The influence of environmental factors on protistan microorganisms in grassland soils along a land-use gradient.

In this study, we investigated the effect of land use intensity, soil parameters and vegetation on protistan communities in grassland soils. We performed qualitative (T-RFLP) and quantitative (qPCR) analyses using primers specifically targeting the 18S rRNA gene for all Eukarya and for two common flagellate groups, i.e. the Chrysophyceae and the Kinetoplastea. Both approaches were applied to extracted soil DNA and RNA, in order to distinguish between the potentially active protists (i.e. RNA pool) and the total protistan communities, including potentially inactive and encysted cells (i.e. DNA pool). Several environmental determinants such as site, soil parameters and vegetation had an impact on the T-RFLP community profiles and the abundance of the quantified 18S rRNA genes. Correlating factors often differed between quantitative (qPCR) and qualitative (T-RFLP) approaches. For instance the Chrysophyceae/Eukarya 18S rDNA ratio as determined by qPCR correlated with the C/N ratio, whereas the community composition based on T-RLFP analysis was not affected indicating that both methods taken together provide a more complete picture of the parameters driving protist diversity. Moreover, distinct T-RFs were obtained, which could serve as potential indicators for either active organisms or environmental conditions like water content. While site was the main determinant across all investigated exploratories, land use seemed to be of minor importance for structuring protist communities. The impact of other parameters differed between the target groups, e.g. Kinetoplastea reacted on changes to water content on all sites, whereas Chrysophyceae were only affected in the Schorfheide. Finally, in most cases different responses were observed on RNA- and DNA-level, respectively. Vegetation for instance influenced the two flagellate groups only at the DNA-level across all sites. Future studies should thus include different protistan groups and also distinguish between active and inactive cells, in order to reveal causal shifts in community composition and abundance in agriculturally used systems.