Extracellular soil DNA (eDNA): a driving force of microbial life in initial stages of soil development?

Proglacial areas represent unique model-systems for monitoring soil formation processes. We correlated biotic and abiotic factors of the Morteratsch Glacier forefield (Switzerland) to characterize initial stages of soil formation (0-150 yrs). To improve the sensitivity of the assessment of primary microbial succession along the chronosequence (2 km), routine molecular analysis of soil microbial community structures and biomass based on intracellular soil DNA (iDNA) were extended to the extracellular soil DNA fraction (eDNA). Soil eDNA has to be considered a driving force of microbial life due to its features as i) quantitatively relevant portion of the soil metagenome; ii) mobile component of the soil mobilome with evolutionary implications (horizontal gene transfer via natural transformation); iii) important component of soil biofilm (physical structure; soil metagenome stability); and iv) source of microbial nutrients. Comparative sequential extraction and DGGE fingerprinting of soil eDNA and iDNA provided evidences that eDNA is a quantitative and qualitative important fraction of the soil metagenome, containing information about microbial communities that were not revealed by iDNA-fingerprinting. As the actively extruded portion of eDNA by specific bacteria along with eDNA from lysed cells constitutes the ‘mobile soil mobilome’, our eDNA/iDNA approach is suggested to be capable to detect rare species as well. The eDNA/iDNA ratio was further evaluated as index of microbial activity (turn-over rate of microbial biomass), in comparison to principal soil enzyme activities, and to give insight into the concept of r/K strategy in microbial primary succession as function of proceeding pedogenesis.