Chemistry Reference
In-Depth Information
21c). Nevertheless, reef corals usually calcify fastest in the light (Kawaguti and Sakumoto
1948), accelerating calcification by a factor of 3 on average (Gattuso et al. 1999). The
reasons for this “light-enhanced calcification” remain controversial despite a half century
of intensive research. Does photosynthesis stimulate calcification by raising pH and
increasing the CaCO
3
saturation state
? Do corals up-regulate calcification when the
resulting proton flux would benefit their symbionts? We will examine these possibilities.
Photosynthesis does not cause rapid calcification
Photosynthesis increases CO
3
2−
levels as suggested by the reaction 2HCO
3
−
→
Ω
CH
2
O
+ O
2
+ CO
3
2−
. This increases the aragonite saturation state
, and could stimulate
calcification. But is this important? Gattuso et al. (1999) estimated the average ratio of
calcification to net photosynthesis (G/P
n
) in reef corals is about 1.3. G/P
n
ratios above 0.8
actually decrease
Ω
in seawater, so photosynthetic alkalinization of the water cannot
produce G/P
n
ratios of 1.3. Photosynthesis also drives down CO
2
concentrations, inhibiting
itself and its ability to increase
Ω
Ω
. Healthy reefs seldom push CO
2
levels below 2
µ
M, or
pH higher than 8.5 (Smith 1973; Frankignoulle et al. 1996), and
Ω
seldom exceeds 6. This
values calculated earlier for the calcifying compartment
(Fig. 15). Non-calcareous plants with high rates of photosynthesis can furthermore grow
side by side with calcareous plants. Finally, increasing
pales by comparison to the high
Ω
by adding Ca
2+
to seawater
(beyond the 10 mM normally present) has little effect on coral calcification rates (Gattuso
et al. 1998). All these examples point toward the conclusion that photosynthesis generally
doesn't cause much calcification by elevating
Ω
.
Photosynthesis may however stimulate calcification by supplying food (Muscatine
1990) and oxygen (Rinkevich and Loya 1984) to the corals. Corals may also regulate
calcification based partly on how efficiently the protons from calcification convert HCO
3
−
to CO
2
(Fig. 21a). A model based on this idea produces Gattuso's average value for G/P
n
=
1.3 at a typical reef pH value of 8 (McConnaughey et al. 2000). Below pH 8.5, G/P
n
appeared to be relatively insensitive to pH and PCO
2
, which could influence how reefs
respond to glacial-interglacial and modern industrial CO
2
changes. This is a controversial
topic, but the global CO
2
increases of recent decades often seem to have had little effect on
coral skeletal growth (Lough and Barnes 1997; Bessat and Buiges 2001).
Calcification relieves CO
2
stress
High G/P
n
ratios like Gatusso's average of 1.3 prevent most of the depletion of
dissolved molecular CO
2
that would otherwise accompany photosynthesis. The
photosynthetic kinetics shown in Figure 21b suggest that this could significantly
stimulate coral photosynthesis. (The half saturation constant K
M
for zooxanthellate
photosynthesis appears to lie between about 6 and 60
Ω
M CO
2
; Legatt et al. 1999, 2002.)
If zooxanthellae can fix more carbon when the coral calcifies rapidly, the coral has good
reason to ramp up calcification during the day. The coral discharges the protons from
calcification into its internal coelenteron cavity. Symbiotic zooxanthellae facing the
coelenteron are well positioned to benefit from this proton flux (Fig. 21b). Al-Horani et
al. (2003) observed only minor alkalinization and sometimes mild acidification of the
coelenteron, even in illuminated corals (Fig. 13). This supports the idea that the protons
from calcification keep coelenteron pH down and CO
2
levels up. Proton discharge into
the coelenteron might also explain why branching and foliose corals such as Acropora
(Fig. 21c) calcify fastest at their non-photosynthetic apical polyps, where Gladfelter
(1983b) observed inward ciliary currents. Such currents could deliver acidified, CO
2
rich
water from the branch tips to the highly photosynthetic tissues further down the branch.
McConnaughey and Whelan (1997) and McConnaughey et al. (2000) summarized
evidence that calcification might stimulate photosynthesis. In contrast, Gattuso et al.
µ
