Fig. 2: Substrate kinetic parameters towards ammonia (NH3) at varying pH.
From: Ammonia oxidizers offset acidification stress via adaptive substrate affinity in aquatic ecosystems
a–g The dependence of ammonia oxidation rates on NH3 concentration at varying pH. The station or strain names are labeled at the top of the graph, with the salinity values indicated next to their respective names. The legend indicates the degree of pH decrease and the pH value at this level. Solid lines in (a–g) represent the best fitting of Michaelis–Menten model or Haldane substrate inhibition model at various pH. The shaded area represents the 95% confidence interval. h–j Normalized changes in kinetic parameters towards NH3 per 0.1-unit pH drop (Vmax represents the maximum rate, Km represents the half-saturation constant, and α represents the substrate affinity). Due to failure to fit the equation under in-situ pH at SM station and pH−0.192 at JRE1 station, parts of kinetic parameters are missing, and the result of pH−0.400 at the SM station is the change between two pH downregulated groups (pH−0.419 and pH−0.819). * means the data related to ammonia-oxidizing bacteria are obtained from published studies on Nitrosomonas europaea, conducted under conditions with pH values ranging from 6.5 to 8.514,15,31. Data are presented as mean ± standard deviation. Errors bars in (a–g) are standard deviation from triplicate incubations (n = 3 biologically independent samples). Data in (h–j) except AOB group, represent parameter estimates calculated from the fits in panels (a–g), error bars indicate propagated standard deviations (derived from the fit uncertainty). For AOB group in panels (h–j), errors bars are obtained from replicate incubations (n = 6 in (h), n = 9 in (i), and n = 6 in (j) biologically independent samples), and individual data points are shown. If error bars are not visible, they are smaller than the size of the data points. / denotes no data. At a given pH, the rates with NH3 as the substrate follow the Michaelis-Menten model or the Haldane substrate inhibition model, and substrate kinetic parameters, including the Vmax(NH3), Km(NH3), and α(NH3), show clear pH dependence.
