  Photosynthesis is an ancient metabolic process that began on the early Earth,
offering plentiful energy to organisms that utilize it, to the extent that they
can achieve global significance. The potential exists for similar processes to
operate on habitable exoplanets and result in observable biosignatures. Prior
to the advent of oxygenic photosynthesis, the most primitive phototrophs,
anoxygenic phototrophs, dominated surface environments on the planet. Here, we
characterize surface polarization biosignatures associated with a diverse
sample of anoxygenic phototrophs and cyanobacteria, examining both pure
cultures and microbial communities from the natural environment. Polarimetry is
a tool that can be used to measure the chiral signature of biomolecules.
Chirality is considered a universal, agnostic biosignature that is independent
of a planet's biochemistry, receiving considerable interest as a target
biosignature for life detection missions. In contrast to preliminary
indications from earlier work, we show that there is a diversity of distinctive
circular polarization signatures, including the magnitude of the polarization,
associated with the variety of chiral photosynthetic pigments and pigment
complexes of anoxygenic and oxygenic phototrophs. We also show that the
apparent death and release of pigments from one of the phototrophs is
accompanied by an elevation of the reflectance polarization signal by an order
of magnitude, which may be significant for remotely detectable environmental
signatures. This work and others suggest circular polarization signals up to
~1% may occur, significantly stronger than previously anticipated circular
polarization levels. We conclude that global surface polarization biosignatures
may arise from anoxygenic and oxygenic phototrophs, which have dominated nearly
80% of the history of our rocky, inhabited planet.
