Figure 4.

The length scale of competition for incoming iron–siderophore complexes is much shorter than the length scale of synergistic production of siderophores. (a) Only the secreter produces siderophores, the non-secreter consumes iron-bound siderophores, but does not contribute to production. The relative increase in the average number of siderophores that need to be produced to take up one iron, relative to if the secreter was alone, depends on the distance between the cells. When the two cells are close, the amount of siderophores required increases significantly (orange area). However, when cells are at distances of greater than 30 µm, the secreter does not need to produce additional siderophores (black area). The non-secreter generally acquires equal amounts of iron as the secreter. (b) Both cells produce siderophores. Secreters can increase the uptake rate of a neighbouring secreter at no or little additional cost, provided that the distance between cells is large enough. For small particles, the presence of an additional cell decreases the number of required siderophores by half, thereby alleviating the disadvantage of siderophore uptake for small particle sizes compared to direct uptake. The benefit is slightly reduced for larger aggregates, but remains synergistic and augments the benefit of secretion compared to direct uptake. Here, κ_L = 10⁻⁶, P = 0.045 amol h⁻¹.