Major gradients in putatively nitrifying and non-nitrifying Archaea in the deep North AtlanticReport as inadecuate

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1 NIOZ - Royal Netherlands Institute for Sea Research

Abstract : Aerobic nitrification of ammonia to nitrite and nitrate is a key process in the oceanic nitrogen cycling mediated by prokaryotes 1 . Besides the two different groups of Bacteria belonging to the beta-and gamma-proteobacteria involved in the first nitrification step, Crenarchaeota have been recently recognized as main drivers of the oxidation of ammonia to nitrite in soil as well as in the ocean as indicated by the dominance of archaeal ammonia monooxygenase amoA genes over bacterial amoA 2,3 . Evidence is accumulating that archaeal amoA genes are common in a wide range of marine systems 3-6 . Essentially all these reports focused on surface and mesopelagic 200-1000 m depth waters where ammonia concentrations are higher than in waters below 1000 m depth. Crenarchaeota are abundant, however, also in the water column below 1000 m depth where ammonia concentrations are extremely low. Here we show, that throughout the North Atlantic, the abundance of archaeal amoA genes decreases drastically from subsurface waters to 4000 m depth and from the subpolar to the equatorial deep waters leading to pronounced vertical and latitudinal gradients in the ratio of archaeal amoA to crenarchaeal 16S rRNA genes. The lack of significant copy numbers of amoA genes in combination with the very low dark carbon dioxide fixation rates in the bathypelagic North Atlantic suggests that the vast majority of the bathypelagic Crenarchaeota are not autotrophic ammonia oxidizers but most likely utilize organic matter, hence live heterotrophically.

Author: Hélène Agogué - Maaike Brink - Julie Dinasquet - Gerhard J Herndl -



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