Fig. 1 

Growth-coupling controls production heterogeneity by restricting the evolutionary landscape. In an uncoupled strain, a trade-off is present between biomass growth and bioproduction for nutrient allocation. With natural selection, the overall direction of its evolutionary trajectory (arrows on teal graph) is towards the maximisation of specific growth rate (μ) at the expense of specific productivity (q_P). In a simplified scenario where a producer (teal) mutates into a non-producer (dark red) with a specified probability, producers with a lower μ swiftly becomes dominated by non-producers (after time = t1 on the rightmost graph (solid lines), assuming no nutrient limitation). With growth-coupling, bioproduction is a necessary outcome when nutrients are used for biomass growth. This restricts the evolutionary landscape of the strain (purple graph); though the overall direction of its evolutionary trajectory is the same, productivity remains at the envisioned endpoint (gold concentric circles). Non-producers in the simplified scenario are no longer viable (purple cross), rendering the mutation event abortive to prevent their dominance (dashed dark red line of rightmost graph)