In vitro studies of the impact of the naked soil amoeba Thecamoeba similis Greef, feeding on phototrophic soil biofilms

The effect of grazing by the naked soil amoeba Thecamoeba similis feeding on algae of in vitro phototrophic soil biofilms was investigated in terms of carbon biomass of algae, bacteria and amoebae and compared to shifts in the microbial community determined by 16S rDNA t-RFLP patterns. In a first approach, grazing of T. similis on unialgal Xanthonema hormidioides biofilms growing on regosol was investigated. The growth rate of T. similis was 0.5 divisions per day and the production efficiency reached 25% (allocation of prey-carbon to predator-carbon). The algal biomass decreased from the original 100% down to 7% at the end of the experiment. The increase of egested residues enhanced bacterial growth; the biomass carbon ratio bacteria vs. algae increased from 0.25 to 4.5 indicating the degradation of the algal biofilm, while the 16S rDNA t-RFLP patterns indicated changes in the composition of the microbial community. A subsequent experiment was conducted by inoculating T. similis on a natural biofilm community consisting of trichomes of cyanobacteria, several species of Chlorophyceae and Xanthophyceae. The total algal biomass (including cyanobacteria) remained constant during the experiment. However, 73% of the algal biomass without cyanobacteria was grazed, while the amount of mucous Chlamydocapsa spec. doubled from 32% to 68%. The growth rate of T. similis was 0.4 divisions per day and the production efficiency was only 13%. The 16S rDNA t-RFLP patterns showed a shift of the microbial community. Our results give a first insight into the obviously high potential of naked soil amoebae feeding on algae to change the structures of both the algal and bacterial communities of phototrophic soil biofilms.