FIG 4.

Cross-feeding and its outcomes are dynamic. (A) A high level of cooperation by one partner can lead to excessive and harmful reciprocation by another. In this case, when production rate exceeds consumption rate (blue arrows), a metabolite (blue dot) can accumulate to toxic levels, for example by acidifying the environment (207). The “conc” triangle illustrates the effect of the metabolite on the recipient, ranging from beneficial (blue) to detrimental (red) as the concentration increases. (B) Growth-independent cross-feeding can rescue partners from starvation. Maintenance metabolism alone can lead to metabolite excretion under nongrowing conditions (left). Consumption of the metabolite by a recipient can stimulate recipient growth and reciprocation, creating a positive feedback loop and lifting both partners out of starvation (30). (C) The level of privatization influences the affinity that each partner must have for a communally valuable metabolite for cooperative coexistence to result (225). (Top) Cross-feeding of an intracellularly generated metabolite (left: high privatization) and an extracellularly generated metabolite liberated by an exoenzyme (right: low privatization). (Bottom) Simulated effect of the relative competition, in this case affinity (inverse of Km, which is the substrate concentration when the growth rate is at half the maximum), for the metabolite on the net growth of each partner under high and low privatization conditions. (Simulated trends are adapted from reference 225.)