Microenvironments of soil microorganisms

Summary Ultrastructural studies of soil micro-organisms and the microenvironments surrounding them are reviewed. Soil microfauna, and bacteria, actinomycetes and fungi, fixed and embedded in situ, were examined by electron microscopy (both transmission and scanning). In some cases ultrastructural histochemistry was used to detect and identify the organic matter with which microorganisms were associated and to examine the polymeric microbial materials (enzymes, extracellular polysaccharides) they produced. Although some small organisms (0.3 m diameter) occurred singly in dense fabrics of clay or humified organic matter, larger bacteria occurred in rhizospheres, in small colonies in the larger micropores or associated with substantial deposits of organic matter (faecal pellets, carbohydrate-rich plant cell-wall debris). Whereas rhizospheres had mixed microbial populations, individual microvoids in the bulk soil usually contained only one type of micro-organism. Following chloroform treatment, microorganisms were found only in mucigel deposits or deep in the interiors of micropores, suggesting that these constitute protected sites where microorganisms survive temporarily adverse conditions. Soil microfauna and fungi were mainly confined to the larger voids. Although some live hyphae occurred in the outer regions of aggregates, hyphae deep within soil fabrics were usually devoid of cytoplasmic organelles. Faecal pellets, plant tissues and cell-wall remnants comprised the most frequent, larger organic masses, while the most common micron- and submicron-sized organic matter consisted of fibrous or amorphous humified matter. Unequivocal detection of enzymes was limited to the surface of microorganisms.