Archaea form one of the three domains of life on Earth and together with the bacteria form the prokaryotes. In the marine environment planktonic Archaea consist of two major groups, the Crenarchaeota and the Euryarchaeota of which the former appears to be the most abundant and may account for ca. 20% of all prokaryotic cells in the global ocean. Despite the fact that marine Crenarchaeota constitute a substantial fraction of the picoplankton in the world oceans, little is known about their basic metabolic requirements. Marine Crenarchaeota inherit unique tetraether membrane lipids and recent studies done on core top sediments showed that the distribution of sedimentary tetraether lipids from different geographic locations varies with sea surface temperature (SST). The change of the tetraether lipid distribution was expressed in an index of the lipid isomers, which was named the Tetraether Index of lipids with 86 carbon atoms, the TEX86. This index was considered as a new temperature proxy for reconstruction of SST in paleoenvironments. During my thesis, I focused on two major objectives. First, to shed more light on the physiology of marine Crenarchaeota and second, to validate the newly introduced temperature proxy TEX86. In order to study the carbon acquisition mechanism of marine Crenarchaeota, seawater-filled mesocosm tank were incubated with 13C bicarbonate and the results showed that the 13C was mainly incorporated in the crenarchaeotal membrane lipids. This 13C tracer experiments demonstrated that at least some marine Crenarchaeota actively utilize bicarbonate and appear to be autotrophs. Field studies, applying functional gene analyses and quantitative PCR, revealed that marine Crenarchaeota occur highly seasonal in surface waters during the winter and were actively involved in the marine nitrogen cycle as nitrifiers. In the laboratory temperature experiments were performed on a crenarchaeotal enrichment culture in order to validate the newly introduced temperature proxy TEX86. The results of this experiments showed that the membrane lipid distribution of the enriched marine crenarchaeotal species is mainly determined by incubation temperature, thus, the TEX86 reflect mainly temperature. Marine Crenarchaeota occur throughout the water column so it was essential to study the origin of the tetraether lipids which reaches the sediment. Tetraether lipid studies done on suspended particulate organic matter (POM) samples and sediment trap material from different oceanic regimes showed that the TEX86 correlates best with SST in different depth of the water column. The archaeal lipid signal which reaches the sediment is mainly derived from the upper 100m of the water column probably due to more effective grazing and repackaging processes. In summary, the experimental work and field studies described in the first part revealed that at least some marine Crenarchaeota are chemolithoautotrophic organisms and thus, they may play an important but as yet not recognized part in the marine carbon and nitrogen cycle. The experimental work and field studies described in the second part validates the newly introduced temperature proxy TEX86. Therefore it should be possible to use the TEX86 for reconstruction of the upper water column temperatures in ancient environments
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