|dc.description.abstract||Over half of the world's calcification is carried out by algae or by
organisms which harbour them, such as coccol ithoph ores, foraminiferans,
coralline seaweeds and reef-building corals. Calcification acts as a sink for
inorganic carbon and although rather little is known about the precise
mechanisms of biological CaC03 formation, the process as a whole is thought
to be under threat from atmosphericC02 rise.
This study examined the response of a reef-building coral, Montipora
digitata and a coralline seaweed, Corallina officinalis to the main factors
which influence calcification, namely light, dissolved inorganic carbon (DIC),
pH, nitrate and calcium.
In contrast to the commonly held view, this study demonstrates that both
photosynthesis and calcification were carbon limited in seawater. Since the
degree of stimulation by DIC in the light was different for each process, and
dark calcification also increased with added DIC, it is clear that
photosynthesis and calcification are only loosely coupled.
Simultaneous pH measurements were made on the surface of the
epithelium and at the site of calcification in the coral Galaxea fascicularis
using pH microelectrodes, and demonstrated for the first time that pH at the
site of calcification is not a simple response to seawater pH.
In this study, nitrate inhibition of calcification was shown to be more
powerful in the dark than in the light, indicating that daylength may be a more
significant factor in coral biology than previously realised.
The currently-accepted hypothesis that biological calcification rates are a
simple function of seawater CaC03saturation state was tested
experimentally. Results from both Corallina officinalis and Montipora digitata
a) calcification is far more responsive to changes in inorganic carbon
than to calcium concentrations; and
b) when [C03 2-] is kept constant, increases in [HC03-1 cause dramatic
increases in calcification rates, even at reduced pH.
All of these data suggest that calcification in M. digitata and C. officinalis
is a strongly biologically controlled process, influenced principally by the
seawater bicarbonate concentration and pH, but strongly mediated by light
and combined nitrogen.||en_US