Biological Accumulation of D-amino acids in Endolithic Microbial Communities

Affiliation(s)PIProject periodFunded by
DEES Sun, Henry 05/05/2008 - 12/31/2012 National Aeronautics & Space Administration

Project Description

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, the 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 (i.e. by not separating organisms from necromass), EMCs can not be regarded as containing L-amino acids only. This project 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 plan 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 planned 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 planned 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.