Raman Characterization of the UV-Protective Pigment Gloeocapsin and Its Role in the Survival of Cyanobacteria

Extracellular UV-screening pigments gloeocapsin and scytonemin present in the exopolysaccharide (EPS) envelopes of extremophilic cyanobacteria of freshwater and marine environments were studied by Raman spectroscopy and compared to their intracellular photosynthetic pigments. This Raman spectral analysis of the extracellular pigment gloeocapsin showed that it shared Raman spectral signatures with parietin, a radiation-protective pigment known in lichens. The UV-light spectra also show similarities. Gloeocapsin occurs in some cyanobacterial species, mostly with exclusion of scytonemin, indicating that these pigments have evolved in cyanobacteria as separate protective strategies. Both gloeocapsin and scytonemin are widely and species-specifically distributed in different cyanobacterial genera and families. The widespread occurrence of these pigments may suggest an early origin, while their detection by Raman spectroscopy makes them potential biosignatures for cyanobacteria in the fossil record and demonstrates the usefulness of nondestructive Raman spectroscopy analyses for the search for complex organics, including possible photosynthetic pigments, if preservable in early Earth and extraterrestrial samples.