Fig. 2.

(A) In our mathematical model, species $S_1$ and $S_2$ share a substrate containing nutrients and toxins at concentrations $C_N$ and $C_T$. The species take up the same nutrients, and invest a fraction of these into toxin degradation and the rest into population growth. Toxins cause cell death and population decline. (B) Example results of the model (parameters in SI Appendix, Table S3), shown as the abundance of species $S_1$ (solid line) and concentrations of nutrients and toxins (dashed and dotted lines, respectively). In monoculture, $S_1$ goes extinct due to toxins (Left), but survives in coculture with $S_2$ (Right). (C) The response of one species to the presence of another is measured as the difference in AUC between the coculture and monoculture (color and parameters in SI Appendix, Table S3) and shown as a function of nutrient and toxin concentrations. At high toxin concentrations and intermediate nutrients, interactions are positive (+ve) due to the joint degradation of toxins (as in B). As nutrients are increased or toxins decreased, competition for limited resources dominates (-ve, short for “negative”).