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Appl Environ Microbiol. 1974 December; 28(6): 960-967
Copyright © 1975 American Society for Microbiology. All Rights Reserved.

Microbial Metabolism and Dynamic Changes in the Electrical Conductivity of Soil Solutions: a Method for Detecting Extraterrestrial Life¹

Planetary Biology Division, Ames Research Center, National Aeronautics and Space Administration, Moffett Field, California 94035

ABSTRACT

The addition of 0.5% glucose solutions to 12 different air-dried soils always resulted in increased electrical conductivity and water-soluble Ca and Mg in the soil solutions. The kinetics and magnitude of these changes for at least two and usually all three of these parameters over a 14-day period were clearly distinguishable from the changes in heat-sterilized controls or unsterilized controls without added glucose. In general, maximal values were achieved more rapidly under aerobic than anaerobic incubation. Some soils (less than half) also showed significant increases in water-soluble Na or K when compared with the controls. The 12 different soils studied represented four general soil groups: I, leached acid upland soils; II, saline alkaline soils; III, nonsaline neutral soils; and IV, high organic soils. Viable counts ranged from 10⁴ to 10⁷ per cm³ of air-dried soil. Glucose metabolism by the indigenous soil microbiota was always accompanied by a significant decrease in the pH of soil solutions, but not necessarily by an increase in the viable count. The feasibility of using electrical conductivity and water-soluble Ca and Mg measurements to detect metabolic activity, either alone or in conjunction with other life detection techniques, is discussed.

¹ Dedicated to the late Wolf Vishniac, one of the pioneers of exobiology, who died in the Antarctic in December 1973.