Microbial activity in soils frozen to below −39 °C

Recent research on life in extreme environments has shown that some microorganisms metabolize at extremely low temperatures in Arctic and Antarctic ice and permafrost. Here, we present kinetic data on CO2 and 14CO2 release from intact and 14C-glucose amended tundra soils (Barrow, Alaska) incubated for up to a year at 0 to −39°C. The rate of CO2 production declined exponentially with temperature but it remained positive and measurable, e.g. 2–7 ng CO2–C cm−3 soil d−1, at −39 °C. The variation of CO2 release rate (v) was adequately explained by the double exponential dependence on temperature (T) and unfrozen water content (W) (r2>0.98): v=A exp(λT+kW) and where A, λ and k are constants. The rate of 14CO2 release from added glucose declined more steeply with cooling as compared with the release of total CO2, indicating that (a) there could be some abiotic component in the measured flux of CO2 or (b) endogenous respiration is more cold-resistant than substrate-induced respiration. The respiration activity was completely eliminated by soil sterilization (1 h, 121 °C), stimulated by the addition of oxidizable substrate (glucose, yeast extract), and reduced by the addition of acetate, which inhibits microbial processes in acidic soils (pH 3–5). The tundra soil from Barrow displayed higher below-zero activity than boreal soils from West Siberia and Sweden. The permafrost soils (20–30 cm) were more active than the samples from seasonally frozen topsoil (0–10 cm, Barrow). Finding measurable respiration to −39 °C is significant for determining, understanding, and predicting current and future CO2 emission to the atmosphere and for understanding the low temperature limits of microbial activity on the Earth and on other planets.