Temperature adaptation of bacterial growth and ¹⁴C-glucose mineralisation in a laboratory study

Microbial decomposition of soil organic matter (SOM) is the source of most of the terrestrial carbon dioxide emission. Consequently, our ability to predict how climate warming will affect the global carbon (C) budget relies on our understanding of the temperature relationship and adaptability of microbial processes. We exposed soil microcosms to temperatures between 0 and 54°C for 2 months. After this, bacterial growth (leucine incorporation) and functioning (¹⁴C-glucose mineralisation) were estimated at 8 temperatures in the interval 0-54°C to determine temperature relationships and apparent minimum (T_min) and optimum (T_opt) temperatures for growth and mineralisation. We predicted that incubation at temperatures above the initial T_opt for bacteria would select for a warm-adapted community, i.e. a positive shift in T_min and T_opt for bacterial growth, and that this adaptation of the bacterial community would coincide with a similar shift also for their functioning. As anticipated, we found that exposure to temperatures below T_opt did not change the temperature relationship of bacterial growth or mineralisation. Interestingly, T_opt for glucose mineralisation was >20°C higher than that for growth. For bacterial growth, the temperature relationship for the bacterial community was modulated when soils were incubated at temperature above their initial T_opt (≈30°C). This was shown by an increase in T_min of 0.8°C for every 1°C increase in soil temperature, evidencing a shift towards warm-adapted bacteria. Similarly, the Q₁₀ (15-25°C) for bacterial growth increased at temperature higher than T_opt. We could not detect a corresponding temperature adaptation of the decomposer functioning. We discuss possible underlying reasons for the temperature-responses of bacterial processes. We note that a temperature adaptation will be rapid when exceeding the T_opt, which initially were >20°C higher for glucose mineralisation than growth. This difference could suggest that different responses to warming exposure should be expected for these microbial processes.