Responses of indigenous microorganisms to soil incubation as viewed by transmission electron microscopy of cell thin sections.

Air-dried soils were adjusted to 50% moisture-holding capacity and incubated for 2 weeks at 30 C. Samples were removed at intervals, and their total microbial populations were physically separated and concentrated from the soil debris for sectioning and ultrastructure examination. Although the total numbers of cell sections in these preparations remained relatively constant during the soil incubations, the percentages of dwarf cells (</=0.3 mum in diameter), minute cells (</=0.2 mum in diameter), and cells with a cystlike structure decreased with time followed by a slow increase. During this period, a corresponding increase and decrease occurred in the percentages of cells in the 0.3- to 0.5-mum diameter range, but dividing cells were rarely observed. The percentages of spores and of cells with electron-transparent areas also increased and then decreased during incubation. When nutrients were added to these soils, the initial increases in cell size occurred at what appeared to be a faster rate. But this probably was related to a corresponding increase in total cell numbers which also occurred. The responses of the spores, cystlike cells, and cells with electron-transparent areas to nutrient additions were not consistent although all conditions of incubation, regardless of nutrient addition, increased the occurrence of an enlarged diffuse intine-like layer for the cystlike cells. In addition to the above, incubated soils contained cells, which were mainly in the 0.3- to 0.5-mum cell diameter range, that had an internal membrane surrounding the general area of the nuclear material. Changes in additional fine structure features of the microbial populations are described.