Evidence of complete ammonia oxidation (comammox) by Nitrospira under low dissolved oxygen in a side stream deammonification pilot. bioreactor metagenome

We report metagenomic evidence for the presence of a Nitrospira-like organism with the metabolic potential for complete ammonia oxidation to nitrate (comammox) in a side stream deammonification pilot reactor. Initial 16S rRNA sequencing of the samples indicated a gradual increase in Nitrospira population while the nitrate concentration began to build-up in the reactor under low DO conditions. On day 71 we identified genes for both ammonia and nitrite oxidation in a metagenomic bin taxonomically affiliated to Nitrospira-clade I. The presence of ferredoxin nitrite reductase, a key enzyme for Clade-I Nitrospira was also present in the same bin. So far, comammox activity has only been associated with clade-II Nitrospira. The discovery of comammox under low DO condition has major implications for sidestream wastewater treatment, as an emphasis has been on the out-selection of NOB bacteria to shuttle nitrite to anaerobic ammonia oxidizers.