Phylogenetic and functional marker genes to study ammonia-oxidizing microorganisms (AOM) in the environment

The oxidation of ammonia plays a significant role in the transformation of fixed nitrogen in the global nitrogen cycle. Autotrophic ammonia oxidation is known in three groups of microorganisms. Aerobic ammonia-oxidizing bacteria and archaea convert ammonia into nitrite during nitrification. Anaerobic ammonia-oxidizing bacteria (anammox) oxidize ammonia using nitrite as electron acceptor and producing atmospheric dinitrogen. The isolation and cultivation of all three groups in the laboratory are quite problematic due to their slow growth rates, poor growth yields, unpredictable lag phases, and sensitivity to certain organic compounds. Culture-independent approaches have contributed importantly to our understanding of the diversity and distribution of these microorganisms in the environment. In this review, we present an overview of approaches that have been used for the molecular study of ammonia oxidizers and discuss their application in different environments

Lower-mantle viscosity constrained by seismicity around deglaciated regions

KNOWLEDGE of the viscosity structure of the Earth's mantle is important for constraining models of mantle convection and isostatic rebound. Here we show that seismicity around the margins of deglaciated areas provides a constraint on the viscosity of the lower mantle, in addition to those previously proposed1,2. Calculations using a spherical, viscoelastic Earth model show that the present-day magnitude of the stress fields induced in the lithosphere beneath the (now-disappeared) Laurentide and Fennoscandian ice sheets is very sensitive to the value of the lower-mantle viscosity. Stresses of ∼100 bar, sufficient to cause seismicity, can still remain in the lithosphere for lower-mantle viscosities greater than ∼1022 Pa s; for lower-mantle viscosities of ∼1021 Pa s, only a few tens of bars of stress persist in the lithosphere today. This influence of lower-mantle viscosity on the state of stress in the lithosphere also has implications for the migration of stress from earthquakes, and hence for earthquake recurrence times. © 1991 Nature Publishing Group