Fig. 1

Strains used in this study. Colored arrows indicate the metabolic processes that are performed by each strain. The colors of the arrows indicate whether each strain carries the egfp or echerry fluorescent protein-encoding gene. Note that we performed the evolution experiment for the nitrite (NO₂⁻) cross-feeding strains with both combinations of fluorescent proteins (nitrite producing strain carrying egfp together with the nitrite reducing strain carrying echerry and vice versa). This controlled for any confounding factors that might have emerged due to differences in the metabolic costs associated with expressing the different fluorescent protein-encoding genes. Definitions: Nar, nitrate (NO₃⁻) reductase encoded by the nar gene cluster; Nir, nitrite reductase encoded by the nir gene cluster; Nor, nitric oxide (NO) reductase encoded by the nor gene cluster; Nos, nitrous oxide (N₂O) reductase encoded by the nos gene cluster