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Within soil ecosystems, rice field soil is one of the most intensively sudied systems and has become a well-understood model ecosystem especially for biogeochemical processes and micobial populations. The influence of other important soil organisms, e.g. protozoa, has been widely neglected. To estimate the impact of those organisms towards the biogeochemistry and microbial community (bacteria and archaea) in anoxic rice field soils, we have studied the taxon Ciliophora DOFLEIN, 1904, as an abundant and frequently observed protozoan group
In an Italian rice field soil, 52 taxa were found. Additionally, water saturated (but not flooded) rice field soil was anoxically incubated for at least 160 days. In these anoxic incubations, we found 23 taxa. Only seven of these were known as obligate anaerobes. The other 16 taxa were classified as facultative anaerobes. These finding indicates that tolerance against anoxic conditions is much wider distributed then assumed before. From 34 up to 190 active cells g Dry Weight soil-1 were found. Members of the obligate anaerobic genus Metopus were dominating the ciliate population all the time by cell number and volume.
We found differences in the bacterial biovolume patterns at two time points (day 4 and day 80). At day 4, besides medium sized cells, we found smallest and biggest cell volumes (from 0.25 to 2.25 µm3) which were known as an effect of protistan grazing from aquatic environments. At 80 day, only medium sized cells were found (volume of 0.75 to 1.5 µm3). However, these results were confirmed by microbial and ciliate biovolumes, which were negatively correlated within the first 4 days but not thereafter.
By Fluorescence in-situ Hybridization (FISH) we were able to show a preferential ingestion of bacteria. Archaeal cells were found occasionally inside phagosomes. In addition to methanogenic endosymbionts of genus Metopus, we identified another endosymbiont affiliated to ?Proteobacteria.
In anoxic incubations, methane production became detectable within the first day. One mightn hypothesis that the early onset of methanogenesis were drifen by methanogenic endosymbionts. The potential endosymbiont-based methane production was calculated. One model was based upon values from the literature (Fenchel, T. and Finlay, B.J. 1995: Ecology and evolution in anoxic worlds, Oxford University Press) and explained about 59% within the first two weeks. The second model was based on direct cell counts and explained about 2% of methane production within the first two weeks. The second model was preferred due to the fact that it was based upon most rice field soil specific parameter. To prove this result eukaryotes in anoxic soil were inhibited via specific inhibitors. Surprisingly, this attemp induced a pronounced increase in methane production. The additional produced methane could not been explained by the introduced C (inhibitor) nor by a potential lysis of ciliate biomass.
All data indicate that ciliates from anoxic rice field soil incubations have a clear but time-limited influence onto the microbial community and especially on methanogenesis.