Coolen, M. J. L., & Overmann, J. (2000). Functional Exoenzymes as Indicators of Metabolically Active Bacteria in 124,000-Year-Old Sapropel Layers of the Eastern Mediterranean Sea. Applied and Environmental Microbiology, 66(6), 2589–2598. doi:10.1128/aem.66.6.2589-2598.2000