Strains in the genus Thauera exhibit remarkably different denitrification regulatory phenotypes

Summary Denitrifiers differ in how they handle the transition from oxic to anoxic respiration, with consequences for NO and N 2 O emissions. To enable stringent comparisons we defined parameters to describe denitrification regulatory phenotype ( DRP ) based on accumulation of NO 2 − , NO and N 2 O , oxic/anoxic growth and transcription of functional genes. Eight T hauera strains were divided into two distinct DRP types. Four strains were characterized by a rapid, complete onset ( RCO ) of all denitrification genes and no detectable nitrite accumulation. The others showed progressive onset ( PO ) of the different denitrification genes. The PO group accumulated nitrite, and no transcription of nirS (encoding nitrite reductase) was detected until all available nitrate (2 mM) was consumed. Addition of a new portion of nitrate to an actively denitrifying culture of a PO strain ( T . terpenica ) resulted in a transient halt in nitrite reduction, indicating that the electron flow was redirected to nitrate reductase. All eight strains controlled NO at nano‐molar concentrations, possibly reflecting the importance of strict control for survival. Transient N 2 O accumulation differed by two orders of magnitude between strains, indicating that control of N 2 O is less essential. No correlation was seen between phylogeny (based on 16 S rRNA and functional genes) and DRP .