Measurement of microbial activity in soil by colorimetric observation of in situ dye reduction: an approach to detection of extraterrestrial life

Here we used Earth-derived soils to develop a related life detection system based on direct observation of a biological redox signature. We measured the ability of soil microbial communities to reduce artificial electron acceptors. Living organisms in pure culture and those naturally found in soil were shown to reduce 2,3-dichlorophenol indophenol (DCIP) and the tetrazolium dye 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide inner salt (XTT). Uninoculated or sterilized controls did not reduce the dyes. A soil from Antarctica that was determined by chemical signature and DNA analysis to be sterile also did not reduce the dyes.Observation of dye reduction, supplemented with extraction and identification of only a few specific signature redox-active biochemicals such as porphyrins or quinones, provides a simplified means to detect a signature of life in the soils of other planets or their moons.The detection of microbial life in extraterrestrial locations is an important goal of human exploration of space because of ecological and health concerns about possible contamination of other planets with earthly organisms, and vice versa [1,2]. Several indirect methods for detecting extraterrestrial life have been proposed in the prior literature. These include (a) examining the ratios of stable isotopes in important elements such as carbon and sulphur for discrimination against heavy isotopes and selective use of the lighter isotopes, (b) microscopic observations of highly organized or specifically-shaped structures [3,4], (c) examination of soils for the presence of specific types of organic moieties [5], and (d) the detection of chiral molecules in extraterrestrial samples [6-12].Previously we suggested a thermodynamic approach to the detection of life. Since living entities require a continual source of usable energy, we should be able to detect a chemical signature of life in the form of mixtures of redox molecules such as porphyrins, quinones, and