Remotely Detectable Biosignatures of Anoxygenic Phototrophs

Abstract:

Many astrobiological/exobiological studies have been directed at searching for evidence of life on planetary bodies within our solar system, but the search for life does not have to be restricted to our stellar neighborhood. The field of exoplanet research has grown rapidly over the last several years. Studies have moved beyond detection to assessing the habitability and biosignatures of these worlds. The biosignatures considered thus far focus on biogenic gases and planetary surface features, such as the light reflected from the surface of plants to generate the “red edge” of vegetation. Much work has focused on detecting biosignatures of higher life forms (vegetation) on exoplanets. However, land plants only appeared on the Earth 450 million years ago, and required a long path of photosynthetic evolution. There is a dearth of studies examining how light might interact with much simpler, more evolutionarily ancient pigmented communities, such as photosynthetic microbes. These anoxygenic phototrophs, which have inhabited Earth for nearly 80% of its history, may dominate exoplanets at a similar stage of evolution as the Archean or Paleoproterozoic Earth.

Similar to the remotely detectable “red edge” of chlorophyll a – containing vegetation, we measured the reflectance spectra of pure cultures and environmental samples of purple sulfur, purple non-sulfur, heliobacteria, green sulfur, and green non-sulfur anoxygenic phototrophs. We observed an increase in reflectivity just past the absorption maximum for the bacteriochlorophyll pigments. Since this reflectance feature is shifted into the NIR compared to that of the red edge of vegetation, we’re calling this the “NIR edge” of anoxygenic phototrophs. The bacteriochlorophyll pigments are particularly well suited to absorb the far-red and near-infrared radiation emitted by M dwarf stars, the most common type of star in our galaxy. Therefore these phototrophs serve as model organisms for photosynthesis adapted to alternative spectral environments.