Biocrust (IMAGE) Arizona State University Caption Artist's rendition of the environments of primeval earth in the Archaean, including the well-known aquatic stromatolites that are formed by carbonate-precipitating microorganisms. The close-up inset illustrates terrestrial biocrusts, typically organized by filamentous cyanobacteria, in a photograph of a modern sample. The new study suggests these biocrusts played a vital role in the nitrogen cycle of the early earth. Credit Estelle Couradeau Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.