Limestone algae: ocean acidification makes for a thinner skeleton

Coccolithophorids react to environmental changes

Morphotypes of the chalk alga Emiliania huxleyi with different degrees of calcification. Front: highly cassified organisms, the degree of calcification decreases towards the rear. © Luc Beaufort / CEREGE / Univ. Aix-Marseille / CNRS)
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For the first time, researchers have investigated on a global scale how marine calcareous algae in their natural habitat react to the increasing acidification by increased input of carbon dioxide. In the current issue of the science journal "Nature" they describe that algae of the group Coccolithophoriden form a thinner limestone skeleton when the pH in the sea decreases.

In the marine ecosystem, the changes in the degree of calcification are therefore obviously much more pronounced than previously suspected due to laboratory experiments, the researchers said. The changes have an impact on the global carbon budget because the microalgae studied affect the carbon dioxide exchange between the ocean and the atmosphere.

Increased acidification of the oceans

About one third of the anthropogenic carbon dioxide is taken up by the oceans and reacts to form carbonic acid and its reaction products. The increasing use of fossil fuels has led to increased acidification of the oceans in the last century, altering marine ecosystems. Particularly sensitive are lime-forming organisms such as corals and calcareous algae - so-called coccolithophores. These microscopic algae are part of the phytoplankton and build a skeleton of lime platelets.

The coccolithophores are very common and produce much of the marine lime - a process that has led over geological time scales to lime deposits such as the limestone cliffs on Rügen. The reactions of calcified algae to ocean acidification in their natural environment have never before been investigated on a global scale.

Satellite image of a mass development of calcareous algae, a so-called coccolithophore flower, off the southwest coast of England. © NASA

New method in use

Using a method developed by Luc Beaufort at the French research institute CEREGE, a large number of plankton and sediment samples have now been analyzed, documenting changes in the calcification of coccolithophores in the ocean today and over the past 40, 000 years. display

The results show that coccolithophores form less lime if the water contains less carbonate ions, ie has a lower pH (acid is). "The reactions in the natural system are much stronger than previously assumed, " reports Bjorn Rost from the Alfred Wegener Institute for Polar and Marine Research. Although laboratory experiments have already shown that the degree of calcification decreases with increasing acidification, the algae form a thinner skeleton. In the marine ecosystem there is a shift in species from strongly to weakly calcified species and strains.

Small differences, big consequences

"Even small physiological differences in their responses to environmental changes can have major ecological consequences if this influences their competitiveness, " Rost explains. With increasing ocean acidification, species that need more energy to build up their limestone skeleton can be displaced. Thus, the group of coccolithophores could in the future take up less carbon with uncertain consequences for the global carbon cycle.

However, the study also shows that there may be exceptions to this general trend. In the coastal zone of Chile, where the sustingº conditions prevail in today's oceans - pH values ​​from 7.6 to 7.9 instead of an average of 8.1 - scientists found extremely strongly calcified coccolithophores.

Genetic analysis showed that this is a diverse strain of the coccolithophorid species Emiliania huxleyi. This strain has evidently managed to adapt to environmental conditions that are unfavorable to calcification. However, in view of the current high speed of climate change, it is highly questionable whether other representatives of coccolithophores will be able to adapt to this rate.

(Alfred Wegener Institute for Polar and Marine Research, 05.08.2011 - DLO)