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Applied and Environmental Microbiology, August 2006, p. 5436-5444, Vol. 72, No. 8
0099-2240/06/$08.00+0 doi:10.1128/AEM.00207-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Impact of Protists on the Activity and Structure of the Bacterial Community in a Rice Field Soil
Jun Murase,1,2
Matthias Noll,2,3 and
Peter Frenzel2*
Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 464-8601, Japan,1 Max Planck Institute for Terrestrial Microbiology, Karl-von-Frisch Strasse, Marburg D-35043, Germany,2 Soil Biology, Institute of Terrestrial Ecology, Swiss Federal Institute of Technology (ETH-Zürich), Universitätsstrasse 16, CH-8092 Zürich, Switzerland3
Received 26 January 2006/ Accepted 31 May 2006
Flooded rice fields have become a model system for the study of soil microbial ecology. In Italian rice fields, in particular, aspects from biogeochemistry to molecular ecology have been studied, but the impact of protistan grazing on the structure and function of the prokaryotic community has not been examined yet. We compared an untreated control soil with a 
-radiation-sterilized soil that had been reinoculated with a natural bacterial assemblage. In order to verify that the observed effects were due to protistan grazing and did not result from sterilization, we set up a third set of microcosms containing sterilized soil that had been reinoculated with natural assemblage bacteria plus protists. The spatial and temporal changes in the protistan and prokaryotic communities were examined by denaturing gradient gel electrophoresis (DGGE) and terminal restriction fragment length polymorphism (T-RFLP) analysis, respectively, both based on the small-subunit gene. Sequences retrieved from DGGE bands were preferentially affiliated with Cercozoa and other bacteriovorous flagellates. Without protists, the level of total DNA increased with incubation time, indicating that the level of the microbial biomass was elevated. Betaproteobacteria were preferentially preyed upon, while low-G+C-content gram-positive bacteria became more dominant under grazing pressure. The bacterial diversity detectable by T-RFLP analysis was greater in the presence of protists. The level of extractable NH4+ was lower and the level of extractable SO42– was higher without protists, indicating that nitrogen mineralization and SO42– reduction were stimulated by protists. Most of these effects were more obvious in the partially oxic surface layer (0 to 3 mm), but they could also be detected in the anoxic subsurface layer (10 to 13 mm). Our observations fit well into the overall framework developed for protistan grazing, but with some modifications pertinent to the wetland situation: O2 was a major control, and O2 availability may have limited directly and indirectly the development of protists. Although detectable in the lower anoxic layer, grazing effects were much more obvious in the partially oxic surface layer.
* Corresponding author. Mailing address: Max Planck Institute for Terrestrial Microbiology, Karl-von-Frisch Strasse, Marburg D-35043, Germany. Phone: 49-6421-1787820. Fax: 49-6421-178809. E-mail: frenzel{at}mpi-marburg.mpg.de.
Supplemental material for this article may be found at http://aem.asm.org/.
Applied and Environmental Microbiology, August 2006, p. 5436-5444, Vol. 72, No. 8
0099-2240/06/$08.00+0 doi:10.1128/AEM.00207-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
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