Protozoan predation of bacterial cells in soil aggregates

The development and survival of Aerobacter aerogenes IAM11022 in the inner and outer zones of soil aggregates (1–2 mm) was investigated in relation to a protozoan ( Colpoda sp.). With different dilutions of the bacterial cell suspension, a constant partition ratio of these cells was observed between outer and inner zones of the aggregates. Protozoa inoculated in the same manner were generally recovered only from the outer zone of the aggregates. In the presence of protozoa, prey cell numbers of the outer zone were reduced from more than 10 8 to approx. 10 4 cells · g soil −1 in 12 days. In contrast, 10 8 cells · g soil −1 remained in the inner zone of the aggregates, even after 12 days. The increase in predator cell number was proportional to initial prey densities in the outer zone of the aggregates. At a constant initial prey density (1.8 × 10 7 cells · g soil −1 ), Colpoda sp. multiplied in proportion to the initial number of predators. When the initial density of the predator was low more prey cells survived in the outer zone. Prey persistence was associated with 3 different types of protection: (1) small pore necks of the inner zone space of the aggregates; (2) the division of the outer zone space into compartments; and (3) the distribution of protozoan cells among soil aggregates. The latter two were closely related to the moisture condition of the soil.