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Anthony Fielding

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Figure 1


Anthony Fielding | PhD Candidate

The Influence of Temperature and Dissolved Inorganic Carbon Concentrations on Carbon Stable Isotope Discrimination by Marine Phytoplankton


One of the aims of paleogeologists is to be able to reconstruct past climates. Air bubbles trapped in ice sheets have preserved a record of atmospheric CO2 levels over the last 300 thousand years. However,this time period is rather short compared to the age of the oldest sediments on the ocean floor. A new method of estimating past climatic conditions involves the stable carbon isotope ratio (13C/12C) which is preserved in marine sediments. This method, if reliable, would provide information about climates as far back as 200 million years. Since the marine sediment signal is largely due to phytoplankton, the isotope signal can ultimately be tied to carbon fixation by phytoplankton. While the physiology of this discrimination is not well understood, some general trends appear to hold.

A comparison of temperature and discrimination against 13C indicates that there is more discrimination at lower temperatures.It has been suggested that the temperature effect is acting through CO2 solubility as predicted by Henry's Law. At higher temperatures, less CO2 would dissolve in the seawater. Lower discrimination at higher temperatures might thus reflect limitation of photosynthesis by diffusive CO2 entry, a process which does not discriminate against 13C. However, many of these phytoplankton are capable of actively concentrating carbon to internal levels far exceeding external concentrations. This fact, along with the observation that the temperature-discrimination relation is not perfect, implies that this phenomenon might be more complicated than has previously been proposed.

The ongoing aim of this project is to study the influence of temperature and dissolved inorganic carbon (DIC) concentration on both the cellular discrimination against 13C and the extent of the carbon concentrating mechanism (CCM). Current experiments have examined the relationship between the degree of induction of the CCM and discrimination against 13C.

Cultures were grown under well defined DIC levels independent of temperature and pH. The latter two variables have been postulated to have their own effects on discrimination. CCM induction was determined by examination of photosynthetic kinetics using an oxygen electrode while discrimination was determined by mass spectrometric measurements of medium DIC during source depletion experiments.

Results from the diatom Thalassiosira pseudonana have shown that it induces a CCM over a range of ambient DIC concentrations from 0.25 to 2.3 mM (Fig. 1). Half-saturation constants for photosynthesis ranged from 62 to 1089 mM DIC respectively. These results indicate an induction of the CCM. Whole cell discrimination against 13C ranged from ca. 14.5 per mil to ca. 25 per mil over the same range of ambient DIC concentrations. The data support the hypothesis that discrimination is related to the degree of CCM induction as measured by photosynthetic kinetics.

Future experiments will be performed to clarify this relationship as well as to investigate the influence of temperature, and its interaction with DIC concentrations, on the CCM and discrimination. In addition, the time course of CCM induction will be studied. Proper interpretation of the 13C/12C data requires the understanding of the physiology involved in carbon isotope discrimination.

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