Irrigated rice field soil is subjected to frequent changes in oxygen status due to the water regime by agricultural management. In this study, the community response of microeukaryotes in rice field soil to the oxygen status was explored in a microcosm experiment under defined conditions. Water-saturated soil was incubated under a two-level factorial design of oxygen and organic enrichment with plant residue. The eukaryotic microbial community composition, which was either present or potentially active in the soils, was analyzed using denaturing gradient gel electrophoresis (DGGE) targeting the 18S rRNA gene or reverse-transcribed 18S rRNA. Oxygen availability was a primary factor shaping the microeukaryotic community in both DNA- and RNA-based analyses, revealing a shift within a week of incubation. Plant residue also affected the microeukaryotic community, which was more notable in the active community showing rRNA expression with time. Sequences of amplicons in DGGE bands indicated that protozoa (ciliates, flagellates, and amoebae) were the most prominent microeukaryotes in water-saturated rice field soil both in DNA- and RNA-based analyses. The use of a modified primer for soil protozoa suggested the functional importance of Heterolobosea amoeba in rice field soil, particularly in anoxic soil with organic enrichment.

No abstract

Heterotrophic protists in soil are grazers that control the biomass and community structure of bacteria, thereby enhancing nutrient recycling. Oxygen regulates the microeukaryotic community, but little is known about its response to microoxic conditions. Here we studied the impact of oxygen tension on culturable heterotrophic protists in a rice field soil. The number of protists, dominated by amoeba and flagellates, under oxygen tensions ranging from atmospheric level (21%) to below the Pasteur point (0.08%) were similar (10(4) cells g(-1) dry soil); no protists were detected under anoxic conditions. DGGE fingerprinting of microeukaryotes demonstrated a shift in the community structure depending on the oxygen tension during growth. Both common and specific amoeba and flagellates were identified at different oxygen tensions. Amoeba isolates (Acanthamoeba sp. and Hartmannella sp.) grew to the same extent under the oxygen tensions tested; the Acanthamoeba sp. isolate migrated more slowly under the lowest tension (0.08%). Our results demonstrated that amoeba and flagellates in soil adapt to a wide range of oxygen tensions with a shift in community structure. This suggests an ability to search for food in soil environments such as the oxic-anoxic interface of flooded soil or inside soil aggregates that are inaccessible to ciliates.