From the archives: The debate over calcium carbonate


A rather terrible limerick by Kelly (originally posted 16th April 2012)

There once was a rather heated debate,

over the formation of calcium carbonate.

Was the biology to blame?

Or the scientists wrong to claim?

That climate is buried in coral precipitate.

For those who don’t work with the ocean’s carbonate system, or with calcium carbonate bearing organisms, one of the longest standing and most heated debates revolves around how these organisms actually secrete their skeletons. Scientists just can’t agree on the molecular mechanism behind the precipitation of biogenic calcium carbonate, or calcification as it is commonly known. Is there active transcellular transport of specific cations (i.e calcium), or are the necessary cations obtained through direct seawater transport? And can the internal pH of this calcifying fluid be biologically controlled to favour precipitation?

While this might not sound like cause for contention, there is a lot riding on the outcomes of this research (and has been an element of pugilism along the road to discovery) . Many important archives that we rely on for climate reconstructions  come out of biogenic carbonates; think corals, both surface and deep-sea,and foraminifera. These organisms are ubiquitous in the fossil record and the way we interpret the chemical composition of calcium carbonate relies on our understanding of how it formed in the first place. From these archives our climate reconstructions and modelled scenarios inform policies that ultimately dictate how we will respond to current climate change. And then of course there is ocean acidification (cue dramatic music). Will the great reefs of the world survive the current drop in ocean pH?   Am I giving this whole calcification argument enough weight yet?

There has recently been a flurry of activity in the scientific literature, with some heavy hitters all stepping into the ring. Unfortunately, not all of these articles are freely accessible but I shall give you the run down on, and links to, the current bout in the calcification prize-fight.

In the blue corner, Venn et al., (2011) published an article that finally showed tangible evidence that the extracellular pH under the calcifying tissue of  the surface coral Stylophora pistillata, was elevated 0.5  and 0.2 pH units above that of the surrounding seawater  (in light and dark conditions respectively: think increased energy for cellular processes during your symbiont’s peak photosynthetic  period). This is of great importance as these conditions favoured calcification in contrast to the intracellular pH of the tissue which remained stable.  So we understand more about calcification for one species….

Emerging from the shadows in the black corner, McCulloch et al., (2012) in Nature Climate Change last week, went further to say that due to biological control over the pH of this calcifying fluid, there will be many more corals with enhanced resilience to the effects of ocean acidification. But the authors caution that their model is from the perspective of mineral precipitation kinetics. In reality, corals function as part of complex ecosystems that are exposed to threats and pressures beyond acidification. There point is that there will be many corals with the ability to ‘up-regulate’ their internal pH (proposed to occur through Ca–ATPase pumps that exchange  H+ ions from the site for Ca2+ ions) and will therefore have a much lower sensitivity to proposed acidification scenarios.

(One point that I personally don’t believe has been given nearly enough weight: what is the point in continuing to calcify if the rest of your skeleton is dissolving around you?)

And from the red corner, providing a winning jab for the fidelity of oceanographic proxies,  Gagnon et al., (2012) further argue for the direct seawater transport model i.e. one that supports a relationship between the chemistry of the calcifying fluid and external seawater. They demonstrate the relationship between the seawater medium where they grew  Stylophora pistillata, and the isotopic and elemental distribution of newly calcified carbonate. They used a rare earth element Terbium, Tb, with no known biological function and other major cations (ie calcium, strontium and barium) as tracers. They were able to determine that the uptake dynamics was the same for each tracer and therefore shared a common mechanism.

While I may appear facetious at times, this really is a serious debate. If we are using coral skeletons to reconstruct climate we must be able to accurately translate the geochemical signal in biogenic carbonates. This relies heavily on how well we can constrain the chemistry of the calcifying fluid from which these carbonates precipitate, and what relationship this has to surrounding seawater. Where in the past there has been major disagreement, it would appear that research groups are converging on common theory, although I’m not sure anyone is taking their gloves of just yet.

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