Influence of Soil Treatment on Microbial Community Structure as Determined by Quantifying 16S rRNA

Buckley, D.H. and T. M. Schmidt

Presented at the All Scientist Meeting (1998-07-21 to 1998-07-22 )

Most knowledge about the importance of microbial activities in soil has been obtained through study of microbially-mediated processes, and has provided little information on the distribution, relative abundance, and diversity of prokaryotic populations in soil. Molecular approaches to microbial ecology, such as those utilizing ribosomal RNA molecules, enable the study of microbes as they occur in soil and are beginning to provide insight on the structure of soil microbial communities. The 16S rRNA is an evolutionarily constrained molecule, allowing for the design of oligonucleotide probes specific for phylogenetically related organisms. In this study RNA is being extracted directly from soil and analyzed using rRNA specific oligonucleotide probes to determine the contribution of population-specific rRNA to total prokaryotic community rRNA. The microbial populations being analyzed in this study are the Alpha, Beta and Gamma proteobacteria, the High G+C Gram positive bacteria, and the cytophaga like bacteria. In addition, the overall contribution of Bacterial and Eukaryal rRNA to community rRNA is being determined.The objective of the proposed research is to characterize the distribution and relative activity of the above microbial populations in soils associated with varying plant communities and agricultural treatment regimes. Currently, three replicate plots representing treatments 1, 2, 4, 6, 7, and 8 have been sampled by pooling 5 – 2.5 cm x 10 cm soil cores from each plot. For samples taken in October 1996 the observed rRNA abundances have revealed no detectable differences in rRNA pool sizes associated with populations analyzed in the LTER treatments 1, 2, 4, 6, and 7. In contrast, the rRNA abundances of the alpha and beta proteobacteria, high G+C Gram + bacteria, total Bacteria, and total Eukarya were significantly higher in treatment 8 when compared to all other treatments (Fig. 1). No detectable difference was observed in the rRNA abundance of the gamma proteobacteria in treatment 8 relative to other treatments. No data is currently available for the cytophaga. The populations measured account for approximately 50% of the rRNA present in treatments 1, 2, 4, 6, 7 and 75% of the rRNA in treatment 8. These data tend to support the hypothesis that historical land management practices can influence microbial community structure for many years after a change in land usage. We are currently in the process of taking additional samples from the KBS LTER and also from old field successional sites to confirm and expand our current observations.Return to Contents

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