Fig. 4: The response of the ammonia oxidation rates of two ammonia oxidizer lineages to acidification simulated by three models.

a Concept model graph of substrate availability reduction effect (S Avail). b Substrate affinity increase effect (S Affin). c–d Substrate availability reduction effect combined with substrate affinity increase effect (S Avail + S Affin). e Relative importance of substrate availability reduction effect and substrate affinity increase effect across different stations or cultures (see Methods). f The ratio of the substrate affinity increase effect to the substrate availability reduction effect in ammonia-oxidizing bacteria (AOB)-dominated and ammonia-oxidizing archaea (AOA)-dominated regions. A two-tailed t-test is used to examine whether there is a significant difference between AOB-dominated and AOA-dominated regions. For the AOB-dominated group, data are shown as scatter dot plots with the center line depicting the mean (n = 3 biologically independent samples). For the AOA-dominated group, boxplots depict the 25–75% quantile range, with the center line representing the median (50% quantile); whiskers extend to the minimum and maximum values (n = 9 biologically independent samples). In AOB-dominated regions, reduced substrate availability exerts a major influence, while in AOA-dominated regions, increased substrate affinity plays a more significant role.