Evaluating Pigments as a Biosignature: Abiotic / Prebiotic Synthesis of Pigments and Pigment Mimics in Planetary Environments

Pigments serve a multitude of functions including light harvesting for photosynthesis, protection against radiation and harmful radicals, membrane support, and defense. The ubiquity of pigments– especially within extremophiles found in high-radiation, high-salinity, and dry environments – and their detectability via mission-ready techniques has elevated these molecules as promising targets for life detection missions. The detection of these molecules has been proposed as a “smoking gun” for the detection of extraterrestrial life as the growing opinion has been pigments cannot be generated prebiotic conditions. However, this claim should be regarded with caution given that the prebiotic synthesis of these molecules is considerably understudied, especially when compared to other building blocks of life The molecular structure does not constitute a particular chemical motif and includes any compound capable of absorbing specific wavelengths of visible light. Previous prebiotic studies have demonstrated feasible prebiotic routes for generating pigments including hemes, chlorophylls, and carotenoids. While a total prebiotic synthesis has yet to be demonstrated, these prebiotic studies, and total synthetic procedures for generating these molecules in industry, indicate promising avenues for future research. Herein, we review the biochemistry of pigments, their abiotic synthesis in industry, prebiotic reactions, and implications/recommendations for future work, and molecules which can mimic their spectral signatures during planetary missions. While pigments hold promise as a biosignature, a current understanding of abiotic routes remains understudied and uncertain. This is critical for evaluating the biogenicity of any pigment detected during missions including by the Mars Perseverance rover.