Abstract EANA2024-76

Photosynthetic life developed around 3.4 billion years ago, and it still provides the foundation for virtually all life on Earth. Because photosynthesis has been so successful on our planet, it is a promising metabolism to look for when searching for life on other planets. To capture light energy, photosynthetic organisms use pigments – molecules that selectively absorb light at specific wavelengths in the visible spectrum. Since these biological pigments are exclusively produced by living organisms, their reflectance spectra serve as a potential biosignature. This is especially relevant for life detection on the surface of other planets where stellar light is available as a continuous and inexhaustible source of energy. While the fundamental principles of photosynthesis are universal, pigments that might have developed on these planets will have to adapt to different host star spectra and atmospheric conditions.

Currently, spectrometry is the main method to detect biological pigments over long distances. However, using light intensity alone to distinguish genuine pigment biosignatures from false positives, such as non-biological mineral pigments, is still challenging. Therefore, we propose a novel approach for detecting pigment biosignatures by linking them to homochirality. Homochirality – the predominance of “left-handed” or L-amino acids and “right-handed” or D-sugars – is a hallmark of life on Earth. Homochirality determines both structure and function of biological systems and is crucial for self-replication, making it a powerful and potentially universal biosignature. To detect homochirality, we have developed a new instrument, a spectropolarimeter, which quantifies the degree of circular polarization with wavelength. When light interacts with homochiral molecules, a fraction of it becomes circularly polarized through multiple scattering events. Thus, the detection of circular polarization could be used as an indicator for homochirality, which in turn is an indicator for life. Currently, our instrument is confined to the visible spectrum (400 – 800 nm), and accordingly, we focus on detecting biological pigments within that range. Our goal is to demonstrate that our method can detect photosynthetic pigments, even if the light spectrum available for growth differs from that on Earth.

We studied the cyanobacterial species Synechococcus elongatus (PCC7942), an oxygenic photosynthetic freshwater bacterium. Our measurements successfully detected circular polarization signals that can be attributed to the biological pigment chlorophyll a as well as the accessory pigments phycocyanin and phycoerythrin. To optimise light harvesting, these accessory pigments capture light at wavelengths different than chlorophyll a. In some cyanobacterial species, the accessory pigments are known to adapt to different light conditions through a process called chromatic acclimation. We grew S. elongatus under various light conditions to induce chromatic acclimation and to explore how light quality and quantity affect spectropolarimetric detection. Our findings suggest that spectropolarimetry could complement classical spectroscopy in detecting photosynthetic life adapted to light conditions other than those on Earth.