"Tracking single coccolith dissolution with picogram resolution and implications for CO2 sequestration and ocean acidification" by T. Hassenkam, A. Johnsson, K. Bechgaard & S. L. S. Stipp, PNAS, 108

Proceedings of the National Academy of Sciences, Vol. 108, No. 21, pp. 8571-8576 (May 24, 2011; published online before print May 6, 2011); doi:10.1073/pnas.1009447108

Tracking single coccolith dissolution with picogram resolution and implications for CO₂ sequestration and ocean acidification

1. T. Hassenkam¹, 
2. A. Johnsson, 
3. K. Bechgaard and 
4. S. L. S. Stipp¹

1. Nano-Science Center, Department of Chemistry, University of Copenhagen, 2100 Copenhagen, Denmark

1. Edited* by Bruce Watson, Science Center, Troy, NY, and approved March 18, 2011 (received for review July 7, 2010)

Abstract

Coccoliths are micrometer scale shields made from 20-60 individual calcite (CaCO₃) crystals that are produced by some species of algae. Currently, coccoliths serve as an important sink in the global carbon cycle, but decreasing ocean pH challenges their stability. Chalk deposits, the fossil remains of ancient algae, have remained remarkably unchanged by diagenesis, the process that converts sediment to rock. Even after 60 million years, the fossil coccolith crystals are still tiny (< 1 μm), compared with inorganically produced calcite, where one day old crystals can be 10 times larger, which raises the question if the biogenic nature of coccolith calcite gives it different properties than inorganic calcite? And if so, can these properties protect coccoliths in CO₂ challenged oceans? Here we describe a new method for tracking dissolution of individual specimens, at picogram (10^-12 g) resolution. The results show that the behavior of modern and fossil coccoliths is similar and both are more stable than inorganic calcite. Organic material associated with the biogenic calcite provides the explanation. However, ancient and modern coccoliths, that resist dissolution in Ca-free artificial seawater at pH > 8, all dissolve when pH is 7.8 or lower. Ocean pH is predicted to fall below 7.8 by the year 2100, in response to rising CO₂levels. Our results imply that at these conditions the advantages offered by the biogenic nature of calcite will disappear putting coccoliths on algae and in the calcareous bottom sediments at risk.

http://www.pnas.org/content/108/21/8571.abstract