polymorph minerals: calcite, which has a rhombohedral
crystal structure; and aragonite, which is orthorhombic.
The calcite crystal structure allows some substitution
of magnesium ions (Mg) for calcium ions, and calcite
with >4 mol% MgCO 3 is called “high-Mg calcite.” Normally, calcite is much less soluble in seawater than aragonite, but high-Mg calcite with 12–16 mol% MgCO 3 is
about 20% more soluble than aragonite (Morse et al.,
2006). Many other factors (impurities, structural disorder)
can equally affect the solubilities of calcite, high-Mg calcite, and aragonite.
The saturation states of the various minerals affect the
rates of crystallization and dissolution. The rate of calcium
carbonate formation, for example, is often expressed as
the equation: R = k(OÀ1)
n
, where k is the rate constant
and n the reaction order.
The solubilities of the calcium carbonate minerals have
been of interest to Earth scientists for some time, mainly
because interpreting Earth’s history requires an understanding of the processes controlling production and preservation of calcium carbonate deposits. Waters of low pH,
such as those in the deep sea, have long been known to be
corrosive to calcium carbonates, as first noted by Murray
and Renard (1891).
Marine calcifying organisms typically precipitate either
one mineral or the other, and only a few species have the
capacity to precipitate two. In general, calcite secretors
(e.g., coccolithophorids) occur in higher latitudes where
colder waters can hold more CO 2 and thus have lower carbonate saturation. Organisms that precipitate the more soluble minerals aragonite (e.g., scleractinian corals) and
high-Mg calcite (e.g., crustose coralline algae) are favored
in low latitudes, which have higher saturation. Over geologic time, the dominant mineralogy of shallow water carbonates, as well as their preservation potential, has also
fluctuated with shifts in seawater carbonate chemistry
(Sandberg, 1983). The preferential disappearance of
corals and other marine reef-builders in several major
extinction events is coincident with rapid ocean acidification (Veron, 2008; Knoll et al., 2007).
Effects on calcification of marine organisms
Controlled laboratory experiments have provided most of
the evidence that ocean acidification will reduce calcification in corals and coralline algae. Results of these experiments indicate that calcification rates of most tropical
coral species will decline significantly as ocean acidification proceeds (reviewed in Kleypas and Langdon, 2006;
Langdon and Atkinson, 2005). A wide range of responses
has been observed, but on average, experimental results
under conditions of 2Â preindustrial CO 2 concentrations
fall within two response classes: (1) >40% reduction in
calcification rate and (2) <10% reduction (Langdon and
Atkinson, 2005). Calcification in newly settled coral
polyps may also be impaired at elevated CO 2 (Albright
et al., 2008). There is no evidence yet that corals can
adapt to these changes, but the response is reversible
(calcification will increase if pH is increased). The skeletons of some live coral species dissolve completely when
placed in waters undersaturated with respect to aragonite,
with the coral polyps left in a “naked” or anemone-like
state (Fine and Tchernov, 2007). These can re-precipitate
skeletons when replaced in waters of normal aragonite saturation. The coral polyps appeared unharmed in these
experiments, but naked corals are considered vulnerable
in the wild, and would lose their reef-building function.
Experimental results indicate that calcification rates of
tropical reef-building coralline algae and rhodoliths
respond more strongly to ocean acidification than those
of corals, which may be due in part to the high-Mg calcite
mineralogy of coralline algae skeletons. Some experiments show net dissolution of coralline algae at atmospheric CO 2 concentrations as low as 700 ppm (Kuffner
et al., 2008; Jokiel et al., 2008; Anthony et al., 2008).
The recruitment of tropical crustose coralline algae in
experimental mesocosms is also reduced at higher CO 2
concentrations (Kuffner et al., 2008).
Evidence that ocean acidification is currently affecting
reef-building organisms has been difficult to obtain,
mainly because of the difficulty in measuring calcification
rates over time. Calcification rates derived from coral skeletons of massive Porites species indicate a dramatic
decrease in calcification beginning around the year 1990
(De’ath et al., 2009), but it is unclear whether this decrease
is due to increased temperature, ocean acidification, or
a combination of the two.
The effects of decreased calcification on the survival of
calcifying reef organisms are not well understood. Some
calcifying organisms become absent near a natural submarine CO 2 vent in a shallow region of the Mediterranean
(Hall-Spencer et al., 2008). Calcium carbonate shells and
skeletons probably have a protective function, but may
also allow organisms to secure themselves to the substrate,
compete for space, or optimize light gathering for
photosynthesis.
Effects on coral reef structures
Coral reef structures are produced by the accumulation of
CaCO 3 skeletal debris of corals, coralline algae, and other
reef calcifiers. Ocean acidification slows net calcium carbonate production by decreasing carbonate production
and/or increasing carbonate dissolution (Kleypas et al.,
2001). Dissolution rates of reef sediments increase significantly under elevated CO 2 conditions, particularly in sediments with a high percentage of high-Mg calcite (Yates
and Halley, 2006; Andersson et al., 2007). Monitoring of
net community calcification rates on coral reefs
(Silverman et al., 2009a) indicate that many or most coral
reefs will shift to a state of net dissolution once CO 2 concentration reaches 2Â preindustrial levels (Silverman
et al., 2009b). Other studies suggest that, under elevated
CO 2 conditions, inorganic cementation of coral reef structures is lower, and erosion rates are higher (Manzello et al.,
2008). Erosion rates of common microscopic euendolithic
OCEAN ACIDIFICATION, EFFECTS ON CALCIFICATION
735
crystal structure; and aragonite, which is orthorhombic.
The calcite crystal structure allows some substitution
of magnesium ions (Mg) for calcium ions, and calcite
with >4 mol% MgCO 3 is called “high-Mg calcite.” Normally, calcite is much less soluble in seawater than aragonite, but high-Mg calcite with 12–16 mol% MgCO 3 is
about 20% more soluble than aragonite (Morse et al.,
2006). Many other factors (impurities, structural disorder)
can equally affect the solubilities of calcite, high-Mg calcite, and aragonite.
The saturation states of the various minerals affect the
rates of crystallization and dissolution. The rate of calcium
carbonate formation, for example, is often expressed as
the equation: R = k(OÀ1)
n
, where k is the rate constant
and n the reaction order.
The solubilities of the calcium carbonate minerals have
been of interest to Earth scientists for some time, mainly
because interpreting Earth’s history requires an understanding of the processes controlling production and preservation of calcium carbonate deposits. Waters of low pH,
such as those in the deep sea, have long been known to be
corrosive to calcium carbonates, as first noted by Murray
and Renard (1891).
Marine calcifying organisms typically precipitate either
one mineral or the other, and only a few species have the
capacity to precipitate two. In general, calcite secretors
(e.g., coccolithophorids) occur in higher latitudes where
colder waters can hold more CO 2 and thus have lower carbonate saturation. Organisms that precipitate the more soluble minerals aragonite (e.g., scleractinian corals) and
high-Mg calcite (e.g., crustose coralline algae) are favored
in low latitudes, which have higher saturation. Over geologic time, the dominant mineralogy of shallow water carbonates, as well as their preservation potential, has also
fluctuated with shifts in seawater carbonate chemistry
(Sandberg, 1983). The preferential disappearance of
corals and other marine reef-builders in several major
extinction events is coincident with rapid ocean acidification (Veron, 2008; Knoll et al., 2007).
Effects on calcification of marine organisms
Controlled laboratory experiments have provided most of
the evidence that ocean acidification will reduce calcification in corals and coralline algae. Results of these experiments indicate that calcification rates of most tropical
coral species will decline significantly as ocean acidification proceeds (reviewed in Kleypas and Langdon, 2006;
Langdon and Atkinson, 2005). A wide range of responses
has been observed, but on average, experimental results
under conditions of 2Â preindustrial CO 2 concentrations
fall within two response classes: (1) >40% reduction in
calcification rate and (2) <10% reduction (Langdon and
Atkinson, 2005). Calcification in newly settled coral
polyps may also be impaired at elevated CO 2 (Albright
et al., 2008). There is no evidence yet that corals can
adapt to these changes, but the response is reversible
(calcification will increase if pH is increased). The skeletons of some live coral species dissolve completely when
placed in waters undersaturated with respect to aragonite,
with the coral polyps left in a “naked” or anemone-like
state (Fine and Tchernov, 2007). These can re-precipitate
skeletons when replaced in waters of normal aragonite saturation. The coral polyps appeared unharmed in these
experiments, but naked corals are considered vulnerable
in the wild, and would lose their reef-building function.
Experimental results indicate that calcification rates of
tropical reef-building coralline algae and rhodoliths
respond more strongly to ocean acidification than those
of corals, which may be due in part to the high-Mg calcite
mineralogy of coralline algae skeletons. Some experiments show net dissolution of coralline algae at atmospheric CO 2 concentrations as low as 700 ppm (Kuffner
et al., 2008; Jokiel et al., 2008; Anthony et al., 2008).
The recruitment of tropical crustose coralline algae in
experimental mesocosms is also reduced at higher CO 2
concentrations (Kuffner et al., 2008).
Evidence that ocean acidification is currently affecting
reef-building organisms has been difficult to obtain,
mainly because of the difficulty in measuring calcification
rates over time. Calcification rates derived from coral skeletons of massive Porites species indicate a dramatic
decrease in calcification beginning around the year 1990
(De’ath et al., 2009), but it is unclear whether this decrease
is due to increased temperature, ocean acidification, or
a combination of the two.
The effects of decreased calcification on the survival of
calcifying reef organisms are not well understood. Some
calcifying organisms become absent near a natural submarine CO 2 vent in a shallow region of the Mediterranean
(Hall-Spencer et al., 2008). Calcium carbonate shells and
skeletons probably have a protective function, but may
also allow organisms to secure themselves to the substrate,
compete for space, or optimize light gathering for
photosynthesis.
Effects on coral reef structures
Coral reef structures are produced by the accumulation of
CaCO 3 skeletal debris of corals, coralline algae, and other
reef calcifiers. Ocean acidification slows net calcium carbonate production by decreasing carbonate production
and/or increasing carbonate dissolution (Kleypas et al.,
2001). Dissolution rates of reef sediments increase significantly under elevated CO 2 conditions, particularly in sediments with a high percentage of high-Mg calcite (Yates
and Halley, 2006; Andersson et al., 2007). Monitoring of
net community calcification rates on coral reefs
(Silverman et al., 2009a) indicate that many or most coral
reefs will shift to a state of net dissolution once CO 2 concentration reaches 2Â preindustrial levels (Silverman
et al., 2009b). Other studies suggest that, under elevated
CO 2 conditions, inorganic cementation of coral reef structures is lower, and erosion rates are higher (Manzello et al.,
2008). Erosion rates of common microscopic euendolithic
OCEAN ACIDIFICATION, EFFECTS ON CALCIFICATION
735