Detection of circular polarization in light scattered from photosynthetic microbes

The search for life in the Universe depends on the identification of observable signatures that are unique to biological processes. If these signatures may be sensed remotely, then extensive surveys of planetary surfaces and distant objects may be undertaken without the need (initially) for costly landing spacecraft. A candidate universal biosignature that may lend itself to remote sensing application is homochirality, which, due to the optical activity of biological molecules, is potentially detectable using circular polarization spectroscopy (1)(2)(3)(4)(5)(6). Organic molecules typically exist in two mirror-image forms and are said to be chiral; that is they exhibit handedness. All known living organisms use only left-handed or L-amino acids in proteins and right-handed or D-sugars in nucleic acids and this unique preference for just a single handedness is termed homochirality. Intriguingly, analysis of the Murchison meteorite has shown Lexcesses of 2-9% for a number of -methyl amino acids (7), with slightly smaller excesses found in the Murray meteorite (8). Homochirality is thought to be generic to all forms of biochemical life as a necessity for self-replication (9) and hence it is likely to be a signature of non-terrestrial life. To be detectable remotely using circular polarization, homochirality must imprint itself upon the circular polarization spectrum in scattered light. Here we report on the results of sensitive laboratory measurements of the polarization spectra of light both scattered from photosynthetic microbes and in transmission through the same cultures. For context, we also present polarization spectra of a leaf and a mineral.

There is a vast array of experimentation that may be brought to bear in the case of in-situ tests for the presence of biological processes (10), however in-situ experiments can sample only a tiny fraction of a planetary surface and its immediate subsurface, and they often anticipate a degree of specificity in the biology sought. Few remote sensing methods directly probe signatures of biological life. Trace gases can be observed that could have a biological origin, such as recent detections of localized methane production on Mars (11). Jupiter’s moon Europa is strongly suspected to host a liquid water ocean (12) and infrared spectroscopic features have been shown to be consistent with those of radiation tolerant microbes (13). Beyond the Solar System, methods will be needed to assess whether extrasolar planets harbor life, and remote sensing is a necessity. Attention is being given to atmospheric composition disequilibria and to biological pigmentation spectral features as biomarkers (14)(15)(16). Typical disadvantages of these methods include model dependence and the possibility that the “biosignature” could be produced by abiotic processes, leading to a false positive.

Circular polarization may provide a more direct indication of the presence of biological processes since it is directly attributable to the chirality of the organic molecules. Earlier experiments (1,3,4) looked at leaves and found significant circular polarization. Here we focus on light reflected from photosynthetic bacterial cultures. Photosynthetic life must reside at the surface, use windows of atmospheric transparency and exploit regions of the spectrum where the host star shines brightly, and hence such life forms are maximally observable. The strong electronic absorption bands that are characteristic of photosynthesis are known to exhibit circular dichroism (different absorption coefficients for left-and right-circularly polarized light), hence we may anticipate a conse-quent polarization signature in scattered light, though due to the complexities of the scattering process, this is not entirely obvious in advance. For example, multiple scattering tends to randomize the polarization state; or alternatively, any circular polarization produced in the incident direction might be cancelled by circular polarization produced by subsequent reflection, as this will have the opposite handedness. If the scatterers depolarize the light, then an appropriate balance between the scattering coefficient and the differential absorption coefficient is needed to achieve measurable circular polarization. In this work, we are testing whether that balance can be achieved with microorganisms. Circular polarization can also be caused by optical interaction associated with the chirality of subcellular structures, such as membranes and macromolecules, aspects that clearly relate to the presence of biology, though yielding a distinct spectral signature relative to the circular dichroism of absorption bands (17).

Photosynthetic cyanobacteria arose between 2 and 3 billion years ago (18,19). The enormous evolutionary advantages of photosynthesis coupled to the resilience of these microbes led to planet-wide changes in the demographic abundance of terrestrial life forms and in turn, to major changes in the composition

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