Exobiology and Evolutionary Biology

Biological Accumulation of D-amino acids in Endolithic Microbial Communities (2)

PI: Henry Sun

New evidence indicates that certain D-amino acids can accumulate in endolithic microbial communities (EMCs) to significant levels (Sun et al. accepted, PNAS). These D-amino acids are recalcitrant components of bacterial and cyanobacterial cell walls (peptidoglycans) that build up through time as necromass. In two Antarctic EMCs, enantiomeric D/L ratios of some amino acids reach 0.6, or 38% D. This finding, termed pseudoracemization, suggests that from a life detection standpoint EMCs can not be
regarded as containing L-amino acids only – unless the organisms can be separated from the necromass.
This proposal brings together multiple areas of expertise to study a large collection of specimens from hot and cold (polar) deserts around the globe (Antarctic Dry Valleys, Elsmere Island of high arctic, Atacama of Chile, Death Valley, Timna of Israel, etc.) to assess whether or not pseudoracemization is a general biosignature of endolithic life on Earth. To accomplish this goal we propose to: 1) identify the EMC organisms by culturing and DNA sequencing; 2) quantify D- and L-amino acids in rocks, in cyanobacterial and bacterial cultures, and in their cell walls; 3) determine the correlation between bacterial necromass and D-amino acids in rocks; and 4) determine the portion of the D-amino acid quantity that can be attributed to age-induced racemization, the competing hypothesis to pseudoracemization.
The proposed work has two major potential implications. In a few years the amino acid analyzer “Urey” will search for extant life on Mars containing either L- or D-amino acids or for fossil remains of extinct biota which, after billions of years of racemization, would contain a mixture of L- and D-amino acids. The proposed research may raise a third possibility: living microbial communities with fossil-like D/L ratios due to pseudoracemization. This study could also yield insights into potential biological evolution on the desertifying, early Mars.