The ratio of calcium carbonate to organic carbon production in the surface ocean is thought to be one of the key marine biotic climate variables, through its effect on ocean carbon cycling. This ratio is significantly affected by calcification and photosynthetic carbon fixation in coccolithophores. The abundance of coccolithophores and their rates of calcification and organic carbon fixation are in turn affected by climate-related changes in the ocean. However, there still exists disagreement on the strength of this feedback mechanism, which is due to the complexity of interactions of the factors regulating phytoplankton growth and ecosystem functioning. This review gives a qualitative overview on experimental and field data of coccolithophores, mainly Emiliania huxleyi, that are most relevant to actual oceanographic conditions and are likely to change in the foreseeable future under a changing climate. The focus is on the bottom-up control factors, light, macronutrients, trace metals and CO2, that can be of use in modelling studies. Several trends have been identified that should be considered when attempting to simulate E. huxleyi growth. Light seems to be the central factor determining the occurrence of blooms. At low irradiance the calcite to organic carbon production ratio increases, but appears to decrease again when irradiance becomes severely limiting. Phosphate and nitrate limitation lead to an increase in the ratio of calcite to particulate organic carbon, which is also shown for zinc but not for iron. This is mainly due to the fact that coccolith formation is generally less dependent on nutrient concentration than is cell replication. Finally, CO2-related effects in E. huxleyi and the other bloom- forming coccolithophore species Gephyrocapsa oceanica have been observed. Under high light conditions, calcification decreases with increasing CO2 concentration. Depending on the nutrient status of the cells, the production of particulate organic carbon strongly increases, or decreases under elevated CO2 concentrations. In contrast, under low light conditions no sensitivity of calcification to CO2 was observed, whereas particulate organic carbon production always strongly increases with CO2 under nutrient replete conditions. How different growth conditions taken together finally affect coccolithophore calcification and organic carbon production is discussed for some factors, but needs further investigation.
Zondervan I., in press. The effects of light, macronutrients, trace metals and CO2 on the production of calcium carbonate and organic carbon in coccolithophores – A review. Deep–Sea Research II. doi:10.1016/j.dsr2.2006.12.004
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