38
2 Coral Reef Ecosystem
carbon molecules from sunlight) by marine algae; and calcification. Shifting the
balance between these different carbon species may have dramatic effects on aquatic
life, depending on which carbon species dominates (Bielmyer-Fraser et al. 2018).
The carbonate ion constitutes the essential brick for the formation of shells and
skeletons of invertebrates, including the skeleton of corals (Allemand and Osborn
2019). Marangoni et al. (2019) stated that the tridimensional structure of coral reefs is
built by calcifying organisms that are able to secrete carbonate skeletons through the
biomineralization process. The carbonate skeletons are formed when carbonate ion
reacts with the calcium ions to produce calcium carbonate, referred to as calcification,
as shown by Eq. 2.3.
CO
2−
3(aq) + Ca
2+
(aq) → CaCO 3(s)
(2.3)
Carbonate ion concentrations are often expressed relative to the saturation state
of seawater with respect to aragonite, the principal crystal form of calcium carbonate
crystals deposited by reef-building corals and many other marine calcifiers (HoeghGuldberg 2011). Changes in aragonite saturation state are considered a proxy for
calcification rate (Langdon and Atkinson 2005).
Under acidic conditions, the concentration of carbonate ions reduces in seawater
according to Eq. 2.4.
H
+
(aq) + CO
2−
3(aq) → HCO
−
3(aq)
(2.4)
At lowered pH, corals cannot absorb the calcium carbonate that they need to maintain their skeletons. Orr et al. (2005) reported that as the carbonate becomes depleted
over time; seawater can become undersaturated with respect to important calcium
carbonate minerals that are fundamental building blocks for many important marine
species, such as corals, zooplankton, and shellfish. The chemical modifications have
profound biological impacts by modifying the quantities of carbonate necessary for
the formation of the skeletons but especially by modifying the pH, a key parameter
in physiology (Comeau et al. 2017).
Furthermore, ocean acidification can similarly affect other aspects of physiology
of marine organisms including acid-base balance, energy metabolism, redox balance
as well as behavior (Ishimatsu et al. 2008; Sokolova et al. 2015) and hence the
possible interactions with pollutants such as metals.
Fluctuations in seawater chemistry as a result of ocean acidification can affect
parameters such as solubility, speciation and distribution of metals in water and
sediments, and thus affect the potential metal toxicity to marine organisms. The
concentrations of the dissolved metals in the marine environment are typically low
due to the relatively low solubility of trace metals in seawater and absorption of the
metals on the sediments (Ivanina and Sokolova 2015).
Most metals are associated with organic matter; with a significant fraction in
the form of metal-organic complexes (Zeng et al. 2015). Lowered pH affects the
adsorption of metals to organic material as most organic particles in seawater are
negatively charged (Millero et al. 2009) and hence surface sites become less available
2 Coral Reef Ecosystem
carbon molecules from sunlight) by marine algae; and calcification. Shifting the
balance between these different carbon species may have dramatic effects on aquatic
life, depending on which carbon species dominates (Bielmyer-Fraser et al. 2018).
The carbonate ion constitutes the essential brick for the formation of shells and
skeletons of invertebrates, including the skeleton of corals (Allemand and Osborn
2019). Marangoni et al. (2019) stated that the tridimensional structure of coral reefs is
built by calcifying organisms that are able to secrete carbonate skeletons through the
biomineralization process. The carbonate skeletons are formed when carbonate ion
reacts with the calcium ions to produce calcium carbonate, referred to as calcification,
as shown by Eq. 2.3.
CO
2−
3(aq) + Ca
2+
(aq) → CaCO 3(s)
(2.3)
Carbonate ion concentrations are often expressed relative to the saturation state
of seawater with respect to aragonite, the principal crystal form of calcium carbonate
crystals deposited by reef-building corals and many other marine calcifiers (HoeghGuldberg 2011). Changes in aragonite saturation state are considered a proxy for
calcification rate (Langdon and Atkinson 2005).
Under acidic conditions, the concentration of carbonate ions reduces in seawater
according to Eq. 2.4.
H
+
(aq) + CO
2−
3(aq) → HCO
−
3(aq)
(2.4)
At lowered pH, corals cannot absorb the calcium carbonate that they need to maintain their skeletons. Orr et al. (2005) reported that as the carbonate becomes depleted
over time; seawater can become undersaturated with respect to important calcium
carbonate minerals that are fundamental building blocks for many important marine
species, such as corals, zooplankton, and shellfish. The chemical modifications have
profound biological impacts by modifying the quantities of carbonate necessary for
the formation of the skeletons but especially by modifying the pH, a key parameter
in physiology (Comeau et al. 2017).
Furthermore, ocean acidification can similarly affect other aspects of physiology
of marine organisms including acid-base balance, energy metabolism, redox balance
as well as behavior (Ishimatsu et al. 2008; Sokolova et al. 2015) and hence the
possible interactions with pollutants such as metals.
Fluctuations in seawater chemistry as a result of ocean acidification can affect
parameters such as solubility, speciation and distribution of metals in water and
sediments, and thus affect the potential metal toxicity to marine organisms. The
concentrations of the dissolved metals in the marine environment are typically low
due to the relatively low solubility of trace metals in seawater and absorption of the
metals on the sediments (Ivanina and Sokolova 2015).
Most metals are associated with organic matter; with a significant fraction in
the form of metal-organic complexes (Zeng et al. 2015). Lowered pH affects the
adsorption of metals to organic material as most organic particles in seawater are
negatively charged (Millero et al. 2009) and hence surface sites become less available