Wilken, Susanne (2007) Der Einfluss von Eisenlimitierung auf die Stabilität und den Silikathaushalt von Diatomeen. (Diploma thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 87 pp.

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Abstract

lron availability in the ocean has been shown to affect the silica content of diatoms with cells grown under iron limitation containing more silica and, therefore, probably having thicker frustules. To date, however, it remains unknown if a higher silica content results in increased valve strength and thus affects grazing protection. In the present study, the effect of iron availability on valve structure and stability of the cosmopolitan diatoms Coscinodiscus wailesii, C. granii and Thalassiosira rotula and the Southern Ocean species T. tumida, Porosira pseudodelicatula and Fragilariopsis kerguelensis was investigated. Stability measurements were carried out under the microscope using calibrated glass needles pressed against the cell with weil defined forces by a micromanipulator. Furthermore, the valve structure of C. wailesii and F. kerguelensis was analysed using scanning electron microscopy and, for C. wailesii, confocal laser scanning microscopy (KLSM). For the analysis with KLSM the fluorescent dye Rhodamine 123, which is incorporated into newly formed silica structures during silica deposition, was used. Additionally, elemental compositions of both Coscinodiscus-species as well as P. pseudodelicaula and F. kerguelensis were measured. With the exception of both Thalassiosira species cells grown under iron deplete conditions showed a significantly higher stability. For F. kerguelensis these results of the stability measurements were further corroborated by observed differences in valve structure. Cells grown under iron limitation showed a visibly higher degree of silicification with significantly thicker costae. C. wailesii cells showed the same trend towards thicker interstriae and smaller pores under iron limitation. The elemental composition was relatively variable between species and the two iron treatments. The only consistent trend for all species tested was a higher Si-content per cell and per cell surface under iron limitation, although the differences between both treatments were relatively small and not significant. In the three central species for which elemental ratios were measured the stability showed an exponential increase with increasing Si-content per cell surface. The increased stability of the frustules suggests a better grazing protection during iron limitation. This effect seems to be strengest in strongly silicified species. A better grazing protection can lead to an increase in net population growth by reducing mortality rates even though growth rates are small. An altered grazing protection will have an impact on grazing rates and, therefore, also on phytoplankton community structure and higher trophic levels. This might affect the ratio of recycling to expert production.

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