# Kinetic plasticity of nitrite-oxidizing bacteria containing cytoplasmic nitrite oxidoreductase

## TL;DR

This paper shows that certain bacteria can dynamically adjust their ability to process nitrite based on environmental conditions, which explains their presence in unexpected environments.

## Contribution

The study reveals a novel mechanism of kinetic plasticity in cNXR NOB, enabling dynamic nitrite affinity regulation in response to pH and nitrite concentration.

## Key findings

- cNXR NOB like Nitrobacter winogradskyi exhibit high nitrite affinity at low concentrations and acidic pH.
- Kinetic plasticity in cNXR NOB is driven by a switch between nitrite transporters and enhanced HNO2 diffusion at low pH.
- This mechanism explains the ecological success of cNXR NOB in acidic, low-nitrite environments.

## Abstract

Nitrite-oxidizing bacteria (NOB) use either periplasmic (pNXR) or cytoplasmic (cNXR) nitrite oxidoreductase to oxidize nitrite, and this distinction influences nitrite affinity and energy yield. cNXR-containing NOB have historically been considered low-affinity, copiotrophic nitrifiers adapted to high nitrite and neutral pH. Here, we report a previously uncharacterized pH- and substrate-dependent modulation of nitrite affinity in cNXR NOB i.e. not observed in pNXR NOB and is not a universal microbial trait. Nitrobacter winogradskyi Nb-255, grown at low nitrite (1 mM), had a high apparent affinity (Km(app) = 25.9 μM; specific affinity ao = 440.5 l g cells−1 h−1) comparable to oligotrophic pNXR NOB. However, when grown at high nitrite (10 mM), these cells showed a low affinity at pH 7.5 (Km(app) = 388.0 μM) but exhibited a rapid increase in affinity upon immediate exposure to pH 5.5 (Km(app) = 19.2 μM) without prior acid adaptation. In contrast, pNXR NOB exhibited consistent kinetic behavior across different pH conditions, underscoring that this kinetic plasticity is unique to cNXR NOB. Kinetic inhibition assays revealed that this plasticity is mechanistically underpinned by a shift from a low-affinity nitrite/nitrate antiporter (NarK) to a high-affinity nitrite channel (NirC), coupled with enhanced HNO2 diffusion at low pH, together increasing intracellular nitrite availability. These findings establish that cNXR NOB can dynamically tune nitrite affinity via transporter-level regulation in response to nitrite concentration and pH. This novel mechanism provides a mechanistic explanation for the unexpected prevalence of Nitrobacter in acidic, low-nitrite environments, highlighting its ecological relevance.