Figure 1.

Hypothesis for how substrate concentration and toxicity govern the structure of the community engaged in metabolic division of labor (MDOL). In a community degrading an organic compound through MDOL, the final product was assumed to be the sole carbon source and was synthesized by the strain performing the second step. Therefore, this strain will obtain more nutrients, while the other strain has to collect products released from this population. Thus, the last population was named “Embezzler” (denoted as the red cells). (A) Increasing the concentration of the substrate improves the flux of the pathway. Since the product consumption ability of Embezzler cells is limited, increasing the concentration will lead to higher final product leakiness, favoring the growth of the first population. (B) Introducing substrate biotoxicity also favors the first population because it converts this toxic substrate (denoted as skull and bones), resulting in lower intracellular substrate concentration compared to that of the Embezzler cells. Thus, the first population was named “Detoxifier.”