Dynamics of 18O Incorporation from H 2 18 O into Soil Microbial DNA

Heavy water (H (2) (18) O) has been used to label DNA of soil microorganisms in stable isotope probing experiments, yet no measurements have been reported for the O-18 content of DNA from soil incubated with heavy water. Here we present the first measurements of atom% O-18 for DNA extracted from soil incubated with the addition of H (2) (18) O. Four experiments were conducted to test how the atom% O-18 of DNA, extracted from Ponderosa Pine forest soil incubated with heavy water, was affected by the following variables: (1) time, (2) nutrients, (3) soil moisture, and (4) atom% O-18 of added H2O. In the time series experiment, the atom% O-18 of DNA increased linearly (R (2) = 0.994, p < 0.01) over the first 72 h of incubation. In the nutrient addition experiment, there was a positive correlation (R (2) = 0.991, p = 0.006) between the log(10) of the amount of tryptic soy broth, a complex nutrient broth, added to soil and the log(10) of the atom% O-18 of DNA. For the experiment where soil moisture was manipulated, the atom% O-18 of DNA increased with higher soil moisture until soil moisture reached 30%, above which O-18 enrichment of DNA declined as soils became more saturated. When the atom% O-18 for H2O added was varied, there was a positive linear relationship between the atom% O-18 of the added water and the atom% O-18 of the DNA. Results indicate that quantification of O-18 incorporated into DNA from H (2) (18) O has potential to be used as a proxy for microbial growth in soil.