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Appl Environ Microbiol. 1982 April; 43(4): 747-752

Interactions of Bacterial and Amoebal Populations in Soil Microcosms with Fluctuating Moisture Content

R. J. Bryant, L. E. Woods, D. C. Coleman, B. C. Fairbanks, J. F. McClellan and C. V. Cole^||

Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado 80523

ABSTRACT

Sterilized soil samples (20 g of soil per 50-ml flask), amended with 600 µg of glucose-carbon and 60 µg of NH₄-N · g of dry soil^–1, were inoculated with bacteria (Pseudomonas paucimobilis) alone or with bacteria and amoebae (Acanthamoeba polyphaga). We used wet-dry treatments, which involved air drying the samples to a moisture content of approximately 2% and remoistening the samples three times during the 83-day experiment. Control treatments were kept moist. In the absence of amoebae, bacterial populations were reduced by the first drying to about 60% of the moist control populations, but the third drying had no such effect. With amoebae present, bacterial numbers were not significantly affected by the dryings. Amoebal grazing reduced bacterial populations to 20 to 25% of the ungrazed bacterial populations in both moisture treatments. Encystment was an efficient survival mechanism for amoebae subjected to wet-dry cycles. The amoebal population was entirely encysted in dry soil, but the total number of amoebae was not affected by the three dryings. Growth efficiencies for amoebae feeding on bacteria were 0.33 and 0.39 for wet-dry and constantly moist treatments, respectively, results that compared well with those previously reported for Acanthamoeba spp.

Present address: High Plains Grassland/Station Research, U.S. Department of Agriculture, Cheyenne, WY 82001.

Present address: Department of Zoology and Entomology, Colorado State University, Fort Collins, CO 80523.

Present address: Agronomy Department, New Mexico State University, Las Cruces, NM 88003.

^|| Present address: Natural Resource Ecology Laboratory, U.S. Department of Agriculture, Colorado State University, Fort Collins, CO 80523.