Water and temperature relations of soil A ctinobacteria

Summary Actinobacteria perform essential functions within soils, and are dependent on available water to do so. We determined the water‐activity (a w ) limits for cell division of S treptomyces albidoflavus , S treptomyces rectiviolaceus , M icromonospora grisea and M icromonospora ( JCM 3050) over a range of temperatures, using culture media supplemented with a biologically permissive solute (glycerol). Each species grew optimally at 0.998 a w (control; no added glycerol) and growth rates were near‐optimal in the range 0.971–0.974 (1 M glycerol) at permissive temperatures. Each was capable of cell division at 0.916–0.924 a w (2 M glycerol), but only S . albidoflavus grew at 0.895 or 0.897 a w (3 M glycerol, at 30 and 37° C respectively). For S . albidoflavus , however, no growth occurred on media at ≤ 0.870 (4 M glycerol) during the 40‐day assessment period, regardless of temperature, and a theoretical limit of 0.877 a w was derived by extrapolation of growth curves. This level of solute tolerance is high for non‐halophilic bacteria, but is consistent with reported limits for the growth and metabolic activities of soil microbes. The limit, within the range 0.895–0.870 a w , is very much inferior to those for obligately halophilic bacteria and extremely halophilic or xerophilic fungi, and is inconsistent with earlier reports of cell division at 0.500 a w . These findings are discussed in relation to planetary protection policy for space exploration and the microbiology of arid soils.