Antarctic soil no. 726 and implications for the Viking labeled release experiment

Abstract

The failure of the Viking Molecular Analysis experiment (Biemann et al., 1976) to detect organic matter on Mars is primarily responsible for the widespread belief (Klein, 1979; Young, 1979) that a chemical, rather than a biological, agent caused the response obtained by the Labeled Release (LR) life detection experiment (Levin & Straat, 1976a, 1977, 1979a). In discussions concerning whether the relative sensitivities of the Molecular Analysis and the LR experiments could permit a biological interpretation of the Viking LR results, it was stated (Biemann et al., 1976) that lo6 typical prokaryotic cells contain the minimal amount of organic matter detectable by the gas chromatograph-mass spectrometer (GCMS) used in the Molecular Analysis experiment. However, it was pointed out (Biemann, 1976) that the mass of organic detritus from dead cells present in terrestrial soils containing living organisms exceeds the living biomass by ten to one hundred fold. The accumulated dead organic matter would, thus, produce a signal in the GCMS even though the living cells, by themselves, would not (Biemann, 1976). The sensitivity of the radiorespirometric method, its ability to detect small populations of microorganisms even prior to the onset of growth, and the biological nature of the response have been demonstrated (Levin, 1963, 1966; Levin & Heim, 1965; Levin & Straat, 19766). Although the LR technique had detected (Levin & Straat, 1976b) as few as 50 colony-forming units in a terrestrial soil sample, it was doubted that LR-detectable levels of indigenous microorganisms could, in fact, exist in a soil in which the total organic content was below the GCMS limit of detection. We wish to report a new development that provides the first experimental evidence resolving this doubt.