"Positive effects of bacteria–protozoa interactions on plant growth are well documented (Bonkowski 2004) and recent findings strongly indicated that grazing induced shifts in bacterial diversity and function are responsible for plant growth promoting effects of bacterial grazers (Bonkowski and Brandt, 2002; Kreuzer et al., 2006; Mao et al., 2007). In fact, shoot and root biomass of A. thaliana increased significantly in presence of amoebae and the early growth response of plants was not linked to increased nutrient availability from consumed bacterial biomass (Krome et al., 2009). Our results confirm that grazing-induced changes in bacterial community composition are strongly interlinked with protozoan effects on plant growth. These findings have important implications for the success of applied studies, such as plant inoculations with growth-promoting bacterial strains.
In conclusion, protozoan grazing rapidly and significantly affected the diversity, activity and function of rhizosphere bacteria. Dominant bacterial groups were reduced, marginal groups gained competitive advantage, leading to greater evenness of grazed communities. However, the treatment-specific banding pattern in DGGE gels indicates that distinct mechanisms based on specific feeding preferences and competitive outcomes structured bacterial community composition in a well-defined way, despite bacterial communities were highly diverse. Our model system has been shown to warrant standardized experimental conditions to further investigate the mechanisms responsible for structuring of bacterial communities and its coupling to plant growth promotion by protozoa. Undoubtedly, protozoa need to be considered an important structuring force in investigations on plant–microbial interactions."