RNA is considered to be a short-lived molecule, indicative of cellular metabolic activity, whereas DNA is thoughtto turn over more slowly because living cells do not always grow and divide. To explore differences in the ratesof synthesis of these nucleic acids, we used H218O quantitative stable isotope probing (qSIP) to measure theincorporation of18O into 16S rRNA, the 16S rDNA,amoAmRNA and theamoAgene of soil Thaumarchaeota.Incorporation of18O into the thaumarchaealamoAmRNA pool was faster than into the 16S rRNA pool,suggesting that Thaumarchaea were metabolically active while using rRNA molecules that were likely synthe-tized prior to H218O addition. Assimilation rates of18O into 16S rDNA andamoAgenes were similar, which wasexpected because both genes are present in the same thaumarchaeal genome. The Thaumarchaea had sig-nificantly higher rRNA to rDNA ratios than bacteria, though the18O isotopic signature of thaumarchaeal rRNAwas lower than that of bacterial rRNA, further suggesting preservation of old non-labeled rRNA. Through qSIP ofsoil with H218O, we showed that18O incorporation into thaumarchaeal nucleic acids was generally low, in-dicating slower turnover rates compared to bacteria, and potentially suggesting thaumarchaeal capability forpreservation and efficient reuse of biomolecules
