with which calcium carbonate is deposited by reef plants
and animals. It depends in large part on O aragonite , the
degree to which the sea water is saturated with the ionic
precursors of aragonite calcium carbonate – the form comprising the skeletons of reef-building corals:
Ca
2þ
þ CO 3
2À
$ CaCO 3
(1)
There is ample Ca
2+ for calcification in all the world’s sea
waters (Gattuso et al., 1999). (Indeed, excess Ca
2+ is toxic
to cellular processes, and biogenic calcification, now the
keystone process for coral reef existence, is believed to
have initially evolved in the Cambrian as a Ca
2+ detoxification mechanism; Brennan et al., 2004; Kleypas and
Langdon, 2006). But the amount of carbonate (CO 3
2À )
available can become limiting, because it is very easily
converted to the unusable bicarbonate (HCO 3
À ) in the
presence of H
+ ions: (the lower the pH, the less the
CO 3
2À ). H
+ ions are evolved when CO 2 dissolves in water,
forming carbonic acid (H 2 CO 3 ), which then dissociates:
CO 2 þ H 2 O $ H 2 CO 3 $ H
þ
þ HCO 3
À
$ CO 3
2À
þ H
þ (2)
More CO 2 in solution (Equation 2) means more H
+
is evolved and more CO 3
2À is robbed from the feedstock
(Equation 1) that could otherwise have been joined
with Ca
2+ in the skeleton of a reef-building organism.
Temperature also affects the solubility of CO 2 (and all
other gases) in seawater: the warm waters of the tropics
absorb less CO 2 than cool temperate waters, and therefore,
less CO 3
2À is robbed from calcification in the tropics,
historically to the great advantage of corals.
Solar radiation
Solar radiation is a primary environmental driver of production on coral reefs, both organic matter (tissues of
plants and animals) and inorganic matter (skeletons), the
latter at rates of up to tens of tonnes of calcium carbonate
per hectare per year (Kinsey, 1983). The part of the solar
radiation spectrum that drives this production is referred
to as photosynthetically active radiation (PAR – wave
lengths 400–700 nm). Quanta in this range are absorbed
by the photosynthetic systems of the zooxanthellae, driving their production of the energy-rich “photosynthate”
(glycerol and glucose). Around 95% of the photosynthate
is translocated from the zooxanthellae to the cells of the
coral host and used throughout the coral colony to build
coral tissues, gametes, and aragonite. Calcification occurs
at its maximum rate for 4 h around local solar noon
(Chalker, 1983), during which time the irradiance of shallow corals with PAR is “saturating” with respect to the
ability of the photo systems within the zooxanthellae to
use it. The excess irradiance elicits “photoinhibition” in
the coral, viz, a reduced photosynthetic efficiency, capacity, or both, compared to the performance at optimal levels
of irradiance (Winters et al., 2003). At the high latitude
limits of coral distribution, vigorous growth of
“phototrophic” corals (those for which photosynthesis is
the primary energy source) is precluded by the meager
annual quota of PAR both at the water surface (due to short
winter day lengths and low sun angles – Figures 1 and 2a)
and at depth (due to reflection and absorption – Baker and
Smith, 1982).
Temperature
Corals are “poikilothermic” organisms (they cannot regulate their temperature and must operate at the ambient
temperature). Diverse coral communities and reefs occur
in waters with sea temperatures in the range 18–28
C
(Figure 2b). Throughout this range, there are suites of
corals for which local temperatures are conducive to tissue
growth, skeleton growth, and reproduction: the same coral
species can host different types (Clades) of zooxanthellae
to suit its particular environmental setting. For example, in
the tropics, the world’s most widely distributed
zooxanthellate coral species Plesiastrea versipora (Veron,
2000) hosts zooxanthellae that are predominantly of the
ubiquitous Clade C (Rodriguez-Lanetty et al., 2001),
a highly efficient photosynthesizer (Cantin et al., 2009)
that confers a fast growth rate on the many coral species
in which it occurs. However, to survive in temperate
waters, P. versipora hosts predominantly zooxanthellae
of Clade B, which can function and survive at lower temperatures than Clade C (Howe and Marshall, 2002). Likewise, some coral species occupy warmer habitats by
hosting warm-adapted zooxanthellae, in this case, predominantly Clade D (Berkelmans and van Oppen, 2006;
Cantin et al., 2009). Compared to Clade C, however, the
tolerance to marginal temperatures in both cases comes
at the cost of a reduced growth rate (Howe and Marshall,
2002; Little et al., 2004). This trade-off has permitted
corals to spread widely along the latitude-correlated attenuation of the parameters that govern coral growth: viz,
temperature, aragonite saturation, and light. Whereas the
zooxanthella complement of adult corals is typically dominated by a single clade, juveniles in some coral species
host a mixture of clades (Gómez-Cabrera et al., 2008).
The extent to which the identity of the clade dominant in
the adult represents a form of selection to the type most
suited to the individual coral’s micro-environment has
yet to be established (Gómez-Cabrera et al., 2008).
With normal local seasonal changes in temperature
($5–7
C – Figure 2c), the coral’s metabolism is
channeled variously into growth and replication of polyps,
gametogenesis, skeletogenesis, and manufacture of stored
energy reserves (Harriott, 1993). However, a few days of
temperatures outside a particular reef’s normal variability
can be stressful and sometimes lethal for corals, be
they abnormally high temperatures or abnormally low.
Temperatures only 1–2
C above its normal summer maximum or below its normal winter minimum can cause
coral bleaching (Brown, 1997): hot water bleaching
(Goreau and Hayes, 1994) or cold water bleaching
(Muscatine et al., 1991). In both cases, the anomalous
284
CORALS: ENVIRONMENTAL CONTROLS ON GROWTH
and animals. It depends in large part on O aragonite , the
degree to which the sea water is saturated with the ionic
precursors of aragonite calcium carbonate – the form comprising the skeletons of reef-building corals:
Ca
2þ
þ CO 3
2À
$ CaCO 3
(1)
There is ample Ca
2+ for calcification in all the world’s sea
waters (Gattuso et al., 1999). (Indeed, excess Ca
2+ is toxic
to cellular processes, and biogenic calcification, now the
keystone process for coral reef existence, is believed to
have initially evolved in the Cambrian as a Ca
2+ detoxification mechanism; Brennan et al., 2004; Kleypas and
Langdon, 2006). But the amount of carbonate (CO 3
2À )
available can become limiting, because it is very easily
converted to the unusable bicarbonate (HCO 3
À ) in the
presence of H
+ ions: (the lower the pH, the less the
CO 3
2À ). H
+ ions are evolved when CO 2 dissolves in water,
forming carbonic acid (H 2 CO 3 ), which then dissociates:
CO 2 þ H 2 O $ H 2 CO 3 $ H
þ
þ HCO 3
À
$ CO 3
2À
þ H
þ (2)
More CO 2 in solution (Equation 2) means more H
+
is evolved and more CO 3
2À is robbed from the feedstock
(Equation 1) that could otherwise have been joined
with Ca
2+ in the skeleton of a reef-building organism.
Temperature also affects the solubility of CO 2 (and all
other gases) in seawater: the warm waters of the tropics
absorb less CO 2 than cool temperate waters, and therefore,
less CO 3
2À is robbed from calcification in the tropics,
historically to the great advantage of corals.
Solar radiation
Solar radiation is a primary environmental driver of production on coral reefs, both organic matter (tissues of
plants and animals) and inorganic matter (skeletons), the
latter at rates of up to tens of tonnes of calcium carbonate
per hectare per year (Kinsey, 1983). The part of the solar
radiation spectrum that drives this production is referred
to as photosynthetically active radiation (PAR – wave
lengths 400–700 nm). Quanta in this range are absorbed
by the photosynthetic systems of the zooxanthellae, driving their production of the energy-rich “photosynthate”
(glycerol and glucose). Around 95% of the photosynthate
is translocated from the zooxanthellae to the cells of the
coral host and used throughout the coral colony to build
coral tissues, gametes, and aragonite. Calcification occurs
at its maximum rate for 4 h around local solar noon
(Chalker, 1983), during which time the irradiance of shallow corals with PAR is “saturating” with respect to the
ability of the photo systems within the zooxanthellae to
use it. The excess irradiance elicits “photoinhibition” in
the coral, viz, a reduced photosynthetic efficiency, capacity, or both, compared to the performance at optimal levels
of irradiance (Winters et al., 2003). At the high latitude
limits of coral distribution, vigorous growth of
“phototrophic” corals (those for which photosynthesis is
the primary energy source) is precluded by the meager
annual quota of PAR both at the water surface (due to short
winter day lengths and low sun angles – Figures 1 and 2a)
and at depth (due to reflection and absorption – Baker and
Smith, 1982).
Temperature
Corals are “poikilothermic” organisms (they cannot regulate their temperature and must operate at the ambient
temperature). Diverse coral communities and reefs occur
in waters with sea temperatures in the range 18–28
C
(Figure 2b). Throughout this range, there are suites of
corals for which local temperatures are conducive to tissue
growth, skeleton growth, and reproduction: the same coral
species can host different types (Clades) of zooxanthellae
to suit its particular environmental setting. For example, in
the tropics, the world’s most widely distributed
zooxanthellate coral species Plesiastrea versipora (Veron,
2000) hosts zooxanthellae that are predominantly of the
ubiquitous Clade C (Rodriguez-Lanetty et al., 2001),
a highly efficient photosynthesizer (Cantin et al., 2009)
that confers a fast growth rate on the many coral species
in which it occurs. However, to survive in temperate
waters, P. versipora hosts predominantly zooxanthellae
of Clade B, which can function and survive at lower temperatures than Clade C (Howe and Marshall, 2002). Likewise, some coral species occupy warmer habitats by
hosting warm-adapted zooxanthellae, in this case, predominantly Clade D (Berkelmans and van Oppen, 2006;
Cantin et al., 2009). Compared to Clade C, however, the
tolerance to marginal temperatures in both cases comes
at the cost of a reduced growth rate (Howe and Marshall,
2002; Little et al., 2004). This trade-off has permitted
corals to spread widely along the latitude-correlated attenuation of the parameters that govern coral growth: viz,
temperature, aragonite saturation, and light. Whereas the
zooxanthella complement of adult corals is typically dominated by a single clade, juveniles in some coral species
host a mixture of clades (Gómez-Cabrera et al., 2008).
The extent to which the identity of the clade dominant in
the adult represents a form of selection to the type most
suited to the individual coral’s micro-environment has
yet to be established (Gómez-Cabrera et al., 2008).
With normal local seasonal changes in temperature
($5–7
C – Figure 2c), the coral’s metabolism is
channeled variously into growth and replication of polyps,
gametogenesis, skeletogenesis, and manufacture of stored
energy reserves (Harriott, 1993). However, a few days of
temperatures outside a particular reef’s normal variability
can be stressful and sometimes lethal for corals, be
they abnormally high temperatures or abnormally low.
Temperatures only 1–2
C above its normal summer maximum or below its normal winter minimum can cause
coral bleaching (Brown, 1997): hot water bleaching
(Goreau and Hayes, 1994) or cold water bleaching
(Muscatine et al., 1991). In both cases, the anomalous
284
CORALS: ENVIRONMENTAL CONTROLS ON GROWTH