Fig. 4. Phase portraits of Eqs. (14) from Supplementary Note 1 illustrating a gradual transition are shown for six distinct values of f₁, the influx rate of iron(II).

For values of f₁ = 17 (a), 12 (b), 11 (c), 10 (d), 9 (e), and 4 (f), the stable equilibrium (green dot) moves continuously from a world that is dominated by APB to one that is dominated by cyanobacteria. Parameter values are f₂ = 10, c = 1, a = 10, b = 12, u₁ = u₂ = 1, and α₁ = α₂ = 1. The GOE is gradual.