Environmental control of stable carbon isotope systematics in Emiliania huxleyi.

摘要

The carbon isotope fractionation in the coccolithophore Emiliania huxleyi constitutes the basis for the paleo-pCO ₂ barometry. Under the premise that the carbon isotope fractionation is dependent on the availability of dissolved CO ₂, measurements of the carbon isotope ratio of sedimentary alkenones can potentially produce a proxy record of ancient atmospheric CO ₂ levels. However, recent studies, including this thesis have suggested that other factors than CO₂ may influence the carbon isotope fractionation in Emiliania huxleyi and hence the validity of the proxy.; In this thesis work the effects of irradiance on carbon isotope fractionation were studied in batch cultures of non-calcifying Emiliania huxleyi . It was found that the biomass becomes more 13C depleted as the light intensity decreases. This is in agreement with utilization of CO₂ via passive diffusion where fractionation is a function of the rate of diffusion of CO₂ into the cell relative to the rate of carbon utilization. However, results reported in the literature for a calcifying strain show the opposite trend with a 13C enrichment of the biomass. These results suggest that the carbon utilization of the calcifying strain of Emiliania huxleyi differ from that of the non-calcifying strain. This is supported by observations in the literature, which indicates a connection between the process of calcification and the supply of carbon for photosynthesis.; A mechanism for the effect of calcification on carbon isotope fractionation in light limited cells is presented here. The mechanism is based on the fact that the calcification and photosynthesis respond differently to light limitation. This difference leads to an imbalance in the rate of calcification to the rate of photosynthesis ratio (C/P), which ultimately affects the availability of CO₂ inside the cell. Apart from light, the availability of nutrients has also been shown to affect calcification. Nutrient starved cells will enhance calcification to the degree that the C/P ratio changes, thus affecting the internal concentration of CO ₂.; In conclusion, the results presented in this thesis challenge the classical belief that the carbon isotope fractionation in Emiliania huxleyi is a direct function of the availability of dissolved CO ₂ by suggesting that the observed isotope fractionation is a result of a complex interaction between environmental factors such as irradiance and the availability of nutrients. In particular, a correlation between phosphate concentration and carbon isotope fractionation has been found. (Abstract shortened by UMI.)