Abstract
The majority of marine nitrite-oxidising bacteria (NOB) belong to three uncultured genera within the Nitrospinaceae family and thus the ecophysiology of these main nitrate producing microorganisms in the ocean remains poorly resolved. Here, we report on the environmental characterization of one of these genera, which we name Candidatus Nitromaricola. A single species within this genus, Candidatus Nitromaricola pontica, comprises >95% of the nitrite-oxidising community in the permanently stratified Black Sea. This provided a unique opportunity to study this globally abundant genus in situ, across pronounced environmental gradients of oxygen and inorganic nitrogen concentrations. We show that Ca. Nitromaricola pontica is capable of oxidising nitrite at similar rates in both oxic and apparently anoxic waters, although it had a higher abundance in the more oxygenated waters of the Black Sea. The genome of Ca. Nitromaricola pontica indicates that like other NOB, it is a chemolithoautotroph, oxidising nitrite, and fixing inorganic carbon. However, Ca. Nitromaricola pontica and other members of the Ca. Nitromaricola genus lack canonical high affinity terminal oxidases found in some other NOB genera. Single-cell analyses confirmed active autotrophic growth and showed that ammonium and urea, rather than nitrite, were the main nitrogen sources for biomass production. Our findings identify Ca. Nitromaricola as a globally abundant marine NOB and highlight its cosmopolitan lifestyle.