Amino Acid Analyses of Desert Varnish from the Sonoran and Mojave Deserts

Abstract

There has long been a debate as to whether desert varnish deposits are microbially mediated or are deposited by inorganic processes. Several researchers have cultured bacteria from the surface of desert varnish suggesting that bacteria are intimately associated with varnish coatings and may play a role in their formation. To test this hypothesis, we have collected scrapings of desert varnish from the Sonoran Desert in Arizona and the Mojave Desert in California and analyzed them for amino acids. Thirteen amino acids were found in desert varnish indicating a biogenic component of these varnishes. Two protein amino acids that were not detected in any of the varnishes are cysteine and tryptophan. Two nonprotein amino acids,β-alanine andγ-amino butyric acid, were found. These are known to be formed by enzymatic decarboxylation, thereby indicating possible organismal activity in varnish. Some D -enantiomers of the amino acids were also found. In addition to small amounts of the D -enantiomer of aspartic acid, which is rapidly formed by racemization and was present in most samples, D -alanine and D -glutamic acid were found. These latter two amino acids are components of the peptidoglycan cell wall material of bacteria. L -lysine was also detected, but not diaminopimelic acid. The combination of L -lysine, D -alanine, and D -glutamic acid is characteristic of the peptidoglycan from Gram-positive bacteria. Although the presence of these biomarkers does not prove that Gram-positive bacteria produce the coatings, finding them is consistent with the hypothesis that they may play a role in desert varnish formation.