Wozu bauen Coccolithophoriden eine Kalkschale? Dient sie zum Schutz gegen Fressfeinde und Pathogene?
Physik, Chemie und Biologie des Meeres
Zusammenfassung der Projektergebnisse
Coccolithophores are a group of marine phytoplankton which cover themselves with a shell composed of small calcium carbonate platelets. Coccolithophores are important for marine ecosystems and elemental cycles as they contribute 1-10% of marine primary production and ~50% to calcium carbonate sediments on the deep seafloor. There has been significant research ever towards understanding the calcification process and its biogeochemical implications in the Earth System. However, the question: “why do coccolithophores calcify?” has received much less attention. Answering this question is important to assess the prevalence of coccolithophores in the future ocean where calcification rates are thought to change. In this project, called “CalciWhy”, we investigated if the calcareous shell of coccolithophores serves as a defence mechanism protecting them against viral infection, microzooplankton grazing, and copepod grazing. We first developed an innovative experimental protocol that allowed us remove the calcareous shell and compare viral infection/grazing on cells with the shell (“armoured cells”) to those without the shell ( “naked cells”). Applying this protocol, we could show that the shell provides limited protection against viral infection in Emiliania huxleyi, the most abundant coccolithophore on Earth. Although there was evidence for reduced infection if the virus was exposed to the host in a certain stage of the host’s cell cycle, reduced infection rates led to no change in the final cell abundances at the end of the experiments. This indicated that the protection, which can occur under specific circumstances seems of little ecological benefit. Experiments with Oxyrrhis marina, our microzooplankton model organism, revealed that the shell effectively reduces grazing rates in three coccolithophore species. Thus, we find convincing evidence that the shell serves as a protection against microzooplankton grazing. However, because O. marina did not differentiate between armoured and naked cells prior to ingestion, the protection by the shell does not lead to a competitive advantage of coccolithophores in natural plankton communities (it would lead to a competitive advantage if the grazer would be able to select between armoured and naked cells). Thus, future research with a grazer that has the ability to differentiate are needed to determine if the protection provided by the shell can also lead to a competitive advantage of coccolithopohres. Experiments with Acartia tonsa, our copepod model organism revealed that the shell does not protect Gephyrocapsa oceanica against copepod grazing. Since G. oceanica has the thickest shell of all the coccolithophores we considered in CalciWhy, there is no reason to believe that results with more weakly armoured coccolithophore species would be substantially different. However, we emphasize that other copepod species may differentiate between armoured and naked cells in which case the shell could constitute a protection against these grazers. Altogether, CalciWhy led to significant progress in our understanding of the ecological benefits of calcification in coccolithophores. Our results will help to understand and eventually predict the occurrence of coccolithophores in marine plankton communities of the past, present, and future.
Projektbezogene Publikationen (Auswahl)
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Why marine phytoplankton calcify, Science Advances., 2(7), e1501822–e1501822
Monteiro, F. M., Bach, L. T., Brownlee, C., Bown, P., Rickaby, R. E. M., Poulton, A. J., Tyrrell, T., Beaufort, L., Dutkiewicz, S., Gibbs, S., Gutowska, M. A., Lee, R., Riebesell, U., Young, J. and Ridgwell, A.
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The calcium carbonate shell of Emiliania huxleyi provides limited protection against viral infection. Front. Mar. Sci. 7:530757
Haunost, M., Riebesell, U. and Bach, L.T.
