Environmental Engineering Reference
In-Depth Information
and hydroxide ions (OH
−
). A major portion of the CO
2
molecules is taken up by the zooxanthellae (ZOOX). The
hydroxide ions help stabilize the pH of the coelenteron by
reacting with protons (H
+
ions).
Fig. 6.22
The uptake of bicarbonate ions from seawater
by the ectoderm of a coral polyp. The ions diffuse through
both cellular layers and the mesoglea, after which they
enter the coelenteron. A part of the ions splits into CO
2
Source
Furla et al. (
2000
)
Building a coral skeleton is a complex process
and can be divided into two major steps:
Step 1. Uptake of calcium and bicarbonate ions
The ion uptake occurs from the water
column through the oral pore and
through the ectoderm. Next, the ions
enter the gastrovascular cavity or coe-
lenteron. This process is diffusion
dependent and therefore passive, where
no energy is required (Fig.
6.22
).
A part of the ions splits into carbon
dioxide and hydroxide ions (OH
−
).
A major portion of the carbon dioxide
molecules is taken up by the zooxan-
thellae. The hydroxide ions help stabilize
the pH of the coelenteron by reacting
with protons (H
+
-ions) into water (H
2
O).
Step 2. Growth of skeletal mass
The next step is to transport the bicar-
bonate ions to the so-called calicoblastic
fl
called adenosine tri phosphate (ATP).
ATP itself is produced in the power
reactors of a living cell, the mitochon-
dria. The calicoblastic cells in the outer
skin layer are highly enriched with these
cell organelles (Fig.
6.22
), and work
hard on a daily basis to allow corals to
build their skeleton. ATP is produced by
oxidizing carbohydrates and fatty acids,
and the yielded energy is used to trans-
port mainly calcium ions over the cali-
coblastic layer (Fig.
6.23
). The needed
carbohydrates are produced by the zoo-
xanthellae and provide up to 95 % of the
energy budget. The transport of bicar-
bonate ions takes place by the exchange
of negatively charged molecules from
the external environment (represented as
A
−
). This principle is called antiport.
The pumping of calcium ions over the
cellular membrane is also carried out by
an antiport system; the only difference is
that the calcium ions as well as the
protons (H
+
) have to be transported
across a gradient. ATP, in the end, is the
energy carrier for this process allowing
the calcium/H
+
-pump to perform its task.
uid; this is the stagnant layer of water
located directly beneath a coral polyp,
where aragonite deposition takes place.
This process is energy demanding,
which requires an energy carrier. This is
provided by very commonly used mol-
ecule by all life on our planet, and it is
