Stable Isotope Probing with 18 O-Water to Study Microbial Functions and Population Dynamics in Soil.

Oxygen atoms from water are incorporated into DNA when it is newly formed so that when soil is incubated with ¹⁸O-water the DNA of growing organisms becomes labeled with ¹⁸O atoms.  In stable isotope probing (SIP), labeled DNA is separated from non-labeled DNA along a cesium chloride gradient formed in an ultracentrifuge.  Because ¹⁸O-atoms are heavier than ¹⁶O-atoms and, consequently, the buoyant density of labeled DNA is greater than non-labeled DNA, water can be used as a labeled substrate in SIP experiments.  The technique allows study of the impact of environmental parameters on growth and mortality of microbial populations in soil.  For instance we have found that fungi grow faster than bacteria in a Ponderosa Pine forest soil when a dry soil is rewet.  We were also able to use SIP with ¹⁸O-water to identify a bacterial strain, Rhodococcus sp. strain RHA1, that degraded toluene in a junkyard soil.  Finally we used SIP to study the impact of ammonium availability on population dynamics of ammonia oxidizing bacteria and archaea in soil.