Autochthony And Zymogeny

Identifying a soil bacterium as a chemoheterotroph, according to the classification scheme for metabolism outlined above, provides little information on how competitive that particular bacterium will be under particular conditions of substrate supply. The great 19th century soil microbiologist Winogradsky addressed this issue through reference to the comparative kinetics of growth, which relates substrate concentration to specific growth rate. In Fig. 5.8, which illustrates some simple and contrasting growth curves, population X (diamonds) will outcompete populations Y and Z at high substrate concentrations. At intermediate substrate concentrations, population Y (circles) will outcompete populations X and Z. At low substrate concentrations, however, population Z (triangles) will outcompete the other populations. Soil bacteria that exhibit the growth kinetics of population X, with a relatively high maximum specific growth rate (^max) and substrate affinity (Ks), will be more competitive at high substrate concentrations and are termed "zymogenous." Not surprisingly, soil microenvironments such as the early rhizos-phere are dominated by zymogenous bacteria, such as fluorescent pseudomonads, which grow rapidly on the simple C substrates (primarily glucose). Soil bacteria that exhibit the growth kinetics of population Z, with a low ^max but relatively low Ks (i.e., a high substrate affinity), are termed "autochthonous." The bacterial populations found in some of the less accessible soil microenvironments (e.g., the smaller pores inside soil aggregates), where substrate C flow is rarely more than a trickle, are generally dominated by autochthonous bacteria. The spatial variability of soil with regard to microbial populations highlights the importance of substrate (and nutrient) availability (sometimes referred to as bioaccessibility) as a driver of both the diversity and the function of the bacterial community. The latter can be demonstrated by studying the mineralization of differentially located (in terms of soil pore size class), radiolabeled C substrates (Killham et al., 1993).

Of course, in reality, there are degrees of autochthony and zymogeny, as indicated by the growth kinetics of the populations in Fig. 5.8. There is a continuum of growth kinetics that ensures that the most competitive soil bacteria will change with substrate concentration. Successions will therefore often occur in environments

Substrate concentration FIGURE 5.8 Comparative Monod kinetics used to assess substrate competition for growth between soil microbial populations.

such as the rhizosphere, where C flow changes, although the picture is further complicated by the proliferation of substrates with varying recalcitrance, enzyme specificity, and availability.

OLIGOTROPHY, COPIOTROPHY, AND THE R-K CONTINUUM

The physiologically based autochthony-zymogeny classification pioneered by Winogradsky is often considered analogous to the r-K continuum commonly used in plant and animal ecology, based on logistic models, and to the scheme of oligotrophy-copiotrophy. The K strategists and oligotrophic bacteria are adapted to growth under conditions of C/nutrient starvation ("oligocarbotrophy" specifies C starvation, while the terms "oligonitrotrophy," "oligophosphotrophy," etc., specify the type of nutrient starvation). Copiotrophs are adapted to nutrient excess. Although the two schemes are used in similar ways, the contrast in physiological versus logistic approaches does distinguish them. Furthermore, oligotrophy is also considered to include unusual forms of C (and nutrient) scavenging, such as exploiting gaseous C sources such as CO2 (e.g., through anapleurotic CO2 fixation) and volatile organic acids.

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