their photosynthetic by-products, the largest energy source
of corals, is a decline in coral productivity and skeletal
growth (Langdon and Atkinson 2005; Pandolfi et al. 2011).
Without Symbiodinium, corals can only survive for a limited
time before the onset of tissue necrosis and ultimately their
death. Hope for the survival of the reef exists in two forms;
some species of symbionts are more thermotolerant than others, and if temperatures return to base-level the coral can
recover by re-establishing symbiosis with Symbiodinium.
Some species of corals with different symbiont associations have managed to inhabit areas with extreme temperatures, highlighting their temperature resilient capacity (van
Oppen et al. 2015). These symbionts have been accredited
with a particularly important role in the overall thermal tolerance of corals, where temperature adapted symbiont clades
can reduce the overall bleaching response (Berkelmans and
van Oppen 2006; Sampayo et al. 2008; Howells et al. 2016).
In recent years, advances in technology have made it possible to uncover some of the underlying mechanisms allowing
for thermotolerance of corals and their symbionts, which
potentially play important roles in their adaptation and acclimatization potential (Richier et al. 2008; DeSalvo et al.
2010; Kvitt et al. 2011; Bellantuono et al. 2012; Kenkel et al.
2013). By understanding these mechanisms it is hoped that
we can aid corals in future response and survival, in light of
global warming (Magris et al. 2015; van Oppen et al. 2015).
Thus, it can be seen that temperature drastically impacts the
survival of corals and if temperatures continue to increase as
predicted, corals will struggle to recover from bleaching
events and subsequently large-scale losses of coral reefs will
be observed.
Whilst the direct effects of elevated temperatures on corals are frightening enough, they extend, as with seagrass and
mangroves, into related atmospheric changes. Increased precipitation and sea level rise have two primary impacts, a
decrease in salinity and alteration of nutrient fluxes. In the
case of salinity, a decrease may actually directly influence
thermotolerance of corals, whereas, recent studies have indicated that more saline environments could increase coral
holobiont temperature resilience (Gegner et al. 2017). Whilst
not directly impacting thermotolerance, eutrophication of
the water column (as the result of declines in seagrass and
mangroves areas) can cause imbalances in coral-symbiont
relationships and ultimately leads to the breakdown of symbiosis (D’Angelo and Wiedenmann 2014). If the symbiotic
relationship is already compromised by imbalances in nutrient exchange between the two partners, it is less likely to
withstand additional temperature stress. Consequently, if
corals undergo repeated long-term bleaching, calcification
rates will be continuously affected. Since calcification is a
costly process that requires a lot of energy, the lack of sufficient nutrients, due to symbiont absence, not only reduces
growth but also increases the porosity of the skeleton
(Tambutté et al. 2015). As storm frequency and intensity
increase, coral skeletons will be less resilient to withstand
turbulent waters, potentially leading to fractures and breaks,
effectively destroying the reef. Thus, the existence of coral
reefs is currently threatened not only by temperature increase,
but also by most associated atmospheric changes which
accompany global warming.
We can clearly see that the existence of mangroves, seagrass and coral reefs is currently threatened not only by temperature increase, but also by many associated changes in
abiotic factors. However, the extent to which certain changes
affect the ecosystem depends wholly on the system in question, as some are more resilient to certain abiotic stressors
than others (Guannel et al. 2016). Sea level rise has shown
contrasting impacts on mangrove, seagrasses and coral reefs,
whereby seagrass showed the most resilience over longer
periods of time (Albert et al. 2017). On the other hand, temperature increase of the water column is having the most detrimental effects on corals whose algal partners can escape
suboptimal conditions, whilst the animal and its skeleton are
left behind. Although mangroves, seagrass and coral reefs
may respond differently to temperature and associated
changes, the end point appears to be a decline in all three
either through migration to new locations or permanent loss.
As each ecosystem provides a service to help mitigate global
warming impacts, the slow disappearance of one could
increase the stress experienced by its neighbors. This is particularly important in terms of increased storm intensities
and frequencies which have the potential to significantly
impact sedimentation and nutrient enrichment, especially in
regions where losses of ecosystems and their associated buffering, trapping, and absorbing capacities have occurred
(Golbuu et al. 2003; Unsworth et al. 2012). Another global
H. S. Earp et al.
Coral reefs act as natural barriers protecting the coastline
from currents, waves and storms by dissipating their force
and hence overall destructive impact (Moberg and Folke
1999). Calcite skeletons allow coral reefs to be robust structures that can withstand strong currents. However, studies
have shown that increasing temperature, along with ocean
acidification, are causing changes in skeletal growth and
robustness (Tambutté et al. 2015). To understand how, one
must first look at the physiology of corals.
Symbiodinium (i.e., dinoflagellates) residing in coral tissue provide the majority of the coral hosts’ energy demands,
allowing them to successfully thrive in oligotrophic waters
(Yellowlees et al. 2008; Baker et al. 2015). The distribution
of coral reefs is heavily limited by sea temperature, where
the sensitivity to temperature stress depends on the physiological tolerance limits of both endosymbiotic partners
(Putnam et al. 2012). When temperatures exceed tolerance
limits, the most common response is the expulsion of symbionts from the tissue of the coral hosts, an event known as
coral bleaching. The response to the loss of symbionts and
their photosynthetic by-products, the largest energy source
of corals, is a decline in coral productivity and skeletal
growth (Langdon and Atkinson 2005; Pandolfi et al. 2011).
Without Symbiodinium, corals can only survive for a limited
time before the onset of tissue necrosis and ultimately their
death. Hope for the survival of the reef exists in two forms;
some species of symbionts are more thermotolerant than others, and if temperatures return to base-level the coral can
recover by re-establishing symbiosis with Symbiodinium.
Some species of corals with different symbiont associations have managed to inhabit areas with extreme temperatures, highlighting their temperature resilient capacity (van
Oppen et al. 2015). These symbionts have been accredited
with a particularly important role in the overall thermal tolerance of corals, where temperature adapted symbiont clades
can reduce the overall bleaching response (Berkelmans and
van Oppen 2006; Sampayo et al. 2008; Howells et al. 2016).
In recent years, advances in technology have made it possible to uncover some of the underlying mechanisms allowing
for thermotolerance of corals and their symbionts, which
potentially play important roles in their adaptation and acclimatization potential (Richier et al. 2008; DeSalvo et al.
2010; Kvitt et al. 2011; Bellantuono et al. 2012; Kenkel et al.
2013). By understanding these mechanisms it is hoped that
we can aid corals in future response and survival, in light of
global warming (Magris et al. 2015; van Oppen et al. 2015).
Thus, it can be seen that temperature drastically impacts the
survival of corals and if temperatures continue to increase as
predicted, corals will struggle to recover from bleaching
events and subsequently large-scale losses of coral reefs will
be observed.
Whilst the direct effects of elevated temperatures on corals are frightening enough, they extend, as with seagrass and
mangroves, into related atmospheric changes. Increased precipitation and sea level rise have two primary impacts, a
decrease in salinity and alteration of nutrient fluxes. In the
case of salinity, a decrease may actually directly influence
thermotolerance of corals, whereas, recent studies have indicated that more saline environments could increase coral
holobiont temperature resilience (Gegner et al. 2017). Whilst
not directly impacting thermotolerance, eutrophication of
the water column (as the result of declines in seagrass and
mangroves areas) can cause imbalances in coral-symbiont
relationships and ultimately leads to the breakdown of symbiosis (D’Angelo and Wiedenmann 2014). If the symbiotic
relationship is already compromised by imbalances in nutrient exchange between the two partners, it is less likely to
withstand additional temperature stress. Consequently, if
corals undergo repeated long-term bleaching, calcification
rates will be continuously affected. Since calcification is a
costly process that requires a lot of energy, the lack of sufficient nutrients, due to symbiont absence, not only reduces
growth but also increases the porosity of the skeleton
(Tambutté et al. 2015). As storm frequency and intensity
increase, coral skeletons will be less resilient to withstand
turbulent waters, potentially leading to fractures and breaks,
effectively destroying the reef. Thus, the existence of coral
reefs is currently threatened not only by temperature increase,
but also by most associated atmospheric changes which
accompany global warming.
We can clearly see that the existence of mangroves, seagrass and coral reefs is currently threatened not only by temperature increase, but also by many associated changes in
abiotic factors. However, the extent to which certain changes
affect the ecosystem depends wholly on the system in question, as some are more resilient to certain abiotic stressors
than others (Guannel et al. 2016). Sea level rise has shown
contrasting impacts on mangrove, seagrasses and coral reefs,
whereby seagrass showed the most resilience over longer
periods of time (Albert et al. 2017). On the other hand, temperature increase of the water column is having the most detrimental effects on corals whose algal partners can escape
suboptimal conditions, whilst the animal and its skeleton are
left behind. Although mangroves, seagrass and coral reefs
may respond differently to temperature and associated
changes, the end point appears to be a decline in all three
either through migration to new locations or permanent loss.
As each ecosystem provides a service to help mitigate global
warming impacts, the slow disappearance of one could
increase the stress experienced by its neighbors. This is particularly important in terms of increased storm intensities
and frequencies which have the potential to significantly
impact sedimentation and nutrient enrichment, especially in
regions where losses of ecosystems and their associated buffering, trapping, and absorbing capacities have occurred
(Golbuu et al. 2003; Unsworth et al. 2012). Another global
H. S. Earp et al.