The structured nature of microbial ecosystems makes their study difficult, and simple laboratory analogues are needed. Two gel-stabilized gradient systems are described in which solute transfer is by diffusion alone. In the first, organisms grow in a semi-solid agar gel located above a source layer of full-strength agar containing the diffusible solute, which was glucose in the experiments reported here. Changes in physicochemical parameters, various solutes and cell concentration have been monitored in cultures of NCTC 10342 and NCIB 4037 grown in this system. In experiments with a range of bacteria, banded growth was noted for several strains. The microbiology of the water at the base of an oil storage tank was investigated in the second model, in which gas oil was poured over a semi-solid layer containing agar, a basal salts medium, cells and a steel plate. After incubation for up to 90 d the system had differentiated into aerobic and anaerobic regions, and activities included hydrocarbon catabolism, oxygen removal, sulphate reduction, and the growth of a large population of aerobic and anaerobic heterotrophs. The value of these models is discussed with reference to microbial ecology.


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