  The identification of a universal biosignature that could be sensed remotely
is critical to the prospects for success in the search for life elsewhere in
the universe. A candidate universal biosignature is homochirality, which is
likely to be a generic property of all biochemical life. Due to the optical
activity of chiral molecules, it has been hypothesized that this unique
characteristic may provide a suitable remote sensing probe using circular
polarization spectroscopy. Here, we report the detection of circular
polarization in light scattered by photosynthetic microbes. We show that the
circular polarization appears to arise from circular dichroism of the strong
electronic transitions of photosynthetic absorption bands. We conclude that
circular polarization spectroscopy could provide a powerful remote sensing
technique for generic life searches.
