Ocean Acidification (Holy Carbonate Chemistry, Batman!)

Copyright (c) 2009 Richard Ling

Copyright (c) 2009 Richard Ling

So there’s another consequence for our fossil fuel addiction…it’s called ocean acidification.  Ocean acidification is actually a bit of a misnomer (the oceans are actually a bit on the basic side, so during ‘ocean acidification’ they are actually becoming less basic, rather than acidic, for now), but I digress.  During what some have begun to call the Anthropocene (the period of time in which humans have begun to have major environmental and climatic effects—think the Industrial Revolution to well, now), human-produced carbon emissions have risen substantially…about 40% or so from preindustrial levels of about 280 parts per million.  This really shouldn’t be news to you, especially with the widespread coverage of carbon dioxide’s role in climate change.  However, ocean acidification is an issue that really hasn’t been disseminated very well, and it has some potentially dire consequences.  Here’s the deal:  the ocean acts as our planet’s only true carbon sink, about a third or so of atmospheric carbon dissolves into the ocean.  As all of this carbon (usually in the form carbon dioxide) dissolves into the sea, it almost immediately reacts with water and undergoes a series of reactions.  I’ll spare you the chemical details for now, but the bottom line is that through these carbonate chemical changes, the ocean’s pH is reduced (becoming less basic), and the ratio of dissolved inorganic carbon has been changed.  What does that mean?  Well, organisms in the ocean that calcify, usually to make skeletons, tests, or shells (e.g. some species of corals, certain types of plankton, etc.), make use of the carbonate ion in seawater.  Marine calcifying organisms precipitate calcium carbonate (limestone); calcium is not limiting in the ocean, but carbonate is thought to be.  As the oceans acidify, the concentration of carbonate falls, potentially making it more difficult for organisms to form calcium carbonate (in a broad sense).

Let’s take the case of corals.  Scleractinian corals are those that can precipitate a calcium carbonate skeleton, and are sometimes referred to as reef-building corals.  The particular type of calcium carbonate that these organisms precipitate is called aragonite (some other organisms use calcite, the other mineralogical species, but let’s just consider corals for now).  Under acidified conditions, the oceans become less saturated in respect to aragonite, meaning that aragonite does not precipitate from seawater as readily, potentially bad news for marine organisms with aragonite skeletons such as scleractinian corals.  The pH of the ocean has already dropped about 0.1 below pre-anthrocene levels and is expected to drop another 0.3-0.4 units by the end of this century.  This may not seem like a lot, but for organisms that have evolved to occupy very narrowly defined environmental parameters (e.g. temperature, sediment load, light, etc.), this would be a tremendous physiological test.  Corals in many locales are already under thermal stress due to rising sea surface temperature and face the threat of bleaching (due to dissociation from their symbiotic dinoflagaellate algal cells–but that’s another story).

Quite a few studies have sought to quantify ocean acidification’s effects on scleractinian corals, and the overall theme is that calcification appears to be reduced with decreasing pH.  This trend seems to be quite robust and linear in some cases.  However, as with most things, the devil’s in the details.  Earlier studies seeking to simulate ocean acidification in the laboratory do so with acid additions in seawater.   Does this lower the pH? Sure.  Does it decrease the aragonite saturation state?  Absolutely.  But how acid additions accomplish this is not really comparable with what is happening in the ocean.  Adding acid to seawater not only decreases carbonate concentrations, but also bicarbonate (another major ion in seawater).  When carbon dioxide dissolves into the ocean, carbonate decreases, but bicarbonate actually goes up (carbonate ions are increasingly converted to bicarbonate).  A recent study (Jury et al. 2009) actually took another approach, in that they adjusted seawater chemistry with a combination of bubbling in carbon dioxide and manipulating total alkalinity.  They found that calcification rates of corals (they used Madracis mirabilis sensu in their laboratory experiments) did not respond consistently to pH or aragonite saturation state.  Ocean acidification in respect to scleractinian corals appears to be a much more complicated issue than previously thought; and it seems that the responses of these organisms are not determined by aragonite saturation state alone. However, this is a complex issue in which multiple parameters are involved and different coral species may be sensitive to different facets of seawater chemistry.  A more holistic view of calcification and marine chemistry will have to be adapted by researchers seeking to elucidate these tenets of global change.

Ocean acidification isn’t limited to corals.  All marine calcifying organisms are thought to be potentially affected, especially as carbon emissions escalate.  Effects on less charismatic calcifying creatures, such as coccolithophores (a type of plankton) and pteropods (a gastopod), may have profound effects on oceanic geochemical dynamics, climate change, and even food webs.  In addition, the lesser-known brethren of shallow corals, deep-sea corals, may be affected as well.  More to come on these residents of the deep later.

Note (24 March 2011):  It has come to my attention that this essay could be misconstrued to mean that the effects of ocean acidification have been overstated.  This is not the case.  This essay highlighted a specific paper that reported interesting results that may indicate that this phenomenon is not as simple as previously thought.  Unknowns abound, particularly at the ecosystem level.  However, the evidence for ocean acidification’s effects on marine communities is extensive and I believe it to be one of the most important anthropogenic impacts—large-scale disturbances in the carbon cycle are nothing to be trifled with and what we are currently doing is geologically unprecedented.  See my further treatment of OA here, at The Urban Times, and here.

The following references (and the works cited therein) provide a nice review of the topic, and information provided by them was drawn upon for this post.

Image:  Richard Ling on Flickr (cc 2.0)

ResearchBlogging.org

Jury, C., Whitehead, R., & Szmant, A. (2009). Effects of variations in carbonate chemistry on the calcification rates of Madracis auretenra (=Madracis mirabilis sensu Wells, 1973): bicarbonate concentrations best predict calcification rates
Global Change Biology DOI: 10.1111/j.1365-2486.2009.02057.x

Orr, J., Fabry, V., Aumont, O., Bopp, L., Doney, S., Feely, R., Gnanadesikan, A., Gruber, N., Ishida, A., Joos, F., Key, R., Lindsay, K., Maier-Reimer, E., Matear, R., Monfray, P., Mouchet, A., Najjar, R., Plattner, G., Rodgers, K., Sabine, C., Sarmiento, J., Schlitzer, R., Slater, R., Totterdell, I., Weirig, M., Yamanaka, Y., & Yool, A. (2005). Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms Nature, 437 (7059), 681-686 DOI: 10.1038/nature04095

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3 thoughts on “Ocean Acidification (Holy Carbonate Chemistry, Batman!)

  1. Thank you for this. I was sitting at work, avoiding the phone, and trying to figure out why I didn’t get it. In my daytimer I’ve got cacium carbonate + hydronium + bicarbonate, and I’m staring at it. I don’t know what happens when you add more of the last two terms — do you get more calcium bicarbonate? Anyway, the fact that nobody seemed to mention more bicarbonate with acidification made me even more confused. It’s nice to see it mentioned here.

  2. Pingback: The Plight of Staghorn Coral « Anthozoa

  3. Pingback: Acidification and Extinctions « Anthozoa

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