| Summary: | <I>Emiliania huxleyi</I> is the most abundant coccolithophore in the World's oceans today. These organisms have the ability to calcify and photosynthesise, and may represent an important oceanic sink for dissolved inorganic carbon. As a result, they have the potential to influence the oceanic carbon cycle. It has been proposed that calcification may improve carbon acquisition under CO<SUB>2</SUB> <SUB>(aqueous)</SUB> limiting conditions, but may involve high Ca<SUP>2+</SUP> transport costs. In this thesis, results presented show that calcification increases in <I>E. huxleyi</I> (strain L) in response to low phosphorus and/or nitrate, high external pH, and high dissolved inorganic carbon. This information was used to produce calcifying and non-calcifying cells of the same strain allowing comparisons, which avoided the genetic and psychological differences between strains. Calcification correlated with an increase in photosynthetic efficiency (Φ><SUB>PSII</SUB>), relative electron transport rate (REF<SUB>PSII</SUB>) and the proportion of photons captured and converted to chemical energy (qP). The mechanisms underlying these correlations are unknown, but may involve the H<SUP>+</SUP> produced during calcification being used to supply an internal CO<SUB>2</SUB> source. High calcifying cells were found to have a lower internal pH than low-calcifying cells, and were able to take up HCO<SUB>3</SUB><SUP>-</SUP>. This may influence CO<SUB>2</SUB> availability within the cell, or nutrient requirements and assimilation. Results from X-ray microanalysis and Electron Energy Loss Spectroscopy indicate that the endomembrane system may provide an efficient pathway for Ca<SUP>2+</SUP> transport to the coccolith vesicle. Potentially, this avoids transport of Ca<SUP>2+</SUP> against a concentration gradient, and resolves what was thought to be one of the major cost-aspects of calcification. The most likely advantage of calcification is that it may allow low levels of photosynthesis to occur efficiently, thus increasing survival time during co-limitation by CO<SUB>2</SUB>, phosphorous and/or nitrate. Calcification may enable the cell to reduce the nutrient requirements for photosynthesis, or allow internal nutrient recycling.
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