SPATIAL ORDER IN MICROBIAL ECOSYSTEMS

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Abstract
Summary: (1) It is suggested that microbiologists are concentrating too much at present on homogeneous laboratory models such as the chemostat to simulate natural microbial ecosystems, which are not usually homogeneous but structured in time and space.(2) Natural microbial ecosystems that are spatially heterogeneous are reviewed and discussed.(3) Microbial interactions are briefly discussed in so far as they are relevant to spatially organized ecosystems.(4) Concepts such as ‘niche’ and ‘habitat’ are defined and discussed in terms of their applicability to microorganisms growing at specific points in solute gradient systems. The definitions currently accepted lack precision because they do not take into account the importance of spatial and temporal coordinates. Use of the words ‘compartment’ and ‘domain’ is advocated. The former corresponds to a cell, the latter to a region around it where the compartment has some influence, acting either as a source or as a sink for particular solutes. The terms ‘niche’ and ‘habitat’ are redefined for the activity domains of a compartment and the habitat domains of the ecosystem. The importance of the vectorial flow of solutes is stressed.(5) Current methodology in microbial ecology is briefly reviewed.(6) Methods specifically designed for investigating heterogeneous ecosystems are described, These include: (i) The gradostat, a system of interlinked culture vessels allowing the establishment of steady‐state bidirectional solute gradients. (ii) Percolating columns. (iii) Capillary methods. (iv.) The thin film fermenter. (v) The use of two‐dimensional diffusion plates or stopped time‐dependent gradient plates. (vi) Gel‐stabilized diffusion models established vertically in glass containers such as beakers. (vii) The use of one‐dimensional gradient‐plate techniques to determine the habitat characteristics of different microbial species. (viii) Investigations into naturally structured systems such as the bacterial colony.

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