The carbon isotope fractionation in the coccolithophore Emiliania huxleyi constitutes the basis for the paleo-pCO₂ barometry. Under the premise that the carbon isotope fractionation is dependent on the availability of dissolved CO₂, measurements of the carbon isotope ratio of sedimentary alkenones can potentially produce a proxy record of ancient atmospheric CO₂ 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 CO₂.
To study the effect of these environmental parameters on carbon isotope fractionation, C₃₇:₂-alkenones were extracted from samples of marine particulate organic matter. The particulate organic matter was collected
together with information of the environmental conditions during three cruises in the North-East Pacific and during a Pacific transect from Victoria B.C. to Guam. Results from the NE Pacific show a lower carbon isotope fractionation in samples collected at the bottom of the euphotic zone compared to samples collected in the mixed layer. This may be an expression of the effect of light limitation. In this work carbon isotope fractionation shows no correlation with dissolved CO₂. Instead, a correlation with the ratio of phosphate concentration
to concentration of dissolved CO₂ ([PO3/4⁻/[CO₂]aq.) was observed. Nitrate availability appears to play an important role in maintaining this relationship as in the absence of nitrate the carbon isotope fractionation is lower than can be predicted from the relationship relating carbon isotope fractionation to [PO3/4⁻/[CO₂]aq. The C₃₇:₂-alkenone based results from the Pacific transect shows a strong correlation between carbon isotope fractionation and phosphate. This correlation is independent of the concentration of dissolved CO₂, implying a nutrient dominated control of isotope fractionation. However, this control may not be typical as the transect passed through waters with very low nutrient levels. Therefore, the results seen here may be a consequence of extreme nutrient conditions.
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 CO₂ 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. / Graduate
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/9068 |
| Date | 14 February 2018 |
| Creators | Eek, Magnus |
| Contributors | Whiticar, Michael J. |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Available to the World Wide Web |
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