Temperature sensitivity of bacterial growth in a hot desert soil with large temperature fluctuations

Hot desert ecosystems are characterized by high soil temperatures with large fluctuations annually and diurnally. Thus, one could hypothesize that not only the microbial community would be adapted to high temperatures, but also have a large temperature range conducive for growth. We determined the temperature sensitivity of the soil bacterial community from the Chihuahuan Desert, Big Bend National Park, Texas, USA, using leucine incorporation as a proxy for bacterial growth. Soil samples were taken during early spring and mid-summer from the surface (0-5 cm) and deeper (15-20 cm) soil layers. Mean winter soil temperature preceding the spring samples was 15 °C and in summer 36 °C at both depths, but with larger amplitude in the top soil than deeper down. Tmin was significantly lower in the top 0-5 cm than at 15-20 cm, −1.2 and 0.0 °C, respectively. Topt also was higher in the top soil than deeper down, 42.9 and 41.4 °C, respectively, resulting in a larger temperature range for growth (Topt − Tmin) in the top soil reflecting the larger temperature fluctuations in this layer. There were no significant differences in cardinal temperatures for bacterial growth in soils sampled in early spring and mid-summer despite large seasonal differences in temperatures, showing that long periods of colder temperatures was less important than periods of high temperatures as selection pressure for temperature sensitivity. Comparing with earlier results from Antarctic soils (Rinnan et al., 2009), which in contrast represent an extremely low temperature environment, we suggest that the range of temperature cardinal temperatures for soil bacterial communities globally varies from around −15 to 0 °C for Tmin and 25 to 45 °C for Topt.