Summary
To date, six candidate genera of anaerobic ammonium‐oxidizing (anammox) bacteria have been identified, and numerous studies have been conducted to understand their ecophysiology. In this study, we examined the physiological characteristics of an anammox bacterium in the genus ‘
C
andidatus
J
ettenia’. Planctomycete
KSU
‐1 was found to be a mesophilic (20–42.5°C) and neutrophilic (
pH
6.5–8.5) bacterium with a maximum growth rate of 0.0020 h
−1
. Planctomycete
KSU
‐1 cells showed typical physiological and structural features of anammox bacteria; i.e.
29
N
2
gas production by coupling of
15
NH
4
+
and
14
NO
2
−
, accumulation of hydrazine with the consumption of hydroxylamine and the presence of anammoxosome. In addition, the cells were capable of respiratory ammonification with oxidation of acetate. Notably, the cells contained menaquinone‐7 as a dominant respiratory quinone. Proteomic analysis was performed to examine underlying core metabolisms, and high expressions of hydrazine synthase, hydrazine dehydrogenase, hydroxylamine dehydrogenase, nitrite/nitrate oxidoreductase and carbon monoxide dehydrogenase/acetyl‐
CoA
synthase were detected. These proteins require iron or copper as a metal cofactor, and both were dominant in planctomycete
KSU
‐1 cells. On the basis of these experimental results, we proposed the name ‘
C
a
.
J
ettenia caeni’ sp. nov. for the bacterial clade of the planctomycete
KSU
‐1.