Reef Science: Development Highlights
K. Schneider, J. Erez. 2006. The effect of carbonate
chemistry on calcification and photosynthesis in the hermatypic
coral Acropora eurystoma. Limnol. Oceanogr., 51(3),
The rise in atmospheric CO2 has caused significant
decrease in sea surface pH and carbonate ion (CO32-)
concentration. This decrease has a negative effect on calcification
in hermatypic corals and other calcifying organisms. We report
the results of three laboratory experiments designed specifically
to separate the effects of the different carbonate chemistry
parameters (pH, CO32-, CO2
[aq], total alkalinity [AT], and total inorganic carbon [CT])
on the calcification, photosynthesis, and respiration of the
hermatypic coral Acropora eurystoma. The carbonate
system was varied to change pH (7.9 - 8.5), without changing
CT; CT was changed keeping the pH constant, and CT was changed
keeping the pCO2 constant. In all of these experiments,
calcification (both light and dark) was positively correlated
with CO32- concentration, suggesting
that the corals are not sensitive to pH or CT but to the CO32-
concentration. A decrease of ~30% in the CO32-
concentration (which is equivalent to a decrease of about
0.2 pH units in seawater) caused a calcification decrease
of about 50%. These results suggest that calcification in
today's ocean (pCO2 = 370 ppm) is lower by ~20%
compared with preindustrial time (pCO2 = 280 ppm).
An additional decrease of ~35% is expected if atmospheric
CO2 concentration doubles (pCO2 = 560
ppm). In all of these experiments, photosynthesis and respiration
did not show any significant response to changes in the carbonate
chemistry of seawater. Based on this observation, we propose
a mechanism by which the photosynthesis of symbionts is enhanced
by coral calcification at high pH when CO2 (aq)
is low. Overall it seems that photosynthesis and calcification
support each other mainly through internal pH regulation,
which provides CO32- ions for calcification
and CO2 (aq) for photosynthesis.
Increasing alkalinity increases the calcification rate and
has been linked to an increase in the supersaturation value
for calcium carbonate. However, it was not determined if the
increase in the calcification rate by increasing alkalinity
was caused by the induced changes in bicarbonate, carbonate
Authors of the above abstract have separated the effects
of the individual components (CO2, bicarbonate,
carbonate) by designing their experiment to vary the parameters
one at a time, while at the same time minimizing changes in
other parameters of the CO2-bicarbonate-carbonate
They conclude that it is primarily the carbonate concentration
of the alkalinity components that determines the calcification
rate. At a constant alkalinity, a pH reduction of 0.2 reduced
the calcification rate in Acropora eurystoma by 50%.
Keeping the alkalinity constant but reducing the pH lowers
the carbonate and increases the bicarbonate concentration.
If we take the above information about carbonate for granted,
it means that systems with a low pH would show a lower rate
of calcification than a system with a normal pH, if the alkalinity
in both systems is the same. That could be compensated by
maintaining a much higher alkalinity in the low pH system,
because that would increase the carbonate concentration to
Systems with a high pH but also a somewhat low alkalinity
level could show increased calcification rates because carbonate
concentration will be higher than normal.
D.I. Kline, N.M. Kuntz, M. Breitbart, N. Knowlton, F.
Rohwer. 2006. Role of elevated organic carbon levels and microbial
activity in coral mortality. MEPS Vol. 314:119-125.
Coral reefs are suffering a long-term global decline, yet
the causes remain contentious. The role of poor water quality
in this decline is particularly unclear, with most previous
studies providing only weak correlations between elevated
nutrient levels and coral mortality. Here we experimentally
show that routinely measured components of water quality (nitrate,
phosphate, ammonia) do not cause substantial coral mortality.
In contrast, dissolved organic carbon (DOC), which is rarely
measured on reefs, does. Elevated DOC levels also accelerate
the growth rate of microbes living in the corals' surface
mucopolysaccharide layer by an order of magnitude, suggesting
that mortality occurs due to a disruption of the balance between
the coral and its associated microbiota. We propose a model
by which elevated DOC levels cause Caribbean reefs to shift
further from coral to macroalgal dominance. Increasing DOC
levels on coral reefs should be recognized as a threat and
Often coral mortality occurs even though the commonly measured
nutrients (nitrate, phosphate, ammonia) are within normal
ranges. Dissolved organic carbon (DOC) is not commonly measured
around reefs, but the authors state that it does cause substantial
mortality. It's very likely that they state in the publication
the concentrations, and possibly also the origin and composition,
of the DOC they are referring to.
They propose that the highly accelerated growth rate, caused
by the DOC, of bacteria living on the corals' surface might
disrupt the balance between the coral and its associated microbiota,
thereby resulting in mortality. They associate elevated DOC
in the Caribbean to the decline in the number of coral colonies,
and conclude the abstract by recommending routine measurement
E.D. Lapid, N. E. Chadwick. 2006. Long-term effects of
competition on coral growth and sweeper tentacle development.
MEPS Vol. 313:115-123.
Outcomes of competition between corals vary temporally and
spatially, and depend in part on the agonistic mechanism involved.
Competition may impact coral growth, reproduction and energy
reserves, however few experimental studies have quantified
these effects. We conducted a 1 yr laboratory experiment on
competition between 2 massive corals, Platygyra daedalea
and Favites complanata. Colonies of P. daedalea
developed sweeper tentacles and extensively damaged the F.
complanata colonies, causing them to loose ca. 55% of
their soft tissue and eventually killing 30% of F. complanata
colonies. Skeletal accretion rate varied widely among corals
within each treatment, and correlated negatively with the
percent tissue damaged on competing colonies of F. complanata.
On isolated control colonies, sweeper tentacles developed
randomly throughout the year, and then reverted back to feeding
tentacles. They appeared to serve as probes to detect the
approach of competitors. Development of sweeper tentacles
is a powerful aggressive/defensive mechanism that may enable
brain corals to dominate some reef areas in the Indo-Pacific
Platygyra daedalea apparently uses its feeding tentacle
as a probe to detect approaching competitors. If a competitor
is detected, sweeper tentacles develop as demonstrated in
these experiments. The sweeper tentacles can wipe out the
competing coral. Isolated colonies of F. complanata
as a control experiment showed a random variation in the development
of feeding and sweeper tentacles throughout the year.