The Chemical and
Biochemical Mechanisms of Calcification
.
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A wide variety of
organisms in reef tanks lay down calcium carbonate structures, including corals, mollusks,
and algae. These structures provide a variety of functions, including protection and body
support, and the process of calcification itself may increase photosynthetic efficiency.
All reef aquarists are versed in the fact that such organisms remove calcium and carbonate
from the water column in order to provide materials for calcification. Exactly how
calcification takes place, however, is rarely considered. To a large extent the lack of
attention to this detail probably relates to the fact that many of the chemical and
biochemical details are simply not known. This article, the third in a series on calcium in
reef tanks, will explore what is known and what isn’t about how calcification takes
place.
In addition to the mechanistic details, this
article will explore a variety of issues that relate directly to reefkeeping. These issues
include the impact of changes in calcium, alkalinity, and pH on calcification. It will
also briefly discuss how ions such as strontium get into calcium carbonate skeletons, and
how phosphate inhibits calcification.
Basics of Calcification
We’ll start by describing what most
scientists agree on about how calcification takes place (whether it has been
experimentally proven or not). The details can be different for different organisms, so
the focus will be on corals, but calcifying algae are similar in many respects.1
In the case of corals, calcification takes
place external to the organism. If one thinks of corals as tissue coating a calcium
carbonate skeleton, then calcification takes place underneath the lowest layer of tissue
(the calicoblastic epithelium, also called the basel epithelium) in a very thin water
space called the extracytoplasmic calcifying fluid2 (ECF; Figure 1). The
calicoblastic epithelium is attached to the crystal surface (the epitheca in many corals)
with desmocytes, also called desmosomes3. These can be thought of as sheets of
tissue running between the calicoblastic epithelium and the skeleton. The ECF is partially
in contact with the surrounding water column. That is, the ions in this area can leak out
into the water column, and ions there can leak in.2 Nevertheless, it is
protected enough for calcification to be rapid there and slow to nonexistent to leak out
in the bulk water.
Figure 1. The site of
calcification in a coral. At the bottom is the coral skeleton and at the top is the
coelenteron of the coral. Calcium makes its way from the water column to the coelenteron,
across the calicoblastic epithelium, and into the ECF where it is precipitated as calcium
carbonate
Overall, corals must somehow get calcium from the water
column to the ECF. They also must get carbonate to the same location. For calcium, the
pathway is fairly clear, if not the exact transport mechanisms. Calcium travels from the
water column, into the coelenteron (the gastric cavity of the coral), enters the
calicoblastic epithelium, and then is transferred to the ECF. Additionally, it is
generally agreed that somewhere along the line, active transport involving chemical energy
is required, and that it is most likely involved in the transfer of calcium from the
calicoblastic epithelium to the ECF.
The transport
of carbonate is less clear, with scientists disagreeing on the details. Some suggest that
bicarbonate enters the coelenteron from the water column and from metabolic processes in
nearby cells. The bicarbonate combines with a proton to form CO2, and the CO2
freely diffuses across the calicoblastic epithelium to the ECF, where it is converted into
carbonate. Other scientists believe that bicarbonate itself is actively pumped from the
coelenteron across the calicoblastic epithelium to the ECF. Once in the ECF, it is
converted into carbonate.
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Once in the ECF, corals must
still combine the calcium and carbonate to form aragonite (a particular crystal form of
CaCO3). Corals cause growth of the calcium carbonate skeleton by raising the
supersaturation of calcium carbonate to the point where precipitation is likely. In a previous article in this
series, the factors that induce crystallization and those that inhibit it were
described. In this context, corals do the following to initiate and control precipitation:
1. They boost the calcium concentration by
actively pumping calcium into the ECF.
2. They may pump bicarbonate into the ECF as
the source of carbonate.
3. They may allow CO2 to freely
diffuse into the ECF as the source of carbonate.
4. They pump protons out of the ECF to
convert CO2 and bicarbonate into carbonate.
5. They maintain a fresh crystal face of
aragonite as a seed for calcification.
6. They may partially exclude magnesium,
phosphate, and organics that inhibit crystal formation.
7. They may help direct the crystallization
with their own organic molecules, including proteins and polysaccharides, secreted into
the ECF.
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Conditions in the
ECF
Unfortunately, the calcium concentration in
the ECF is not known. Nor is the carbonate concentration. Nor, for that matter, is the pH,
or even the alkalinity. In fact, is there any experimental information about it? To be
honest, there isn’t much. Most of the information available involves theoretical
predictions, such as McConneaughey and Whelan’s computation of the supersaturation
that could result from a particular calcium and proton transporter that they believe
exists (see details below).1 Apparently, the technical hurdles to measuring
these values in the very thin ECF have proven too great.
One of the things that is known is the flux
of calcium across the ECF. Values as high as 1.7 mol/cm2/h have been measured
based on the overall calcification rate.2 One way to gauge the magnitude of
this process is to compare it to seawater. Normal seawater is approximately 10 mM calcium.
One cubic centimeter consequently contains 10 mmole of calcium.
If the ECF had calcium at normal seawater concentration, the calcium would be totally
depleted from approximately 1.7 mm of fluid above the skeleton in an hour. Of course, the
ECF is not nearly this thick.
Unfortunately, the thickness
of the ECF is also not well established, and likely not even constant.3 The
calicoblastic epithelial cells themselves are about 1 micron thick, and the ECF area
appears by microscopy to be even thinner. If we take a value of 1 micron for discussion
sake, then the calcium within the ECF is totally depleted every 2 seconds. Consequently,
while we do not know the calcium concentration (and hence the supersaturation), we do know
that calcium is streaming across the ECF like water through a fire hose.
Calcium Transport from the
Calicoblastic Epithelium to the ECF: Proton Antiport
If both calcium and a source of carbonate get
into the ECF, it must come from the calicoblastic epithelium. How these materials get into
the ECF is an area of debate. This first section will present the theory published by
McConneaughey and Whelan1 (Figure 2).
Figure 2. The site of
calcification in a coral showing the proposed Ca++/2H+ transporter
(green oval) that sends calcium from the calicoblastic epithelium into the ECF. The
diffusion of CO2 from the coelenteron to the ECF and subsequent conversion into
carbonate is also shown.
Their proposal is that there
is a calcium/proton antiporter that takes calcium from the interior of the calicoblastic
epithelium and pumps it into the ECF. At the same time that each calcium ion enters the
ECF, two protons are pumped back out of the ECF into the cell. Since both of these
processes are transporting against a concentration gradient, it requires substantial
energy. The energy in this case is provided by one adenosine triphosphate molecule (ATP)
in the cell being broken down into adenosine diphosphate (ADP) and phosphate (a common
driving mechanism for many transporters). These authors also point out that the actual
transporter may be the related transporter where two calcium ions are transported for each
four protons and one ATP molecule. If the actual transporter is one of these two,
distinguishing between them is nontrivial since they have the same stoichiometry but
different ATP requirements.
Relatives of these transporters are very
common in other biological systems where calcium and protons are being exchanged, as on
muscle cell membranes. In fact, their commonality may be the only reason that they have
been suggested to be involved in calcification, and there appears little to no evidence
that this transporter is actually present on the skeletal side of the calicoblastic
epithelium.
One reason that active transport of calcium
from the cell to the ECF is required is that the calcium concentration within the
calicoblastic epithelium, as in nearly all cells, is much lower than it is in seawater.
Typical concentrations of free calcium inside of cells are below 1 uM (though total
concentrations can be mM when taking storage depots such as mitochondria into account)
while in seawater the concentration is 10 mM. Transporting calcium ions against this
gradient of 10,000x requires substantial chemical energy, and hence the need for ATP.1
We can, in fact, determine exactly how much
of a gradient this transporter might theoretically pump against:
One ATP, used perfectly efficiently, is
capable of driving calcium up a gradient of:
(1) E = 2.3RT[pCao - pCai]
Where E is the energy of hydrolysis of ATP,
Cao is the calcium concentration in the ECF (o for Outside the cell), and Cai is the
calcium concentration inside of the cell. Since E is about 50 kJ/mol, and RT at 25 °C is 2.48 kJ/mole, we can solve for
the gradient:
(2) pCao - pCai = 8.8
That is, the calcium can be pumped up against
a theoretical gradient of 6.3 x 108 . Consequently, if all that the transporter
did was move calcium, and it did it with perfect efficiency, there is more than enough
energy to drive the calcium from the cell to levels far above normal seawater in the ECF.
However, we also need to take into account
the transfer of the protons. The energy required to pump them out depends on the pH inside
the cell (pHi) and in the ECF (pHo):
(3) E = 2.3RT[2pHi - 2pHo]
where the "2" reflects the fact
that 2 protons are being pumped. To gauge the effect, let’s assume that the pH in the
cell is 7.0 and in the ECF is 9.0:
(4) pHi - pHo = -2
So
(5) E = 2.3RT[2pHi - 2pHo] = -22.8 kJ/mole
If we take the 50 kJ/mole from the original
ATP hydrolysis and subtract out the energy required to pump the protons, we have:
Consequently, we can see that there is enough
energy available in one ATP molecule to pump calcium up a gradient of 104 and
two protons up a pH gradient of 2 units. This gradient is adequate to cause
supersaturation in the ECF (assuming that CO2 freely diffuses into the ECF or
that H2CO3 otherwise gets there in adequate quantities).
Figure 3. A Stylophora pistillata coral
similar to those used in many of the calcification studies described in this article. The
picture was taken by Andy Hipkiss of a coral in his aquarium.
Calcium Transport from the Calicoblastic
Epithelium to the ECF: Other Mechanisms
There
are a variety of other proposed mechanisms of calcium transport from the calicoblastic
epithelium to the ECF.2 Tambutte et al4 have extensively
studied the various calcium pools in one coral (Stylophora pistillata; Figure 3)
and believe that this transport is dependent on a calcium-ATPase. This transporter, or a
related Ca++-ATPase, has also been proposed by Isa et al5 and
Ip et al6 . Unfortunately, there is little further characterization of
these molecules in the context of a calcifying coral.
If the calcium-ATPase described by these authors is
present in the bottom membrane of the calicoblastic epithelium as suggested, then it will
have the same theoretical potential to boost calcium concentrations as the Ca++/
2H+ transporter described above, without the proton transport (though there is
some concern about how low the concentration in the calicoblastic cell can get before this
transporter can no longer grab hold of calcium to begin with).2 Consequently,
this Ca++-ATPase may be theoretically able to boost calcium to levels that
induce supersaturation and would lead to formation of calcium carbonate.
Carbonate Transport from the Calicoblastic
Epithelium to the ECF: CO2
In McConneaughey and Whelan’s model, they propose
that CO2 diffuses freely from the coelenteron to the ECF. CO2, in
fact, does readily cross cell membranes when in its unhydrated form (CO2, not
the hydrated form H2CO3). When the CO2 hits the high pH
ECF, it is converted into carbonate, releasing two protons:
(8) H2CO3 ßà CO3- - + 2H+
The intrinsic simplicity of this model is
that these can be the exact two protons pumped out by the Ca++/ 2H+
transporter (Figure 2). Consequently, the Ca++/ 2H+ transporter
maintains a balance between calcium entering the ECF and protons leaving it.
The interconversion of CO2 and H2CO3
is a relatively slow process, taking seconds to minutes. Such a slow process would be
problematic for a coral that needs to rapidly interconvert these two forms, and they
consequently use an enzyme to speed up the process. Carbonic anhydrase is widely used by
organisms including humans. In the context of corals, it is an important aspect of
calcification.
When an inhibitor of carbonic anhydrase
(e.g., acetazolamide or ethoxyzolamide) is added, calcification is decreased in a number
of systems including Stylophora pistillata4 and a hydroid7 .
This result does not say whether the required carbonic anhydrase is in the ECF or
elsewhere (such as in the calicoblastic epithelium), but it is consistent with diffusion
of CO2 into the ECF.
Carbonate Transport from the
Calicoblastic Epithelium to the ECF: HCO3-
Of course, simplicity of a theory, as much as
we might like it, does not necessarily mean reality. Other authors have provided evidence
that active bicarbonate transport is important, not just CO2 diffusion. In
fact, these two processes are not mutually exclusive: perhaps both contribute in different
places, to different extents, or in different organisms.
Many authors have tried to distinguish
between carbon coming from bicarbonate (or carbonate) in the water column and CO2
coming from metabolic processes. In most instances, authors conclude that both contribute
to the pool of carbon going into the calcium carbonate skeleton. In Stylophora
pistillata, 70-75% of the carbon going into calcification comes from metabolic
processes, and 25-30% from bicarbonate in the water column8 . Still, the source
of the carbon does not necessarily indicate how it gets to the site of calcification, as
CO2 and bicarbonate are readily interconverted, especially in the presence of
carbonic anhydrase.
Furla et al9 also claim
that dissolved inorganic carbon (DIC; e.g., H2CO3/ HCO3-/CO3-
-) is actively transported and that an inhibitor of anion transport (DIDS) halts the
process:
"Seawater DIC is transferred from the
external medium to the coral skeleton by two different pathways: from seawater to the
coelenteron, the passive paracellular pathway is largely sufficient, while a
DIDS-sensitive transcellular pathway appears to mediate the flux across calicoblastic
cells."
And also
"Irresp. of the source, an anion
exchanger performs the secretion of DIC at the site of calcification."
Still, both Furla et al and
McConneaughey and Whelan may be partially correct: if bicarbonate is delivered to the ECF
by an anion exchanger, there will rapidly become an excess of protons in the ECF, driving
down the pH and inhibiting calcification. A removal mechanism involving Ca++/2H+
antiport solves this problem, though not with the perfectly balanced efficiency of the CO2
diffusion mechanism. Consequently, some active transport of protons from the ECF to the
coelenteron must take place.2 Whether it takes place via McConneaughey and
Whelan’s antiporter or some other mechanism remains to be established. In either
case, how this large flux of protons across the calicoblastic epithelium takes place
without disrupting the pH is unknown.2
Entry of Calcium into the
Calicoblastic Epithelium
The next question to answer is how the
calcium enters the calicoblastic epithelium. Clearly, it does not just freely flow from
the coelenteron because that space is largely in equilibrium with the external fluid (at
least with respect to calcium in seawater; 10 mM calcium). If calcium did enter freely,
the cell contents would also be at 10 mM calcium. While this might seem like an efficient
process, cells do not survive well at 10 mM calcium, and need to keep the free
concentration substantially lower as calcium plays a critical role in numerous cell
processes and cannot be permitted to be unusually high.
Figure 4. The site of
calcification in a coral showing the proposed Ca++ channel from the coelenteron
into the calicoblastic epithelium (red oval) in addition to those features shown in Figure
3.
Nevertheless, huge amounts of
calcium are flowing across the calicoblastic epithelium, and somehow the inflow of calcium
must be gated. In fact, there is a calcium channel that regulates the flow of calcium into
the cell. This voltage dependent channel has been characterized, and a portion of it has
even been cloned.10
Figure 5. A Galaxea sp. coral similar to those
used in some of the calcification studies described in this article. This picture was
taken by John Link of a coral his aquarium.
This calcium channel does not pump calcium, but only
allows it to flow in the direction of the concentration gradient (which is from the
coelenteron into the calicoblastic epithelium; Figure 4). What it does do is open and
close this calcium pathway in response to the voltage across the cell membrane. Such
voltage dependant channels are very common in creatures of every type, but how they work
on a molecular level is beyond the scope of this article. In short, however, the cell must
change its surface electrical potential in response to its internal calcium concentration
somehow, and this voltage change opens or closes the calcium channels.
The evidence that this channel is important in calcification is substantial. Chemical
inhibitors specific to these types of channels inhibit calcification in corals such as Galaxea
fascicularis11 (Figure 5) and Stylophora pistillata.4 Immunohistochemistry
has also been used to localize the cloned portion of the channel to the calicoblastic
epithelium (as well as the oral ectoderm).
The Role of Organics
Organic molecules are known to play a
substantial role in the formation of calcium carbonate in many organisms, including
abalone shells12 and other mollusk shells13 . These materials can be
proteins, glycoproteins, mucopolysaccharides, and phospholipids (and likely others that
have not yet been identified). They help to induce the nucleation and growth of aragonite
and are often referred to as the "organic matrix" because much of skeleton of
corals is comprised of these organic materials.
In the case of corals, we have relatively
little information about exactly what these organic materials are doing. The structures of
some of these proteins contain an unusually large number of aspartic acid residues. These
amino acids are capable of binding to calcium, but whether that is a critical function or
not has not been established. Here is some speculation about what these organics might be
doing with respect to calcification:
1. They may help control the concentration of
free calcium in the ECF, and thereby help control the rate of precipitation of CaCO3.
2. They may control the location of crystal
growth by binding free calcium and ferrying it to the location where the coral wants
precipitation to take place.
3. They may bind to the aragonite crystal
face and thereby control the rate of precipitation.
4. They may bind to the aragonite crystal
face and thereby prevent precipitation in places where the coral does not want the
skeleton to grow.
5. They may bind to the aragonite crystal
face and thereby inhibit binding of magnesium, phosphate, or other ions that are known to
inhibit the growth of calcium carbonate crystals.
Regardless of the mechanisms involved, the need for these
organics in calcification is easily verified. Allemand et al14 have
studied the role of such materials in Stylophora pistillata. Interestingly, they
find that inhibitors of protein synthesis reduce the rate of calcification considerably.
For example, reducing protein synthesis by 60-85% reduced calcification by 50%. A similar
result was found by inhibiting glycoprotein synthesis. These results did not come about
because of reduced metabolism, but rather by specific effects of reduced protein and
glycoprotein synthesis. The most important conclusion in their paper may be that the
rate of skeletogenesis may be more limited by the rate of biosynthesis and exocytosis of
organic matrix proteins rather than by calcium deposition.
Interestingly, the
apparently large need for a particular amino acid (aspartic acid) to synthesize these
proteins is satisfied by external sources, not by either the coral itself or the
zooxanthellae. For this reason, it might be interesting to see what added aspartic acid
does to calcification rates in reef tanks.
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Implications for
Reefkeeping: Calcium Concentration
Reefkeeping hobby lore has it that boosting
the calcium concentration above natural levels of 410 ppm does little to enhance
calcification in most corals. That idea is supported by experiments on Stylophora
pistillata where calcification becomes limited by calcium at levels below natural
levels, but is not increased above about 360 ppm.4 The relationship between
external calcium concentration and calcification rate displays exactly the behavior to be
expected if an active transport process were limiting the calcification rate, and that
this transport process is saturated with calcium at concentrations above 360 ppm.
Using some of the information provided in
previous sections, we can understand why this may be the case. Again, for Stylophora
pistillata, as the calcium level is increased in an artificial seawater medium from 0
to 800 ppm, the calcium uptake by the coelenteron increases in a linear fashion.4 The
uptake by most of the tissues other than the calicoblastic epithelium also increases in a
linear fashion. There is no data specific to the calicoblastic epithelium, but the data
show that calcification does not increase above 360 ppm calcium.
If the calcium is let into the calicoblastic
epithelium by a calcium channel, then the influx of calcium is dependent on the
concentration in the coelenteron, and the proportion of time that the calcium channels are
open. Since the cells themselves control the gating of the calcium channels, they
presumably can control their internal calcium levels at will UNLESS there is not enough
calcium outside of the cells to go through the gate, cross the calicoblastic epithelium
cells, and get to the active transporter that sends it into the ECF. Consequently, one
interpretation is that at external calcium concentrations below 360 ppm, the calcium flux
into the calicoblastic cells becomes the rate-limiting step in calcification.
There is a second interpretation that is also
possible, however. In this scenario, calcium enters the calicoblastic epithelium through
the gated channels, but is not controlled very well in the cell. As the calcium
concentration in the coelenteron drops, the concentration inside of the cell drops
(regardless of whether there is a large efflux or not), making it harder for the active
transport to pump the calcium into the ECF, and thereby decreasing the rate of
calcification.
The difference between these two scenarios is
rather esoteric, and probably not of interest to most reefkeepers, but it is
intellectually stimulating nevertheless. The difficulty in distinguishing these two
scenarios comes about because the nature of the control of the calcium level in these
cells is unknown. How exactly the large influx is regulated in relation to the large
efflux is not understood and has apparently never been investigated.2
Consequently, we cannot yet know whether calcification drops primarily because the influx
through the gates cannot keep up with the efflux rate when calcium concentrations in the
coelenteron are low, or whether it drops primarily because the active transport of the
calcium into the ECF cannot keep up when the calcium concentration in the calicoblastic
cell is low.
Implications for Reefkeeping: pH
It is well known in the scientific
literature, if not in the reefkeeping hobby, that calcification is slowed considerably as
the pH is lowered below natural levels.15,16 This result is especially
concerning and is a hot topic of research because of the decrease in the pH of the oceans
as CO2 is added to the atmosphere. The predictions of
reduced calcification in coral reefs in the future are substantial. Again, taking the
previous sections as a backdrop, we can begin to understand why.
As the pH of the external fluid is lowered,
it becomes harder and harder for cells to excrete the excess protons that come about from
calcification. That is, they take in bicarbonate, strip off a proton, precipitate the
carbonate into their skeleton and then have to do something with that proton. Many of
those protons can be used to make CO2 out of bicarbonate, and may thereby boost the rate of
photosynthesis.1 Still, not all of the protons may be used this way, and some
will be excreted.
In the model of McConneaughey and Whelan,1
protons are pumped into the calicoblastic epithelium, and then allowed to somehow move
down the concentration gradient from the lower pH cell interior (higher proton
concentration) to the coelenteron (higher pH, lower proton concentration). If the seawater
pH drops, the coelenteron pH will likely also drop since it is exchangeable with the
external fluid. As the pH drops in the coelenteron, the efflux of protons from the
calicoblastic epithelium cells will be slowed as that process is likely gradient
dependent. Finally, as the pH drops inside of the calicoblastic epithelium because protons
are not leaving as readily, it becomes harder and harder for the Ca++/2H+
transporter to pump protons out and calcium into the ECF.
In the model of Furla et al9
where bicarbonate is actively pumped into the ECF, protons must either be actively pumped
away, or they must passively diffuse away (the latter probably is not efficient enough to
work as the ECF could never then have a pH higher than the coelenteron, something that
would mitigate against supersaturation and precipitation of aragonite). In either case,
the process may not function as well at lower coelenteron pH as the proton gradient
between the ECF and the coelenteron becomes smaller or more likely, bigger in the wrong
direction. That is, protons will be less inclined to leave the ECF for the coelenteron.
Consequently, the supersaturation of calcium carbonate, which is highly dependent on pH,
will decrease, and hence calcification will also decrease.
Figure 6. A Porites sp. coral similar to those
used in some of the calcification studies described in this article. The picture was taken
by Joe Burger of a coral in his aquarium.
Implications for Reefkeeping: Alkalinity
Unlike the calcium concentration, it is widely believed that
certain organisms calcify faster at higher alkalinity than in normal seawater. This result
has also been demonstrated in the literature, where it has been shown that adding
bicarbonate to seawater increases the rate of calcification in Porites porites17
Figure 6). In that case, a doubling of the bicarbonate concentration resulted in a
doubling of the calcification rate.
Much, though not all, of the carbon source for
calcification comes from external bicarbonate. As the alkalinity is reduced (at a given
pH) the bicarbonate concentration (which comprises the bulk of the alkalinity in seawater)
will also be reduced. Diffusion of bicarbonate or diffusion of CO2 from the
coelenteron can apparently become rate limiting in many corals. In part this may be due to
the fact that both photosynthesis and calcification are competing for bicarbonate, and the
fact that the external bicarbonate concentration is not that large to begin with (when
compared, for example, to the calcium concentration).
One Final Anomaly
As a final thought experiment, consider how
materials other than calcium and carbonate get into the skeletons of corals. A variety of
metals get into these structures, the most notable of these being strontium, but others
include cadmium, manganese, lead, uranium and barium. How do they get there?
Apparently the relative concentrations of
strontium (and other metals) and calcium in the ECF are similar to seawater2 based
on skeletal incorporation rates in a large variety of coral species and locations. Once in
the ECF, they get incorporated as impurities in the growing crystal in direct proportion
to their concentration relative to calcium. The oddity is that these metals are able to
get in and be precipitated in the same ratios in many different corals (though seemingly
not all), and apparently in ratios suggesting that the relative concentrations are similar
to bulk seawater. It has been claimed that the active transporter that sends calcium into
the ECF can readily distinguish calcium from strontium, for example, and that strontium
must enter in another fashion (e.g., in Galaxea fascicularis18 ). Why,
if these metals enter in some other way, is the relative concentration so constant across
many species of corals?
Consequently, it is a current topic of
debate in the literature as to how these other metals become incorporated into aragonite
skeletons. This topic is especially important, as the calcium to strontium incorporation
rate in ancient coral skeletons has been widely discussed as a paleothermometer, with the
incorporation rate varying in a known fashion with temperature. Understanding how the
incorporation actually takes place would seem to be an important aspect of that
hypothesis.
Another important issue has to do with the
inhibition of calcification by phosphate and phosphate-containing organics. Phosphate is
known to inhibit the precipitation of calcium carbonate from seawater.19, 20, 21 Phosphate
also decreases calcification in corals, such as Pocillopora damicornis22 and
entire patch reefs23 . This inhibition is likely related to the presence of
phosphate in the ECF and on the growing crystal surface. Exactly how the phosphate gets in
isn’t well understood. Nevertheless, the next time you are worried about phosphate
levels in your tank, you can think of calcification inhibition in the ECF in addition to
the driving of unwanted algae in your tank.
This inhibition of calcification takes place
at concentrations frequently attained in reef tanks, and may begin at levels below those
detectable by hobby test kits. For example, one research group found that long term
enrichment of phosphate (2 mm; 0.19 ppm; maintained for 3 hours
per day) on a natural patch reef on the Great Barrier Reef inhibited overall coral
calcification by 43%.23 A second team found effects in several Acropora species
at similar concentrations.24
Organic phosphate and phosphonate inhibitors
of calcification have also been studied and probably work by a similar mechanism. HEBP, a
bisphosphonate that is shown below, causes a 36% inhibition of calcification in Stylophora
pistillata at 10 mm, and stops it completely (99%) at 500 mm.25
The Story Continues….
There are a host of additional interesting
topics related to calcification that have not been covered here. This article, for
example, has not delved into the biological details of calcification, such as the nature
of the tissues around the site of calcification. For those interested, some of these
biological and mechanistic details are described by Barnes.3 This article has
also not described the hypothesized relationship between calcification and photosynthesis,
but it is an interesting one and worthy of understanding.1, 2 What this article
has tried to accomplish is to provide a more detailed understanding of the chemical
aspects of calcification taking place in your reef tank.
References
1. Calcification generates protons for
nutrient and bicarbonate uptake. McConnaughey, T. A.; Whelan, J. F. Marine Research,
Biosphere 2 Center, Highway 77, PO Box 689, Oracle, USA. Earth-Sci. Rev. (1997), 42(1-2),
95-117.
2. Photosynthesis and calcification at
cellular, organismal and community levels in coral reefs: a review on interactions and
control by carbonate chemistry. Gattuso, Jean-Pierre; Allemand, Denis; Frankignoulle,
Michel. Observatoire Oceanologique, LOBEPM, UPRESA 7076 CNRS-UPMC, Villefranche-sur-mer,
Fr. Am. Zool. (1999), 39(1), 160-183.
3. The Structure and formation of
growth-ridges in scleractinian coral skeletons. Barnes, D. J. Proc. Roy. Soc. Lond. B.
(1972) 182, 331-350.
4. A compartmental approach to the
mechanism of calcification in hermatypic corals. Tambutte, E. Allemand, D. Mueller, E.
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