Abstract
Nitrification, the oxidative process converting ammonia to nitrite and nitrate, is driven by microbes and plays a central role in the global nitrogen cycle. Our earlier investigations based on 16S rRNA and
amplicon analysis,
quantitative PCR and metagenomics of groundwater-fed biofilters indicated a consistently high abundance of comammox
Here, we hypothesized that these nonclassical nitrifiers drive ammonia-N oxidation. Hence, we used DNA and RNA stable isotope probing (SIP) coupled with 16S rRNA amplicon sequencing to identify the active members in the biofilter community when subjected to a continuous supply of NH
or NO
in the presence of
C-HCO
(labeled) or
C-HCO
(unlabeled). Allylthiourea (ATU) and sodium chlorate were added to inhibit autotrophic ammonia- and nitrite-oxidizing bacteria, respectively. Our results confirmed that lineage II
dominated ammonia oxidation in the biofilter community. A total of 78 (8 by RNA-SIP and 70 by DNA-SIP) and 96 (25 by RNA-SIP and 71 by DNA-SIP)
phylotypes (at 99% 16S rRNA sequence similarity) were identified as complete ammonia- and nitrite-oxidizing, respectively. We also detected significant HCO
uptake by
subgroup10,
,
, and
under conditions that favored ammonia oxidation. Canonical
alone drove nitrite oxidation in the biofilter community, and activity of archaeal ammonia-oxidizing taxa was not detected in the SIP fractions. This study provides the first
evidence of ammonia oxidation by comammox
in an ecologically relevant complex microbiome.
With this study we provide the first
evidence of ecologically relevant ammonia oxidation by comammox
in a complex microbiome and document an unexpectedly high H
CO
uptake and growth of proteobacterial and acidobacterial taxa under ammonia selectivity. This finding raises the question of whether comammox
is an equally important ammonia oxidizer in other environments.