(a) Addition of 0.1 mM tryptophan to P. inhibens cultures results in approximately 25-fold increase in produced IAA. Shown is an LC-MS extracted ion chromatogram (EIC, m/z 176.0706 ± 10 ppm) of an IAA standard of 10 μM (black), IAA detected in bacterial cultures supplemented with tryptophan (blue), and IAA detected in untreated bacterial cultures (red). (b) The addition of isotopically labeled tryptophan (13C and 15N) leads to bacterial production of IAA with full isotopic incorporation, indicated by m/z 187.0011. Inset shows the LC-MS chromatogram of an IAA standard (black) (EIC, m/z 176.0706 ± 10 ppm) and the labeled IAA detected in two biological replicates (blue) (EIC, m/z 187.0011 ± 10 ppm). (c) Cell density (OD600) after 16 hr at 30°C of a tryptophan auxotroph E. coli strain grown in known concentrations of tryptophan and in algal supernatant (see Materials and methods). Dashed line indicates density of the initial inoculum. Error bars represent the standard deviation between four biological replicates.