P34B-07
SPECTROPOLARIMETRY OF PHOTOSYNTHETIC PIGMENTS AS GLOBAL SURFACE BIOSIGNATURES

Wednesday, 16 December 2015: 17:45
2007 (Moscone West)
William B Sparks, Space Telescope Science Institute, Baltimore, MD, United States, Mary N. Parenteau, SETI Institute Mountain View, Mountain View, CA, United States, Robert E. Blankenship, Washington University in St Louis, Departments of Biology and Chemistry, St. Louis, MO, United States, Thomas A Germer, National Institute of Standards and Technology Gaithersburg, Gaithersburg, MD, United States and Victoria Suzanne Meadows, University of Washington, Seattle, WA, United States
Abstract:
Photosynthesis is an ancient metabolic process on the early Earth. The most primitive phototrophs used reductants such as H2, H2S, and Fe(II) and were widespread in marine, intertidal, and likely continental habitats. These anoxygenic phototrophs were the key primary producers for the first ~1 billion years before the evolution of oxygenic photosynthesis at 2.7 Ga. The potential clearly exists for this type of primitive photosynthesis to operate on habitable exoplanets.

Anoxygenic phototrophs are not known to emit gases that are uniquely biogenic in origin, so we focus on surface pigments signatures as having the strongest promise to offer identifiable biosignatures for a pre-oxygenic habitable exoplanet. Following our earlier work that showed photosynthetic cyanobacteria yield a polarization signature potentially useful in remote sensing, here we seek to characterize the remotely detectable polarization biosignatures associated with anoxygenic phototrophs.

The six major pigments of anoxygenic phototrophs (bacteriochlorophylls [Bchls]) absorb in the near-infrared (NIR) from ~705 – 1040 nm. The lower symmetry of the pigment structure relative to chlorophylls shifts the energy absorption bands to longer wavelengths. As a result, Bchls are well suited to absorbing the relatively higher flux of red and NIR radiation of M dwarf stars, the most abundant type of star in the Galaxy, as well as the plentiful flux of typical main sequence stars. Homochirality is a powerful biosignature, and because of the optical activity of biological molecules, it can, in principle, be remotely observed on macroscopic scales using circular polarization spectroscopy. Bchls and Chls are optically active molecules with several chiral centers, strongly interacting with the incident light.

We measured the reflectance and transmission full Stokes polarization spectra of pure cultures of anoxygenic phototrophs and environmental samples of microbial mats, and found strong correlations between spectral and polarization features. This work characterizing polarization biosignatures associated with the pigments of anoxygenic phototrophs informs the search for life on exoplanets at a similar stage of evolution or biogeochemical state as the Archean Earth (i.e., pre-oxygenic photosynthesis).

<< Previous Abstract | Next Abstract