Metabolic Activity of Permafrost Bacteria below the Freezing Point

TLDR

The stationary phase in microbial growth is usually attributed to nutrient limitation. We propose that in permafrost, stationary phase is caused by diffusion barriers in thin unfrozen water layers whose thickness depends on temperature. We measured permafrost bacterial metabolic activity from 5 to −20 °C by tracking 14 C‑acetate incorporation into lipids. The incorporation followed a sigmoidal pattern with a log phase lasting 200–350 days, stationary phase reached by day 550, and doubling times ranging from 1 day at 5 °C.

Abstract

ABSTRACT Metabolic activity was measured in the laboratory at temperatures between 5 and −20°C on the basis of incorporation of 14 C-labeled acetate into lipids by samples of a natural population of bacteria from Siberian permafrost (permanently frozen soil). Incorporation followed a sigmoidal pattern similar to growth curves. At all temperatures, the log phase was followed, within 200 to 350 days, by a stationary phase, which was monitored until the 550th day of activity. The minimum doubling times ranged from 1 day (5°C) to 20 days (−10°C) to ca. 160 days (−20°C). The curves reached the stationary phase at different levels, depending on the incubation temperature. We suggest that the stationary phase, which is generally considered to be reached when the availability of nutrients becomes limiting, was brought on under our conditions by the formation of diffusion barriers in the thin layers of unfrozen water known to be present in permafrost soils, the thickness of which depends on temperature.