CARBON ISOTOPES IN THE BIOSYNTHESIS OF SCYTONEMIN BY CYANOBACTERIA

Many species of colonial and filamentous cyanobacteria synthesize scytonemin in response to exposure to ultraviolet radiation. Scytonemin is most common in lichens, desert biological soil crust, microbialites and intertidal and hypersaline mats. Scytonemin is preserved in sediments, and even when it has been eroded and transported to marine depositional environments its C and N stable isotopic composition can be used to distinguish among biocrust, lichen, and microbialite sources. Scytonemin biosynthesis is tied to that of aromatic amino acids, as it requires tryptophan and 4-hydroxyphenylpyruvic acid as precursors; thus, the production of scytonemin affects the flow of carbon and nitrogen through the Shikimate pathway. We developed a chemical method to oxidize scytonemin forming two products, isatin and 4-hydroxybenzaldehyde, that are amenable to analysis by gas chromatography-isotope ratio mass spectrometry and are derived from the tryptophan and 4-hydroxyphenylpyruvic acid precursors, respectively. 4-hydroxybenzaldehyde and cyanobacterial biomass have a consistent C isotopic difference, whereas the isotopic difference between isatin and biomass is variable and likely reflects differences in the partitioning of C isotopes in the Shikimate biopathway. These two compounds derived from scytonemin can potentially be used to better understand stress response to ultraviolet exposure in cyanobacteria.