Impact of Coccolith Formation on

Impact of Coccolith Formation on the Carbon Cycle

SCIENCE October 20 2008

The Research Article "Phytoplankton calcification in a high-CO₂ world" (M. D. Iglesias-Rodriguez et al., 18 April 2008, p. 336) presents data from lab cultures showing that coccolithophorids produce more limestone skeleton under a high CO₂environment. This contradicts widespread claims that increasing CO₂ in seawater will cause them to dissolve, assuming the internal milieu is the same as the external one, which is not true for any known organism.

Their results are no surprise. In the carbon balance model of coccolith formation (1), based on models of coral calcification (2, 3), photosynthesis removes CO₂, increasing internal pH. This promotes calcification as a mechanism of pH homeostasis, obviating the need for metabolically-costly alkalinity efflux or proton influx. If this is the dominant mode of pH regulation, the photosynthesis to calcification ratio must be close to 1, as found in corals and calcareous algae, including coccolithophores (4). Any factor increasing photosynthesis leads to equal increases in calcification. Non-photosynthesizing deep-sea corals and mollusks (except Tridacnids), must generate alkalinity through costly proton and bicarbonate pumping, but their internal milieu is also greatly different than surrounding water. Deep sea ahermatypic corals have no problem growing skeletons in water undersaturated in skeletal solubility, and are hardly threatened by decreasing ocean pH (5), although this should increase the metabolic energy they must expend.

A serious caveat is that Iglesias-Rodriguez et al.'s cultures were grown under high "nutrient-replete conditions" (nitrate 100 micromolar, phosphate 6.24 micromolar). Only when nutrients are in excess can phytoplankton be CO₂ limited and increase growth rates when CO₂ rises. These conditions are extremely abnormal in surface waters, other than sewage plumes or the most intense upwelling events. Coccolithophores should not show such responses in most ocean waters. This is reminiscent of the "CO₂ fertilization effect" that predicts plants should grow faster as CO₂ rises, as found in highly fertilized greenhouse plants, but not during normal nutrient-limited plant growth. Experiments with elevated CO₂ under natural nutrient levels are needed to assess the real-world carbon cycle implications.

Thomas J. Goreau

Global Coral Reef Alliance, 37 Pleasant Street, Cambridge, MA 02139, USA.

References

1. E. Paasche, Phys. Plant. Suppl. 3, 1 (1964).

2. T. F. Goreau, Biol. Bull. 116, 59 (1959).

3. T. F. Goreau, Annals New York Academy of Sciences 109, 127 (1963).

4. T. J. Goreau, Proc. 3d. Int. Coral Reef Symp. 2, 395 (1977).

5. M. Fine, D. Tchernov, Science 315, 1811 (2007).

Response to T. J. Goreau`s E- Letter

20 October 2008

M. Debora Iglesias-Rodriguez
School of Ocean and Earth Science, University of Southampton, Southampton SO14 3ZH, UK,
Toby Tyrrell, Paul R. Halloran, Rosalind E. M. Rickaby, Peter von Dassow, John A. Raven

We fully agree with T. J. Goreau that calcification by coccolithophores contrasts with that of other marine organisms (e.g. corals and foraminifera). Coccolithophore calcification occurs within the cell, rather than externally to that cell, and consequently coccolithophores are potentially able to exert a much more sophisticated control over their calcification process. However, we strongly disagree with the suggestion by Goreau that calcification and photosynthesis are tightly coupled. Several studies have indicated that the coccolithophore calcification to photosynthesis ratio ranges from considerably less than one, to greater than two (1–4). Specifically, the absence of a tight coupling between photosynthesis and calcification has been clearly demonstrated by analyses recording only a minor reduction in photosynthetic rates when coccolithophores were cultured in a calcium-free medium (thus preventing calcification) (5).

Goreau argues that only when nutrients are plentiful, can phytoplankton be CO₂ limited. The nitrate and phosphate concentrations used in our experiments were chosen to be consistent with those used in previous experiments (6), and indeed represent nutrient-replete conditions. Therefore, to answer this question we refer the reader to our down-core reconstruction of coccolith mass. This record indicates that the calcification response observed in the laboratory under nutrient-replete conditions is applicable also under the nutrient-limited conditions of the real ocean. However, we know that phosphate limitation can increase calcification in Emiliania huxleyi (1). Consequently, we agree that an important further step will be to assess the role played by nutrient limitation (iron, nitrate, phosphate) in influencing the phytoplankton physiological response to elevated CO₂ levels. Despite the challenging experimental limitations, imposed by the vast complexity of marine ecosystems, our study provides an important insight into the acclimation of calcifying phytoplankton to high CO₂ conditions.

M. Debora Iglesias-Rodriguez, Toby Tyrrell

National Oceanography Centre, University of Southampton, Waterfront Campus, European Way, Southampton SO14 3ZH, UK.

Paul R. Halloran, Rosalind E. M. Rickaby

Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK.

Peter von Dassow

Station Biologique de Roscoff, Place George Teissier, BP 74, 29682 Roscoff Cedex, France.

John A. Raven

Plant Research Unit, University of Dundee at SCRI, Scottish Crop Research Institute, Invergowrie, Dundee, Scotland DD2 5DA, UK.

References

1. E. Paasche, S. Brubak, Phycologia 33, 324 (1994).

2. E. Paasche, Phycologia 40, 503 (2002).

3. I. Zondervan, Deep-Sea Res. Pt. II 54, 521 (2007).

4. M. N. Müller, A. N. Antia, J. LaRoche, Limnol. Oceanogr. 53, 506 (2008).

5. L. Herfort, B. Thake, J. Roberts, New Phytol. 156, 427 (2002).

6. U. Riebesell et al., Nature 407, 364 (2000), doi:10.1038/35030078.