Fig. 1. Iron and copper dose-response curves, cellular quotas and steady-state uptake rates of AOA and AOB isolates, Nitrosopumilus maritimus and Nitrosococcus oceani.

Effect of (A) cupric ion concentration [Cu²⁺] (pmol L⁻¹) (B) unchelated Fe concentration (Fe´) on the relative growth rate (μ/μ_max) of the ammonia-oxidising bacterium Nitrosococcus oceani strain C-107 (open circles and dashed line) compared with previously published data on the ammonia-oxidising archaeon Nitrosopumilus maritimus strain SCM1^21,22 (closed squares and solid black line—error bars within points on N. maritimus data). Error bars on N. oceani points represent standard deviation calculated on n = 21 (three replicates were conducted for each treatment in a total of seven acclimated sub-cultures—calculated to be statistically indistinguishable, ANOVA, P < 0.01). Error bars on N. maritimus SCM1 data points in 1B) represent standard deviation on n = 20, as calculated previously.²³ (C) Intracellular Cu and Fe quota in mol cell⁻¹ normalised to mol P cell⁻¹ at maximum relative growth rate, μ_max, in N. oceani and N. maritimus (Cu quota calculated in this study, Fe quota calculated in ref. ²²) compared with published quota data of numerous marine phytoplankton (data and references provided in Supplementary Table). Fe:P and Cu:P at μ_max in N. oceani was measured at 5006 pmol L⁻¹ Fe´ and 0.07 pmol L⁻¹ Cu²⁺, respectively. Cu:P at μ_max in N. maritimus SCM1 was measured at 0.97 pmol L⁻¹ Cu²⁺. Additional Fe and Cu quota under varying [Fe´] and [Cu²⁺] is provided in Table 1 and Supplementary Fig. 1. See Supplementary Materials for further discussion on the use of quota data at μ_max. Error bars on N. oceani data represent standard deviation, n = 3. D log K_in, Fe´ steady state-uptake constant, in L cell⁻¹ h⁻¹ as a function of cellular surface area (μm³) in N. oceani (filled diamond) compared with published data of N. maritimus²² and other marine phytoplankton.⁴⁸ Error bars represent standard deviation, n = 6.