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Ingrid Zondervan, Björn Rost, Richard Zeebe, Ulf Riebesell:
Elevated CO₂ concentrations slow down carbonate production in calcifying marine phytoplankton
(Talk)

The present rise in atmospheric CO₂ concentration causes significant changes in surface-ocean carbonate chemistry. By the end of the next century, the expected increase in atmospheric CO₂ will give rise to an almost three-fold increase in surface-water CO₂ concentrations relative to pre-industrial values (assuming IPCC's 'business as usual' scenario IS 92a). This will cause CO₃^2- concentrations and seawater pH to drop by ca.50% and 0.35 units, respectively. Changes of this magnitude may have significant effects on marine biogenic calcification. To test this, we have examined the response of monospecific cultures of two dominant calcifying phytoplankton species, the coccolithophorids Emiliania huxleyi and Gephyrocapsa oceanica, to changes in carbonate chemistry. For this purpose the carbonate system of the growth medium was manipulated to simulate mean surface-ocean conditions between pre-industrial times and the year 2100. Over this range, the ratio of calcification to organic matter production of E. huxleyi and G. oceanica decreased by 23% and 50%, respectively. Scanning electron microscopy indicates that malformed coccoliths and incomplete coccospheres increased in relative numbers with increasing CO₂. These results suggest that changes in sea-water carbonate chemistry expected to occur over the next century may slow down the production of calcium carbonate in the surface-ocean and its subsequent transport to the deep-sea. CO₂-related changes in coccolith formation have profound implications for the oceanic carbon cycle, both in the future as well as in the geologic past, and may influence the ecology of calcareous phytoplankton.

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