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Denitrification and ammonia oxidation by Nitrosomonas europaea wild-type, and NirK- and NorB-deficient mutants

¹ Department of Microbiology, University of Bayreuth, Universitaetsstrasse 30, 95447 Bayreuth, Germany
² Department of Molecular Cell Physiology, Free University, 1081 Amsterdam, The Netherlands
³ Department of Microbiology, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands

Correspondence
Ingo Schmidt
ingo.schmidt1{at}uni-bayreuth.de

The phenotypes of three different Nitrosomonas europaea strains – wild-type, nitrite reductase (NirK)-deficient and nitric oxide reductase (NorB)-deficient strains – were characterized in chemostat cell cultures, and the effect of nitric oxide (NO) on metabolic activities was evaluated. All strains revealed similar aerobic ammonia oxidation activities, but the growth rates and yields of the knock-out mutants were significantly reduced. Dinitrogen (N₂) was the main gaseous product of the wild-type, produced via its denitrification activity. The mutants were unable to reduce nitrite to N₂, but excreted more hydroxylamine leading to the formation of almost equal amounts of NO, nitrous oxide (N₂O) and N₂ by chemical auto-oxidation and chemodenitrification of hydroxylamine. Under anoxic conditions Nsm. europaea wild-type gains energy for growth via nitrogen dioxide (NO₂)-dependent ammonia oxidation or hydrogen-dependent denitrification using nitrite as electron acceptor. The mutant strains were restricted to NO and/or N₂O as electron acceptor and consequently their growth rates and yields were much lower compared with the wild-type. When cells were transferred from anoxic (denitrification) to oxic conditions, the wild-type strain endogenously produced NO and recovered ammonia oxidation within 8 h. In contrast, the mutant strains remained inactive. For recovery of ammonia oxidation activity the NO concentration had to be adjusted to about 10 p.p.m. in the aeration gas.

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