Future extinction for Ehux?

[U Riebesell et al. "Reduced calcification of marine phytoplankton in response to increased atmospheric CO2", Nature, 407, pg 364-7, 21 September, 2000]

Ehux is a wide-ranging species, found as a member of the phytoplankton community in almost all oceans (see biogeography). Given that it blooms in many areas, and many remote oceans far from land, surely it cannot be seriously affected by human activities? Well not so, if some recent research is correct at least.

Dissolved Inorganic Carbon System and Calcification

One of the most direct effects humankind is having on Earth's environment is to pump colossal amounts of carbon dioxide into the atmosphere, which will severely affect global climate. But it is not the climate effects, but instead a more direct effect of extra CO2 that may spell trouble for Ehux. A large part of the extra CO2 added to the atmosphere does not stay there but instead moves on to cross the sea surface and enter the surface ocean, raising the level of dissolved carbon there. The extra CO2 has the effect of acidifying the ocean, lowering the pH. It is this effect which could lead to problems for Ehux.

The ease with which organisms can form calcium carbonate (CaCO3) structures such as coccoliths is related to the chemistry of the ocean; more precisely how readily the dissolved substrates will "jump out of solution" into solid CaCO3. The substrates for CaCO3 are calcium (Ca) and carbonate (CO3) ions. If these are present in excess in seawater then that water is said to be "supersaturated with respect to CaCO3", which means that CaCO3 will precipitate more or less spontaneously from solution. Or at least it would do if not for high concentrations of ammonium, phosphate and particularly magnesium in seawater, which supposedly inhibit CaCO3 formation. Although the ocean is 5-6 times oversaturated with respect to calcite, spontaneous inorganic precipitation hardly ever occurs. Extraction of the inhibiting compounds from the coccolith producing vesicles is thought to be one mechanism by which coccolithophores manage to precipitate CaCO3.

But nevertheless, if calcium and carbonate concentrations are pushed up high enough, then eventually the inhibiting effects of other compounds are overcome and spontaneous inorganic precipitation of CaCO3 takes place. On the other hand, if concentrations of calcium and/or carbonate were to decrease, the water would become less super-saturated with respect to CaCO3, and organisms would have to expend more energy in order to precipitate it. And below a certain level seawater actually becomes corrosive to CaCO3, tending to dissolve it once made.

Dissolved inorganic carbon in the ocean exists in three forms: dissolved CO2 gas, bicarbonate (HCO3) and carbonate (CO3) ions. The partitioning between the three forms is governed by the pH (see plot). The acidification of the ocean by anthropogenic CO2 is causing carbonate concentrations everywhere to fall (decreasing by 30% relative to preindustrial by 2050), as carbon atoms in carbonate molecules move instead into bicarbonate and/or CO2 molecules, and is hence making it harder for organisms to synthesize their CaCO3 structures.

Calcifying organisms are in trouble.

Ehux Experiments

Ulf Riebesell and colleagues investigated the importance of this by growing Ehux and other (non-calcifying) phytoplankton in seawater equilibrated with high atmospheric CO2 concentrations up to 850 ppmv (pre-industrial = 280, current = 360 ppmv). While the water they grew their cultures in always remained supersaturating with respect to CaCO3, it became much less so due to the high CO2. The results are shown in the images below.

Scanning electron microscopy (SEM) photographs of coccolithophores under different CO2 concentrations. a, b, d, e: Emiliania huxleyi; and c, f: Gephyrocapsa oceanica collected from cultures incubated at CO2 concentration of approx. 12 micro-moles per litre (a-c) and at CO2 concentration of approx. 30-33 micro-moles per litre (d-f), corresponding to pCO2 levels of about 300 ppmv and 780-850 ppmv, respectively. Scale bars represent 1 micron. Note the difference in the coccolith structure (including distinct malformations) and in the degree of calcification of cells grown at normal and elevated CO2 levels.

Ehux's coccoliths were malformed, presumably because the Ehux cells found it harder to instigate calcite precipitation within the cell. Since Ehux's coccoliths presumably serve a useful function (otherwise why produce them), without them they will probably not be able to compete so well against other phytoplankton. Any significant penalty could lead to Ehux being replaced by other phytoplankton in the sea, and hence, eventually, its extinction.

Similar results were obtained for a coral reef in Biosphere II, when it too was subjected to high CO2 conditions. The ability to calcify (form new calcium carbonate) was reduced (Global Biogeochemical Cycles, 2000, 14, 639-).

So is Ehux destined for the dustbin of history? Will it go extinct in the future high-CO2 greenhouse? The answer is that this may well happen, but no-one yet knows for sure.

Past High-CO2 Climates

Despite the general pessimism there is however one ray of hope. If we go far enough back into the geological past we can eventually find high CO2 climates comparable to the more intense CO2 greenhouse we are presently creating. We have to go back many millions of years, to the Cretaceous for instance (~145 to 65 million years ago).

Coccolithophores survived that time (although not Ehux, it hadn't evolved then), and even fared relatively well in the hot, high-CO2 climate, despite the possibly low carbonate ion concentrations. We can glean that much at least from the reasonably abundant coccoliths that fell into in marine sediments forming at that time. Calcium concentrations were higher during the Cretaceous and solid calcium carbonate is present in cores from that time, down to similar depths as to today. This all suggests that the ocean was probably about equally supersaturated with respect to CaCO3 as it is today, despite the high CO2.

To sum up, coccolithophores including Ehux may well be in deep trouble in the decades and centuries ahead, as more and more fossil fuel CO2 diffuses across the sea-surface and makes the surface ocean more acidic. But let's hope that future research proves this to be an unfounded worry - it would indeed be a shame to lose this beautiful natural phenomenon.

Toby Tyrrell, Hanno Kinkel.

In recent years laboratory experiments have been superceded by mesocosm experiments such as the large multi-institution project described here.

Ehux home page

Toby Tyrrell : T.Tyrrell@noc.soton.ac.uk