Carbon Fixation Pathways in Marine Chemoautotrophic, Mesophilic Crenarchaeota: Implications for the Oceanic Carbon Cycle

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Intellectual merit. The meso- and bathypelagic realm of the oceans represents the largest continuous habitats on Earth, yet we know very little about the kind of microbes living in this zone and the global impact of their activities. Traditionally, the role of microbes in this zone has been seen as degraders of organic matter with a concomitant release of CO2. However, recently this perception has been called into question by the finding that planktonic Crenarchaeota belonging to the marine crenarchaeota group 1 (MG1), which dominate the prokaryotic cell numbers in the meso-and bathypelagic zone, could actually be autotrophs. However until now, almost nothing is known about the metabolic capacities of this extensive and probably diverse group of organisms. This project is designed to address the following questions:. How do autotrophic, mesophilic Crenarchaeota fix CO2?. Do mesophilic, chemolithoautotrophic Crenarchaeota exist that obtain energy through sulfide or hydrogen oxidation?. If yes, are there different autotrophic carbon fixation pathways in different metabolic groups?. What differences exist regarding CO2 fixation between mesophilic and thermophilic Crenarchaeota?. What can these differences tell us about the evolution of Archaea?. What are the consequences for our view of global oceanic carbon fixation and the oceanic carbon cycle in general?To address these questions, the investigators propose first to elucidate the carbon fixation pathway used by ammonium-oxidizing archaea that we have currently in culture and that have been obtained from a variety of environments. Secondly, they propose to obtain isolates from the nutricline, the oxygen minimum zone, hydrothermal plumes, and the general vicinity of hydrothermal vents during an already NSF-funded research cruise to the hydrothermal vents at 9N on the East Pacific Rise. The current working hypothesis is that autotrophic carbon fixation within these Crenarchaeota is not carried out via the Calvin cycle, but rather via an alternative CO2 fixation pathway. If true, this would mean that a significant amount of carbon fixation in the ocean is not carried out via the Calvin cycle, leading to a reconsideration of our current view of oceanic CO2 fixation and the oceanic carbon cycle in general. This work specifically focuses on cultivating, isolating and further characterizing these organisms and thus ideally complements ongoing research in other laboratories utilizing environmental genomic techniques. Broader impacts. This work addresses an important, yet poorly understood aspect of the global carbon cycle, a topic that has a significant societal relevance. Results of this work will be essential to refine current carbon cycle models, and potentially fundamentally alter the current view of how carbon is transferred in the ocean. This project has a variety of educational components ranging from teaching at local schools and the involvement of undergraduates to post-doctoral support. It is expected that results will be the topic of media interviews (print and radio) as well as be integrated into coursework and web-pages existing either in the PI's labs or at the institution.