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April 29 2008 High CO2, Calcification and Nutrients “Phytoplankton Calcification in a High-CO2 World” (Iglesias-Rodriguez et al, April 18, p. 336-240) presents data from lab cultures showing coccolithophorids produce more limestone skeleton under high CO2. This contradicts widespread claims that increasing CO2 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 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 CO2r 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. Goreau1 1Global Coral Reef Alliance, 37 Pleasant Street, Cambridge MA 02139 1. Paasche, E., A tracer study of inorganic carbon uptake during coccolith formation and photosynthesis in the coccolithophorid Coccolithus huxleyi, Phys. Plant. Suppl. 3, 1-82 (1964) 2. Goreau, T. F., The physiology of skeleton formation in corals. I. A method for measuring the rate of calcium deposition by corals under different conditions, Biol. Bull. 116, 59-75 (1959) 3. Goreau, T. F., Calcium carbonate deposition by coralline algae and corals in relation to their roles as reef-builders, Annals New York Academy of Sciences, 109, 127-167 (1963) 4. Goreau, T. J., Carbon metabolism in calcifying and photosynthetic organisms: theoretical models based on stable isotope data, Proc. 3d. Int. Coral Reef Symp., 2, 395-401 (1977) 5. Fine, M., & D. Tchernov, Scleractinian coral species survive and recover from decalcification, Science, 315, 1811 (2007)
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