Population dynamics and ecology of ciliates (Protozoa, Ciliophora) in an anoxic rice field soil

Abstract

Wetland rice fields cover 1.46 million km² globally; the flooded soil of these fields is largely anoxic. While biogeochemistry and microbiology have been studied in detail, the microbial loop and especially the dynamics and function of ciliates are largely unknown. We used anoxic microcosms prepared with soil from an Italian rice field and recorded species composition, abundance and volume of ciliates together with numbers, volume and size distribution of bacteria. Ciliates were the dominating protists observed in the microcosms, but could be outnumbered by flagellates if the soil was amended with rice straw. The number of ciliate taxa was 23. Metopus species were dominant, but 16 of the species recorded in the anoxic soil were facultative anaerobes. Another 29 species were found in accompanying experiments that included the oxic soil surface. Total abundance in the anoxic soil was on average 110 cells g⁻¹ dry weight and comparable to that of other soils. The population of ciliates declined around 30 days after flooding, but recovered later. The period before the population declined was characterized by a rapid species turnover, many facultative anaerobes and large species. After recovery, the average cell size was much smaller, but even then a facultative anaerobe, Plagiacampa pentadactyla , was common. About 90% of all species were bacteriovores while the others—mainly Gymnostomatidae—were predators. Grazing ciliates may have controlled bacteria during the first 5 days after flooding, as could be shown by a negative correlation between the respective volumes and by the size spectra of the bacteria.