2.5 Threats to Coral Reefs
41
sulphur to the atmosphere via its sea-to-air exchange from global oceans (Lana et al.
2011). Oxidation of atmospheric DMS by hydroxyl radicals leads to the formation
of sulphate aerosols, which act as cloud condensation nuclei (CNN), thus leading to
greater cloudiness. Thus this can significantly affect solar radiation over the ocean
(Charlson et al. 1987) and Raina et al. (2013) elucidate that cloud production, especially in the tropics, is an important regulator of climate because clouds shade the
earth and reflect much of the sun’s heat back into space.
Charlson et al. (1987) hypothesized that changes in phytoplankton activities can
be mediated by changes in low level cloud cover and sunlight, thus regulating sea
surface temperatures (SSTs). The hypothesis of Charlson et al. (1987) is referred to
as the CLAW hypothesis (named after the first letter of each authors surname), which
proposes that as temperatures increase due to GHG gas warming, greater amounts of
DMS would be emitted into the atmosphere to produce more low level cloud cover,
which would in turn lower sea surface temperatures, a type of ocean thermostat or
climate feedback.
Production of DMSP was found to increase when corals are subjected to water
temperatures that put them under heat stress (Raina et al. 2013) and thus elevated
levels of DMS above stressed reefs (Jones et al. 2007). Hence, Deschaseaux et al.
(2014) reported that DMSP was found to be a biomarker of stress in corals as their
concentrations tend to increase as a response to environmental stress exposure in reef
corals. However, declining coral numbers could lead to a decrease in the production
of DMSP, thus this may impede cloud formation.
Corals are also exposed to environmental pollutants such as metals. Schwarz
et al. (2013) reported that exposure to metals induces oxidative stress in organisms
through cellular biochemical reactions that produce ROS including superoxide (O 2
− ),
hydrogen peroxide (H 2 O 2 ), hydroxyl radical (•OH) and singlet oxygen (
1 O 2 ). An
increase in temperature and metal exposure increases the generation of ROS in aquatic
organisms leading to the oxidation of biomolecules like lipids, proteins and nucleic
acids (Monserrat et al. 2007).
In their study, Deschaseaux et al. (2018) found that DMSP concentrations significantly decreased in both the host and symbionts under the most elevated zinc treatment (1000 µg/L); suggesting that the decrease is a consequence of DMSP oxidation
from increased levels of ROS. The reef-building coral Acropora aspera was used in
the study of Deschaseaux et al. (2018) and it was found that extended exposure to
high Zn concentrations in A. aspera induced a decrease in DMSP concentrations in
the coral host and associated Symbiodinium; indicating that DMSP was consumed
(or stopped being produced) in the coral holobiont under Zn contamination. In their
study, Jones et al. (2007) expounded that raised levels of DMS above stressed reefs
have led authors to suggest the involvement of DMSP is a corals stress response.
However, Yost et al. (2010) in his research reported that Montastraea franksi
showed differential responses to Cu exposure; DMSP levels generally decreased
with Cu dose in both the holobiont and the symbiont fractions whilst the DMSP
levels exclusively increased in the symbiont at the highest Cu dose.
41
sulphur to the atmosphere via its sea-to-air exchange from global oceans (Lana et al.
2011). Oxidation of atmospheric DMS by hydroxyl radicals leads to the formation
of sulphate aerosols, which act as cloud condensation nuclei (CNN), thus leading to
greater cloudiness. Thus this can significantly affect solar radiation over the ocean
(Charlson et al. 1987) and Raina et al. (2013) elucidate that cloud production, especially in the tropics, is an important regulator of climate because clouds shade the
earth and reflect much of the sun’s heat back into space.
Charlson et al. (1987) hypothesized that changes in phytoplankton activities can
be mediated by changes in low level cloud cover and sunlight, thus regulating sea
surface temperatures (SSTs). The hypothesis of Charlson et al. (1987) is referred to
as the CLAW hypothesis (named after the first letter of each authors surname), which
proposes that as temperatures increase due to GHG gas warming, greater amounts of
DMS would be emitted into the atmosphere to produce more low level cloud cover,
which would in turn lower sea surface temperatures, a type of ocean thermostat or
climate feedback.
Production of DMSP was found to increase when corals are subjected to water
temperatures that put them under heat stress (Raina et al. 2013) and thus elevated
levels of DMS above stressed reefs (Jones et al. 2007). Hence, Deschaseaux et al.
(2014) reported that DMSP was found to be a biomarker of stress in corals as their
concentrations tend to increase as a response to environmental stress exposure in reef
corals. However, declining coral numbers could lead to a decrease in the production
of DMSP, thus this may impede cloud formation.
Corals are also exposed to environmental pollutants such as metals. Schwarz
et al. (2013) reported that exposure to metals induces oxidative stress in organisms
through cellular biochemical reactions that produce ROS including superoxide (O 2
− ),
hydrogen peroxide (H 2 O 2 ), hydroxyl radical (•OH) and singlet oxygen (
1 O 2 ). An
increase in temperature and metal exposure increases the generation of ROS in aquatic
organisms leading to the oxidation of biomolecules like lipids, proteins and nucleic
acids (Monserrat et al. 2007).
In their study, Deschaseaux et al. (2018) found that DMSP concentrations significantly decreased in both the host and symbionts under the most elevated zinc treatment (1000 µg/L); suggesting that the decrease is a consequence of DMSP oxidation
from increased levels of ROS. The reef-building coral Acropora aspera was used in
the study of Deschaseaux et al. (2018) and it was found that extended exposure to
high Zn concentrations in A. aspera induced a decrease in DMSP concentrations in
the coral host and associated Symbiodinium; indicating that DMSP was consumed
(or stopped being produced) in the coral holobiont under Zn contamination. In their
study, Jones et al. (2007) expounded that raised levels of DMS above stressed reefs
have led authors to suggest the involvement of DMSP is a corals stress response.
However, Yost et al. (2010) in his research reported that Montastraea franksi
showed differential responses to Cu exposure; DMSP levels generally decreased
with Cu dose in both the holobiont and the symbiont fractions whilst the DMSP
levels exclusively increased in the symbiont at the highest Cu dose.