Temperature defines metabolic states that lead to seasonal adaptation of the soil nitrite oxidizer Nitrobacter vulgaris strain CN101

ABSTRACT Nitrite-oxidizing bacteria (NOB) play a central role in nitrification, yet their temperature-dependent ecophysiology remains poorly understood, particularly in terrestrial environments. In this study, the physiological and transcriptomic responses of Nitrobacter vulgaris strain CN101, isolated from forest soil in Japan, were investigated across different culture temperatures. Temperature-dependent kinetics revealed two distinct activation energies below and above 20°C, suggesting a shift in metabolic state. At lower temperatures, nitrite oxidation rates declined, and transcriptomic analysis showed reduced expression of genes involved in electron transport and protein synthesis, whereas genes related to carbon and nitrogen assimilation remained relatively unchanged. Electron microscopy revealed smaller cell size and simplified membrane structures under low-temperature conditions. Together, these findings indicate that metabolic activity is maintained while biomass production is constrained below 20°C. At higher temperatures, efficient electron transport, increased translational capacity, and enlarged cell morphology were restored, consistent with a growth-promoting state. These coordinated metabolic and structural changes suggest that strain CN101 adopts a temperature-dependent strategy that balances survival and proliferation in seasonally fluctuating forest soils. By integrating physiological, ultrastructural, and transcriptomic analyses, this study provides a concrete example of how temperature influences the ecophysiological adaptation of terrestrial NOB. IMPORTANCE Nitrification is a key microbial process in the global nitrogen cycle, yet how nitrifying microorganisms remain active under fluctuating environmental conditions remains poorly understood. In temperate soils, seasonal temperature variation strongly influences nitrification, but the physiological responses of nitrite-oxidizing bacteria (NOB) remain largely unexplored. In this study, we show that the soil nitrite oxidizer Nitrobacter vulgaris strain CN101 exhibits temperature-dependent physiological states, shifting from a metabolically active but growth-limited state at low temperatures to a state of active growth at higher temperatures. These findings demonstrate how temperature restructures NOB metabolism and physiology. This study provides a representative example of the seasonal ecophysiology of soil nitrifiers and highlights temperature-driven physiological restructuring as a key mechanism shaping nitrification dynamics in terrestrial ecosystems.