We first discuss simple single species growth of a chemolithoautotroph to illustrate the interactions between nutrients and substrates in growth. We show how the macrochemical reaction equation with variable yield coefficients can be decomposed in a number of subprocesses with constant yield coefficients.
We then discuss a simple predator-prey system, where nutrients are accumulated in the prey, which no longer have a constant composi- tion of biomass. The implication is a varying conversion efficiency from prey to predator, with consequences for qualitative aspects of population dynamics. We illustrate the principles with a grazer (a heterotrophic consumers) feeding on algae (autotrophic producers). The algae frequently have an excess of energy in the form of carbohydrates, which are excreted and serve as food supplements for the heterotroph.
This exchange of carbohydrates against nutrients is basic to a symbiosis, where the algae are no longer grazed as long as the grazer is able to extract nutrients from other sources. Depending on parameter values of the system, the co-existence can be very stable, and further integrate into a single entity with mixotrophic properties. This process is basic to the evolution of the eukaryotic cell, and is basic to the organizational structure of metabolism.
Mixotrophs can specialize under particular environmental conditions into autotrophs or heterotrophs, which again can associate in symbiontic relationships. The chemical composition of membranes testify from the frequent occurrence of this process, which can now also be understood quantitatively.