Metabolic activity of planktonic archaea and their contribution to the marine carbon cycle

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The oceans cover about three quarters of the Earth and the water colmn, with an average depth of 4000m, represents one of the World s largest habitate for life. Over the last two decades it has became evident that microorganisms of the domain Archaea are abundant and ecological relevant members of the oceanic picoplankton in all depth. The chemolithoautotrophic life style of the most abundant marine archaeal group derived from the successful cultivation in pure culture. By date all members of this novel phylum Thaumarchaeota generate energy by the aerobic conversion of ammonia to nitrite and fix CO2 as primary carbon source. Exhibiting extremely high subtrate affinity and operating the most energy-efficient aerobic CO2 fixation pathway, marine ammonia-oxidizing archaea are perfectly suited to thrive in nutrient-poor environments as typically for most of the ocean water. Because of their physiology and high abundance ammonia-oxidizing archaea have been recognized as predominant nitrifyers and important primary produzers in the Ocean. Based on its ubiquitous distribution I hypothesis that the recently characterized thaumarchaeal variant of the hydroxypionate/hydroxybutyrate cycle (HP/HB cycle) is the most important CO2 fixation pathway in the dark realm of the ocean below the photic zone. Previous modells quantifying the pelagic chemoautrotrophy flux are mostly based on the out-dated assumption that bacterial and not archaeal nitrifyers are the main contributors. Hence, the research project proposed here aims to re-evaluate the contribution of planktonic archaea to the marine C-cycle and to pelagic chemoautotrophy using novel, organism specific analytical tools. For instance, the in situ activity and turn over rates of thaumarchaeal ammonia oxidizers will be determined by measuring the incorporation of stable C and H isotopes into diagnostic thaumarchaeal lipids. In combination with the distribution of novel marker genes coding for the thaumarchaeal HP/HB cycle and application of the NanoSIMS/HIS-SIMS technology we will investigate the actual contribution of chemolithoautotrophic archaea to the pelagic primary production. Furthermore, I propose to cultivate members of the yet uncharacterized deep sea thaumarchaeota clade at near-environmental conditions to get access to their physiological and genetic characteristics. In sum, this research project aims to obtain novel findings on the biology of one of the World s most abundant organism group and to contribute to the biogeochemical understanding of the marine C-cycle.