¹⁴C-Labeled Organic Substrates

The substrates selection process was conducted over a period of several years. Levin and Straat developed the guidelines for selection of the LR substrates, detailed in Levin and Straat (1976a) (p. 295-299), and then underwent extensive testing to select those substrates best matching the guidelines. This section outlines the basic selection guidelines. Please refer to the paper listed above for complete detail.

On the assumption that life on Mars may have evolved similarly to that on Earth, substrates were selected from those that may have been produced on Mars through prebiotic processes. These compounds would, then, have been available for chemical evolution of more complex life precursors and, finally, living systems. Formate, glycine, alanine, lactate, and glycolate are all simple Miller-Urey products, readily produced in a reducing atmosphere. However, even a nonreducing simulated Mars atmosphere with a mixture of carbon dioxide, carbon monoxide, and trace amounts of water vapor produced acetaldehyde, formaldehyde, and glycolic acid when exposed to ultraviolet light (Hubbard et al., 1971).

The LR substrates were each present at 2.5 x 10^-4 M concentration in unbuffered aqueous solution adjusted to pH 6-6.5. Substrate concentrations were kept low should one or more substrate be toxic (Levin and Straat, 1976a). It was assumed that, were life present, the soil would be at the proper pH (hence no buffer) and would already contain any needed nutrients. The LR substrates chosen for incorporation into the simple solution and their key parameters are shown below:

Note: The ¹⁴C-DL-alanine mixture and the ¹⁴C-DL-sodium lactate mixture both contain equal amounts of each stereoisomer.