DNA-based evidence for ancient microbial life in loess paleosols

Loess-paleosol sequences are terrestrial archives of past climatic fluctuations and may host ancient life specific for these different climatic phases. Knowledge about the recovery of microbial DNA from paleosols is rare. We here ask whether DNA can be found in loess and paleosol layers of up to 120,000 years of age, and whether it carries information that relates to past environments. We analyzed a loess-paleosol sequence near Toshan, Northern Iran, with 26 m thick deposits of loess and intercalating paleosols showing different degrees of soil development due to different climatic phases. Environmental DNA was extracted from paleosol and loess layers, followed by amplicon sequencing and qPCR of the 16S rRNA genes to investigate changes in microbial community composition and abundance. Highest diversity was found in modern surface soil. Paleosols showed higher diversity and bacterial abundance compared to the loess layers. The microbial community composition differed over the loess-paleosol sequence and differences were mainly correlated with age, though with low explanatory power,suggesting that specific microbial remains were indeed archived. In general, microbial communities hosted a large number and high diversity of Actinobacteria, especially in loess-containing layers, while Proteobacteria were found in higher relative abundance in paleosol horizons. Basically, different soil layers seem to host more specific communities and were usually most similar in the successive layers . This suggests that at least part of the microbial community in loess-paleosols originates from earlier times and has to some extent been preserved.