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Emiliania huxleyi, a microalga of global importance!

Micrographs of Emiliania huxleyi taken with a scanning electron microscope. In experiments this calcifying and bloom-forming alga showed different reactions to ocean acidification. The tiny calcified platelets (coccoliths) are formed in special cell organelles and subsequently transported to the outside, where it is integrated into the covering (coccosphere). Photo: Björn Rost, Alfred-Wegener-Institut

The unicellular alga Emiliania huxleyi floats in the sunlit layers of all oceans. It numbers among the more than 300 calcified microalgae (coccolithophores) covered with calcified platelets (coccoliths) that lend each species its characteristic appearance. As a primary producer, the alga is not only an important component of the marine food network, but also plays a major role in the global carbon cycle: On the one hand, it fixes dissolved CO2 via photosynthesis while calcification, on the other hand, leads to the release of CO2 to the seawater. This makes E. huxleyi a model organism for scientists at the Alfred Wegener Institute for Polar and Marine Research for improving the understanding of fundamental processes in the global carbon cycle.

Under favourable conditions Emiliania huxleyi (Fig. 1) can build up large amounts of biomass and dominate phytoplankton communities owing to its high cell division rates. Towards the end of such an “algal bloom”, when the cell division rates decline due to nutrient limitation, more coccoliths are formed than necessary and Emiliania sheds individual coccoliths. No longer part of the coccosphere, the discarded coccoliths scatter sunlight and give the seawater a milky colour. Such phenomena may cover areas of several hundred square kilometres and can even be observed from satellites (Fig. 2). When the cells die and sink, they take the carbon fixed by photosynthesis and calcification with them to the depths, where the carbon is deposited in the form of sediments. For millions of years coccolithophores like Emiliania have formed lime deposits in this way, such as the white cliffs of Dover and the chalk cliffs on Rügen.

Even though Emiliania is a microscopically tiny alga with a size of only 3 to 5 µm, it and other coccolithophores play a key role in the global carbon cycle [1]. This is due to the fact that Emiliania binds large amounts of the greenhouse gas CO2 as organic material through photosynthesis and transports it into the deep sea. As a result of this process, being called the organic carbon pump, the CO2 concentration in the surface water is reduced to such an extent that CO2 can be absorbed from the atmosphere [2].


 

Satellite picture of coccolithophore bloom in the English Channel off the coast of Plymouth (Cornwall). It can be primarily attributed to Emiliania huxleyi.
Photo: NASA

At the same time Emiliania has the opposite effect on the CO2 concentration in seawater due to calcification, which may be astonishing at first, because dissolved inorganic carbon is also removed from the seawater during this process. When calcium carbonate (CaCO3) is formed, however, bicarbonate (HCO3-) and calcium ions (Ca2+) react with each other and CO2 is produced. The production and vertical transport of calcium carbonate, the so-called carbonate pump, does not always cause CO2 to increase in the surface water. In species like Emiliania, the coccoliths may ”ballast” the relatively light organic material due to high density of calcium carbonate and thus can also increase the export via the organic carbon pump. By virtue of these complex processes, coccolithophores have a significant influence on the CO2 uptake capacity of the oceans and thus on our climate.

The anthropogenically induced rise in atmospheric CO2 concentration leads to an increase in dissolved CO2 as well as an acidification of seawater. The way in which this impacts on processes like photosynthesis and calcification and, in the end, on the capacity of Emiliania to bind carbon and transport it into the deep sea is the subject of scientific research. In comparative experiments, scientists of the Alfred Wegener Institute are currently testing how Emiliania and other representatives of the coccolithophores react to ocean acidification. Long-term experiments, furthermore, are aimed at estimating the potential of evolutionary adaptation to the expected conditions in the future. To develop a detailed understanding of key processes like photosynthesis and calcification, both physiological as well as molecular biological methods are applied. The results of these different approaches will help to estimate possible consequences of climate change on the important group of coccolithophores and on marine ecosystems.

Because of its truly global significance – Emiliania huxleyi is the most abundant and one of the few bloom-forming coccolithophores – this species was selected by the German Botanical Society as “Alga of the Year” last year. Its genome is currently undergoing sequencing and analysis by an international consortium that also includes scientists of the Alfred Wegener Institute.

 

[1] G. Hempel, I. Hempel. S. Schiel: Faszination Meeresforschung. Ein ökologisches Lesebuch. Verlag H. M. Hauschild. Bremen 2008. – [2] B. Rost, U. Riebesell: Coccolithophores and the biological pump. Responses to environmental changes. In H. R. Thierstein, J. R. Young (Hrsg.): Coccolithophores: from molecular processes to global impact. Springer-Verlag. Berlin, Heidelberg 2004


 
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