Characterization of Growing Microbial Populations in Mcmurdo Dry Valley Soils through Stable Isotope Probing with 18O-Water.

Initial surveys of microbial diversity in soils from the McMurdro Dry Valleys, Antarctica, through16S rRNA gene sequencing revealed a surprising bacterial richness, including representatives from at least ten different phyla, and a high proportion of unique and rare sequences. However, given the low rates of microbial activity and decomposition rates, the question of whether this richness represents functioning vs. dormant members of the community was raised. We determined if microbial populations grew in the McMurdo Dry Valley soils by characterizing the growing microbial community through quantitative stable isotope probing (qSIP) with 18O-water. In H218O-qSIP, an environmental sample is incubated with 18O-water. Microorganisms that grow and replicate their DNA incorporate 18O into their nucleic acids resulting in DNA with higher densities than non-labeled DNA. The labeled DNA may be separated from non-labeled DNA along a cesium chloride gradient generated in an ultra-centrifuge. We show that DNA of microbial populations in the McMurdo Dry Valley was indeed labeled with 18O water indicating these microorganisms grew in the McMurdo Dry Valley soils. Especially members of the Proteobacteria, Actinobacteria and Bacteriodetes grew in our incubations. When we amended soil with salt only a third of the taxa that grew in control soils replicated cells. Soils amended with carbon or nitrogen supported growth of a similar number of taxa as control soils but their growth rates declined, suggesting that bacterial taxa will be negatively impacted by increases in soil nutrients.