CAENEUS · Crenarchaeota ecology and nutrient utilization in the subsurface ocean (CAENEUS)

Over the past 10 years, it become apparent that Crenarchaeota are not only striving in some extreme environments but that they are ubiquitously present in the aquatic and terrestrial environment including the oceanic water column. In the pelagic realm of the ocean, their relative contribution to the total prokaryotic abundance increases with depth. It has been shown that the mesophilic Marine Crenarchaeota Group I (MCGI) live chemoautotrophically, fixing carbon dioxide as carbon source and using ammonia as an energy source. Based on the abundance of the amoA gene, encoding the ammonia monoxygenase, a key enzyme common to all nitrifiers, it has been deduced that they might be more important nitrifiers than Bacteria. This has never been tested thus far, however. In this proposal, we will investigate the enigma of this MCGI cluster is utilizing ammonia in the deep ocean, where ammonia concentrations are below the detection limit using conventional analytical methods. We will determine the distribution of archaeal and bacterial amoA gene abundance throughout the water column down to abyssopelagic realms in the northern North Atlantic and the tropical Atlantic and distinguish between archaeal and bacterial nitrification and dark CO2 fixation rates. Using three different single-cell approaches in a correlative way, we will specifically focus on the potential shifts in the phylogenetic composition of the MCGI cluster and its autotrophic activity with depth, as ammonia concentrations are probably below the nanomolar level in bathypelagic waters. Taken together, we will determine for the first time, the relative importance of archaeal vs. bacterial nitrification in the ocean. By focusing on two Atlantic sites with contrasting age of deep-water masses and accompanied to that, contrasting deep-water ammonia concentrations, we will be able to investigate the entire range of diversity and metabolic adaptation in Marine Crenarchaeota Group I.