Metabolic activity of permafrost bacteria below the freezing point

Metabolic activity was measured in the laboratory at temperatures between 5 and -20 degrees C on the basis of incorporation of C-14-labeled acetate in to lipids by samples of a natural population of bacteria from Siberian perm afrost (permanently frozen soil). Incorporation followed a sigmoidal patter n 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 degrees C) to 20 days (-10 degrees C) to ca, 160 days (-20 degrees C). The curves reached the stationary phase at different levels, depending on the i ncubation temperature, We suggest that the stationary phase, which is gener ally considered to be reached when the availability of nutrients becomes li miting, was brought on under our conditions by the formation of diffusion b arriers in the thin layers of unfrozen water known to be present in permafr ost soils, the thickness of which depends on temperature.