During the last two or three decades, microbial ecologists have concentrated on quantifying numbers (individual cells or colony forming units) and/or biomass in natural environments, as a measure of 'how much is there'. However, such measures provided little indication of what the microorganisms are doing [1], nor of how the biomass is spatially distributed. Microorganisms are able to deploy the same amount of biomass in different ways, depending on species, resources available and other external environmental factors (see Sections 8.3-8.5).

Many microorganisms (including bacteria and fungi) have modular body forms. When new modules are formed, e.g. by fission or budding, the modules commonly remain attached to the body from which they were formed, but sometimes they become detached and are able to function as separate physiological units [2]. Effectively, they have iterative, indeterminate growth patterns, tend to branch, and are sessile except when in dispersal phases. Such modular organisms contrast with unitary organisms, such as higher animals that generally exhibit noniterative, determinate growth patterns, are nonbranched and are motile. Since modular organisms have simple organisation, tend to lack motility at some stages of their life, and often have external digestion, they are vulnerable to environmental changes. However, they commonly have considerable phenotypic plasticity, i.e. genotype can be expressed differently under different environmental conditions. Thus, microorganisms often exhibit different physiologies and/or morphologies under different environmental regimes. Fungi exhibit considerable morphological and physiological plasticity (Section 8.3.2 and 8.5). Bacteria, on the other hand, are physiologically very plastic, but less so morphologically, though colony characteristics can sometimes change

Biophysical Chemistry of Fractal Structures and Processes in Environmental Systems Edited by Nicola Senesi and Kevin J. Wilkinson © 2008, IUPAC

(Sections 8.3.1 and 8.4). Arguments have been put forward that bacterial colonies are not self-similar [3], but, like many naturally irregular structures, many modular organisms, including fungi and some bacteria under some conditions, are often approximately fractal. Colonies are, however, self-similar only over a finite range of length scales.

Fractal geometry has been used to describe quantitatively inter- and intra-specific differences, within-colony differences, and environmentally induced differences in the morphology of bacteria and fungi (Sections 8.3-8.5). Since microbial morphology is most easily seen on artificial media, many studies have been performed in solid agar culture (Section 8.3), but this reflects the real world poorly, and studies are increasingly being undertaken in more realistic microcosms and even field situations (Sections 8.4 and 8.5). The vast majority of these studies have concerned fungi; hence, these are emphasised, though the work on bacteria is also summarised. Some natural environments are themselves fractal, e.g. soil, and this affects the diversity, density and distribution of the inhabitant microorganisms (Section 8.6). Further, a range of mathematical models have been developed to explain fractal growth of microbial colonies [e.g. 4-8], but consideration of this topic is beyond the scope of this chapter.

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