2015 Annual Science Report
VPL at University of Washington Reporting | JAN 2015 – DEC 2015
Global Surface Biosignatures: Circular Polarization Spectra of Anoxygenic Phototrophs and Cyanobacteria
This new project focuses on characterizing the chiral signature of biological molecules. The phenomenon of chirality is a powerful biosignature and, in principle, can be remotely observed on planetary scales using circular polarization spectroscopy. Molecules such as photosynthetic pigments are optically active and have several chiral centers, and influence the polarization of light. This can be measured using full Stokes spectropolarimetry. The goal of this interdisciplinary project is to characterize the circular polarization spectra of chiral photosynthetic pigments in anoxygenic phototrophs and cyanobacteria as global surface biosignatures.
We experimentally measured the circular polarization spectra of pure cultures of anoxygenic phototrophs. Major representatives (i.e., type strains) were analyzed from all major groups (purple sulfur, purple non-sulfur, green sulfur, filamentous green non-sulfur, and Heliobacteria). The reflectance, transmission, and absorption spectra were obtained for each culture. We observed strong correlations between the absorption maxima of the pigments and features in the circular polarization reflectance spectra (Fig. 1) (Sparks et al., 2015). The strong polarization signal is due to the weakly chiral centers intrinsic to the bacteriochlorophyll pigment structure, as well as to the long-range chiral order of the pigment macrodomains. These results were presented at the 2015 Winter AGU Meeting.
PROJECT INVESTIGATORS:William Sparks
PROJECT MEMBERS:Thomas Germer
RELATED OBJECTIVES:Objective 4.1
Earth's early biosphere.
Co-evolution of microbial communities
Biosignatures to be sought in nearby planetary systems