Coral Reef Science:  Development Highlights

Habib Sekha

K. Schneider, J. Erez. 2006. The effect of carbonate chemistry on calcification and photosynthesis in the hermatypic coral Acropora eurystoma. Limnol. Oceanogr., 51(3), 1284-1293.

Abstract:

The rise in atmospheric CO₂ has caused significant decrease in sea surface pH and carbonate ion (CO₃^2-) 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, CO₃^2-, 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 CO₃^2- concentration, suggesting that the corals are not sensitive to pH or CT but to the CO₃^2- concentration. A decrease of ~30% in the CO₃^2- 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 CO₃^2- ions for calcification and CO₂ (aq) for photosynthesis.

Comments:

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 or both.

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 system.

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 acceptable levels.

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.

Abstract:

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 routinely monitored.

Comments:

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 of DOC.

E.D. Lapid, N. E. Chadwick. 2006. Long-term effects of competition on coral growth and sweeper tentacle development. MEPS Vol. 313:115-123.

Abstract:

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 region.

Comments:

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.