Notes on protozoa in agricultural soil with emphasis on heterotrophic flagellates and naked amoebae and their ecology

Abstract

Heterotrophic flagellates and naked amoebae are usually very numerous in agricultural soils; with numbers in the magnitude of 10,000 to 100,000 (active+encysted) cells per gram of soil. In 'hotspots' influenced by living roots or by dead organic material, the number may occasionally be as high as several millions per gram of soil. An exact enumeration of these organisms is virtually impossible. As they most often adhere closely to the soil particles, direct counting will underestimate numbers since the organisms will be masked. The method usually applied for enumeration of these organisms, the 'most probable number (MPN) method', is based on the ability of the organisms to grow on particular culture media. This method will in many cases underestimate the total protozoan number (active+encysted). It is uncertain how many of the heterotrophic flagellates and naked amoebae are actively moving and how many are encysted at a particular time; the 'HCl-method' which has usually been used to discriminate between active and encysted has proven to be highly unreliable. Despite the methodological difficulties many investigations of these organisms indicate that they play an important role in agricultural soils as bacterial consumers, and to a minor extent as consumers of fungi. Because of their small size and their flexible body they are able to graze bacteria in small pores in the soil in which larger organisms are precluded from coming. Key factors restricting the number and activity of heterotrophic flagellates and naked amoebae in soils seem to be water potential and soil structure and texture. In micro-cosm experiments, small heterotrophic flagellates and naked amoebae regulate the size and composition of the bacterial community. Bacterial activity seems to be stimulated by these organisms in most cases as well as the mineralization of carbon and nitrogen and possibly other mineral nutrients. In the rhizosphere of living plants the activity of protozoa has proven to stimulate uptake of nitrogen in pot experiments, and it has been hypothesized that organic matter liberated by plants in the root zone will stimulate bacterial and protozoan activity, leading to mineralization of organic soil nitrogen which is subsequently taken up by the plants.

Application of the denaturing gradient gel electrophoresis (DGGE) technique as an efficient diagnostic tool for ciliate communities in soil

“…Protist numbers in the unpolluted control soil (Lara et al, 2007b) were reported to be in general at the lower end of what is usually found in soils (Ekelund and Rønn, 1994), which we attribute to the low organic carbon content (0.04%) in the control soil. However, we cannot exclude that a second round of PCR amplification may increase differential amplification in complex template mixtures and thus influence the abundance of a sequence type in a DGGE gel (Suzuki and Giovannoni, 1996).…”

Microbial-faunal interactions in the rhizosphere and effects on plant growth

“…the ratio of total microbial biomass to basal respiration of the active biomass) has been found to be increased [2,10,79]. Protozoa are considered to be effective bacterial predators because of their high turnover rates and high numbers in rhizosphere soil [34,42,135]. Thus, concomitant with an increase in microbial turnover in the presence of microfauna [2,31,32] grazing on microflora may result in a decrease in bacterial numbers.…”

The community structure of soil Sarcodina in Baiyun Mountain, Guangzhou, China

“…Between them, the architecture of the habitable soil pore network which is determined by soil texture and structure, and the moisture in the soil have considered as the two most important factors in the regulation of soil protozoan populations. Their interaction establishes the basic environmental surrounding for soil sarcodina [12]. However, little research has been reported about the effect of multi-physicochemical-factors on sarcodina abundance, though the effects of single-physicochemical-factors of soil parameters such as soil moisture, pH, OM and nutrition salt (TN, TP and TK) on sarcodina abundance have been reported to some extent [20,24,26].…”

“…This accords with other terrestrial regions of the world [4,13,22,28], though their dominant species were different. Ekelund and Rønn [12] considered that testate amoebae are K-selected; whereas, most other Protozoa (smaller naked amoebae and flagellates for example) are r-selected, especially the groups that feed on bacteria. This Kselection may enable testate amoebae to adapt to the permanent drought climate of general terrestrial soils and helps to explain why testate amoebae are the first dominant group of the sarcodina in our analyses.…”