The effects of light, macronutrients, trace metals and CO sub(2) on the production of calcium carbonate and organic carbon in coccolithophores-A review

The ratio of calcium carbonate to organic carbon (C) production in the surface ocean is thought to be one of the key marine biotic climate variables, through its effect on ocean C cycling. This ratio is significantly affected by calcification and photosynthetic C fixation in coccolithophores. The abundance of coccolithophores and their rates of calcification and organic C 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 carbon dioxide (CO sub(2)), which 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 C 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 (POC), which is also shown for zinc but not for iron. This is mainly due to the fact that coccolith formation is generally less dependant on nutrient concentration than is cell replication. Finally, CO sub(2)-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 CO sub(2) concentration. Depending on the nutrient status of the cells, the production of POC strongly increases, or decreases under elevated CO sub(2) concentrations. In contrast, under low light conditions no sensitivity of calcification to CO sub(2) was observed, whereas POC production always strongly increases with CO sub(2) under nutrient-replete conditions. How different growth conditions taken together finally affect coccolithophore calcification and organic C production is discussed for some factors, but needs further investigation.