Low abundant but highly active Nitrosomonas sp. drive nitrification. Low-abundant but highly transcriptionally active uncharacterised Nitrosomonas sp. drive ammonia-oxidation in the Brouage mudflat, France.

Exploring differences in nitrification rates within adjacent sedimentary structures of ridges and runnels on the Brouage mudflat, France, we quantified Potential Nitrification Rates (PNR) alongside amoA genes and transcripts. PNR was lower in ridges (-1.7 fold) than runnels, despite higher (+1.8 fold) ammonia-oxidizers abundance. However, ammonia-oxidizers were more transcriptionally active in runnels (+1.9 fold). Sequencing of amoA genes and transcripts revealed starkly contrasting profiles with transcripts from ridges and runnels dominated (91% in ridges and 98% in runnels) by low abundant (4.6% of the DNA community in runnels and 0.8% in ridges) but highly active phylotypes. The higher PNR in runnels was explained by higher abundance of this group, an uncharacterised Nitrosomonas sp. cluster. This cluster is phylogenetically similar to other active ammonia-oxidizers with worldwide distribution in coastal environments indicating its potential, but previously overlooked, contribution to ammonia oxidation globally. In contrast DNA profiles were dominated by highly abundant but low-activity clusters phylogenetically distinct from known Nitrosomonas (Nm) and Nitrosospira (Ns). This cluster is also globally distributed in coastal sediments, primarily detected as DNA, and often classified as Nitrosospira or Nitrosomonas. We therefore propose to classify this cluster as Ns/Nm. Our work indicates that low abundant but highly active AO could be responsible for the nitrification globally, while abundant AOB Ns/Nm may not be transcriptionally active, and as such account for the lack of correlation between rate processes and gene abundances often reported in the literature. It also raises the question as to what this seemingly inactive group is doing?