Mathematical model approaches describing cellular fluxes of carbon and Ca2 in Emiliania huxleyi

Anthropogenic carbon dioxide (CO2) emissions from fossil carbon storages to atmosphere are causing major impacts on the Earth s climate. The increase of atmospheric temperatures due to a strengthening of the natural greenhouse effect is a well known phenomenon. Moreover, a large fraction of the atmospheric CO2 that has been released by mankind enters the ocean and induces, besides other effects, the acidification of surface waters ( Ocean Acidification ). The effects of increasing partial pressures of atmospheric CO2 on parameters that describe the abiotic environment have been examined extensively. It is now of particular interest to gain a detailed comprehension of the complex interactions between the changing environment and the metabolism of marine organisms. This knowledge is essential when aiming to understand the ongoing changes in their entirety. In the spotlight of this thesis are coccolithophores, unicellular algae that significantly impact the marine carbon cycle. Coccolithophores perform photosynthesis and calcification. Both processes rely on different carbon species and thus have a different effect on the carbonate system of the surface ocean. In connection with the formation of calcite particles, they further impact the sinking rates of particulate carbon from the photic zone into the ocean. The ratio of photosynthesis to calcite precipitation rate varies with the composition of the carbonate system. In order to understand the dependencies of both individual processes on the complex cabonate system, a detailed understanding of cellular carbon fluxes is essential. Until now, it is impossible to measure these fluxes directly. Therefore, mathematical models are used in this thesis to examine these fluxes. All presented models describe Emiliania huxleyi, one of the globally most abundant and important coccolithophores. The first part of this PhD thesis summarises current knowledge concerning the intracellular formation of coccoliths, i.e. the calcite platelets that surround the cells. Former propositions about the provision of carbon and calcium ions (Ca2 ) towards the site of calcite precipitation are reviewed and discussed. Furthermore, abiotic prerequisites on initiation and continuation of calcite precipitation, as well as the biological control of precipitation onset and crystal growth are of particular interest. Gaps of knowledge are discussed. In the second part, different hypotheses concerning the pathway of Ca2 are discussed on basis of analytical calculations. It turns out that a continuous trans- port across the membrane of the compartment, in which calcite precipitation takes place, i.e. the coccolith vesicle, is more likely than is a vesicle-based transport. The latter possibility constitutes the hypothesis currently favoured in literature. A transport via molecules that exhibit high Ca2 binding capacities, as found in other coccolithophores, is thought to be unlikely for E. huxleyi, because there is currently no evidence hinting at comparable molecules. Based on the assumption that Ca2 is transported continuously into the coccolith vesicle, a numerical model is developed, which examines the influence of different membrane transporters on the precipitation rate of calcite. These transporters convey varying substrate stoichiometries and are inserted into the in silico membrane of the coccolith vesicle. The influence of these transporters on the calcite precipitation rate is examined. Since the precipitation rate is known from literature, it is possible to exclude some of the tested transport stoichiometries. Two potential substrate stoichiometries are detected that serve as basic assumptions in the next step of modelling calcite precipitation. In the third part, different numeric models are developed that examine carbon fluxes through cytosol, coccolith vesicle, and chloroplast. Based on current knowledge of external carbon sources for organic matter production and calcite precipitation, it is concluded that both processes are supplied independently of each other with carbon under replete conditions. When external CO2 becomes limiting, how- ever, external bicarbonate ions that usually constitute the substrate for calcite precipitation are used by photosynthesis also. Moreover, an energy-efficient possibility is proposed how CO2 could be accumulated around Ribulose-1,5-bisphosphat (RubisCO), the CO2 fixing enzyme, without being transported against concentration gradients. The general discussion summarises the findings of this thesis and picks up gaps that were not discussed in detail previously. Moreover, measurements and models are proposed that might fill gaps of knowledge. Finally, implications of this work on physiological, ecological, and biogeochemical aspects of coccolithophore research are given