Why Corals Care About Ocean Acidification: Uncovering the Mechanism

Cohen, A.; Holcomb, M.

Oceanography 22(4): 118-127


ISSN/ISBN: 1042-8275
DOI: 10.5670/oceanog.2009.102
Accession: 068510009

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Stony corals build hard skeletons of calcium carbonate (Ca CO3) by combining calcium with carbonate ions derived, ultimately, from seawater. The concentration of carbonate ions relative to other carbonate species in seawater is rather low, so corals expend energy to raise the p H of seawater sequestered in an isolated, extracellular compartment where crystal growth occurs. This action converts plentiful bicarbonate ions to the carbonate ions required for calcification, allowing corals to produce Ca CO3 about 100 times faster than it could otherwise form. It is this rapid and efficient production of Ca CO3 crystals that enables corals to build coral reefs. Ocean acidification reduces the p H and thus the abundance of carbonate ions in seawater. Corals living in acidified seawater continue to produce Ca CO3 and expend as much energy as their counterparts in normal seawater to raise the p H of the calcifying fluid. However, in acidified seawater, corals are unable to elevate the concentration of carbonate ions to the level required for normal skeletal growth. In several experiments, we found that boosting the energetic status of corals by enhanced heterotrophic feeding or moderate increases in inorganic nutrients helped to offset the negative impact of ocean acidification. However, this built-in defense is unlikely to benefit corals as levels of CO2 in the atmosphere continue to rise. Most climate models predict that the availability of inorganic nutrients and plankton in the surface waters where corals live will decrease as a consequence of global warming. Thus, corals and coral reefs may be significantly more vulnerable to ocean acidification than previously thought.