Essential and Substitutable Resources

Just as every resource can be consumed by different species, every species consumes different resources. Some of these resources are essential for growth, while others are substitutable. All chemical elements that are required for cellular growth, such as phosphorus, nitrogen, carbon, potassium, and iron are essential resources. It depends on the species, which chemical forms of these resources it can use. As a carbon source, for example, heterotrophic microorganisms like E. coli can use sugars like fructose. However, E. coli does not rely on fructose only. It can also live on many other sugars, such as maltose, ribose, and galactose. These different sugars therefore constitute substitutable resources for E. coli.

Figure 3 illustrates the difference between essential and substitutable resources. Here, the concentration of one resource is plotted on the x-axis, and the concentration of another resource on the j-axis. Different combinations of resource concentrations will often result in different growth rates. This is indicated by the so-called zero net growth isoclines (ZNGIs) of a species. A ZNGI represents the availability of two resources at which the population density of a species remains constant. For resource availabilities above and to the right of a ZNGI, the population density will increase. For resource availabilities below and to the left, the population density will decrease.

For essential resources, ZNGIs run parallel to both axes, forming a right-angle corner (Figure 3a). The exact position of the ZNGI is determined by the R* values. The example in Figure 3a depicts the ZNGI for two essential resources of a diatom: phosphorus, which is a major component of DNA and RNA, and silicate, which diatoms use to build their outer shells. For substitutable resources, such as two types of sugars, ZNGIs intersect both axes (Figure 3b). The example in Figure 3b depicts the ZNGI for two sugars representing two substitutable resources for E. coli.

phosphorus phosphorus

Silicate availability

silicate

Silicate availability

maltose

1 fructose

Fructose availability

Figure 3 Zero net growth isoclines (ZNGIs). Above and to the right of the ZNGIs, the net growth rate is positive. Below and to the left of the ZNGIs, net growth rate is negative. (a) Phosphorus and silicate, two essential resources for a diatom. (b) Maltose and fructose, two substitutable resources for E. coli.

1 fructose

Fructose availability

Figure 3 Zero net growth isoclines (ZNGIs). Above and to the right of the ZNGIs, the net growth rate is positive. Below and to the left of the ZNGIs, net growth rate is negative. (a) Phosphorus and silicate, two essential resources for a diatom. (b) Maltose and fructose, two substitutable resources for E. coli.

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