Tropical Synapses: Marine Conservation: The Coral Reef

Submitted as the third writing assignment in the Marine Megafauna course at Coursera.com this month.

How Might Ocean Acidification Affect the Coral Reef?

One might think that the coral reef ecosystem, as a warm water ecology confined by requirements for warmer water temperatures to mostly tropical and semi-tropical waters between 30 degrees north and south of the equator, might see little negative impact from global warming. Indeed, one might optimistically expect that Atlantic and Pacific coral reefs might extend their overall habitat, if shallow enough waters permitted, perhaps to 35 or even 40 degrees north and south, as the subtropical seas warm.

Yet there are more pessimistic concerns which moderate optimism about global warming’s relation to coral reef ecosystem health. First, some regional endemic coral species, especially in warmer equatorial waters, may not adapt readily to rapid rises in water temperature, so a decline in local species diversity might occur. Second, thermal stressors might alter disease susceptibility, which has already been suggested to be leading to loss of coral health in the Great Barrier Reef. Third and perhaps more importantly, global warming is occurring in association with a slow but steady increase in atmospheric carbon dioxide, likely from human burning of fossil fuels. When atmospheric carbon dioxide dissolves in the oceans, it does so as the weak acid, carbonic acid, which lowers the alkaline ocean pH. Because the primary structural mineral of the reef (calcium carbonate in the form of crystalline aragonite) is more soluble in an acid environment and so tends to dissolve in more acid watery environments, global ocean acidification creates a problem which may threaten the structural integrity of all coral ocean reef habitats. The Intergovernmental Panel on Climate Change has projected that all of our oceans’ pH will potentially fall by 0.3 to 0.4 pH units over the next century. This report will focus on the conservation issue of ocean acidification and its potential implications for coral reef species.

Coral species are very diverse, and there are soft as well as hard species of coral. Even soft-bodied corals contain a calcite skeleton which, cemented by successive generations of organisms, forms over time a layered stony structure underlying even soft-coral-predominant reefs. For the purposes of this article, we will focus on a single hard coral species, the commonly named “hood” or “smooth cauliflower” coral Stylophora pistillata, which is listed in conservation status by the IUCN as Near Threatened. This coral species has a wide distribution in the tropical and semitropical regions of the Indian and Pacific oceans as well as the Red Sea, but is not generally found in Atlantic or Caribbean reefs. S. pistillata spreads readily within a given ocean basin by larval attachment and colony formation on the under-surface of driftwood or floating pumice. Its polyps tend to be about 1 mm in diameter and several cm long, and though its colonies can be confluent to meters in diameter, they are typically just centimeters in size, sometimes much smaller when seen as young colonies on smaller floating structures. Because it can thus form easily lab-transportable intact colonies, S. pistillata has been a favorite for laboratory studies of coral organism metabolism. Like all hard corals, its coral polyps secrete a calcium carbonate exoskeleton which forms the body of the coral colony and ultimately contributes to the stony base habitat of the reef itself. It is this calcium carbonate (aragonite) mineral exoskeleton which may be threatened by the conservation issue of ocean acidification.

I will focus on the paper “Live tissue imaging shows reef corals elevate pH under their calcifying tissue relative to seawater,” written by Alexander Venn, Eric Tambutté, Michael Holcomb, Denis Allemand, and Sylvie Tambutté and published in PLoS One on May 27, 2011 (onlinehttp://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0020013). This article reports a research study designed to probe the chemical micro-environment at the polyp-mineral reef interface, where the polyp attaches to the rock of the reef. There, the polyp has a specialized outer layer called the calicoblastic epithelium. This layer of cells secretes the mineral deposits which cause the rock of the reef to slowly grow by accretion.

The researchers collected microscopic photographs of the cell-aragonite crystal boundary and measured the micro-environmental pH at this polyp-mineral interface. In particular, the study microscopically sampled both intracellular and extracellular fluid pH at this interface. The study showed that, even in the midst of flowing seawater, the polyp actively created and maintained a more alkaline pH at its active aragonite crystallization region just under the polyp. This crystallization area, which was always in intimate contact with the calicoblastic cells, was kept more alkaline than surrounding seawater pH of 8.15 by about 0.5 pH units in sunlight and 0.2 pH units in darkness. Thus, the article suggests that coral can and does compensate for a more acid ambient seawater pH than is ideal for calcium mineral deposition by actively maintaining a more alkaline pH locally in the region where alkaline pH is most critical for precipitation of aragonite calcium carbonate crystals.

The researchers kept ambient seawater pH at 8.15 and did not assess pH differences in a more acid environment. Nevertheless, this result seems reassuring, since it supports the hypothesis that, as long as a smooth cauliflower coral Stylophora pistillata colony remains healthy in other respects, it should be able to compensate (especially during daylight and with some small additional metabolic energy costs) for the predicted rises in pCO2 and drops in ocean pH over the next hundred years. Thus, tropical reef building may well continue beyond the year 2100 despite predicted increases in atmospheric carbon dioxide. Note that unmitigated optimism is not warranted by this study. We do not expect metabolic flexibility and pH tolerance to be the same across all species of coral, and furthermore, much of the lower supporting portions of today’s barrier reefs are composed of older, less live-populated, deep ocean aragonite rock walls, which might still tend more toward dissolution with drops in ocean pH.

Brief biography of a conservation biologist who is studying the consequences of ocean acidification on reef ecosystems: Jean-Pierre Gattuso.

Internet web page: http://www.obs-vlfr.fr/~gattuso/

Jean-Pierre Gattuso is a senior research scientist at the Laboratoire d'Oceanographie de Villefranche, in France. Dr. Gattuso is the editor of the journal Biogeosciences and has been active on several American and European expert panels on climate change, serving as an expert regarding the potential effects of future ocean acidification. His research interests include the cycles of carbon and carbonate in coastal ecosystems and the responses of ocean organisms and ecosystems to changes in the overall environment, including ocean acidification.

Marine Conservation: The Coral Reef

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