Cadmium toxicity on communities of ammonia-oxidizing microorganisms

The oxidation of ammonia to nitrite, which constitutes the initial and rate-limiting step in the nitrification process, plays a pivotal role in the transformation of ammonia within soil ecosystems. Due to its susceptibility to a range of pollutants, such as heavy metals, pesticides, and pharmaceuticals, nitrification serves as a valuable indicator in the risk assessment of chemical contaminants in soil environments. Here, we analyzed the effects of cadmium (Cd) treatment on soil potential nitrification rate (PNR), and the abundance of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) communities. The results showed that, under 1 day incubation, the soil PNR with Cd 0.5 mg kg^-1 was a little higher but not statistically significant than that with zero mg kg^-1. Then, the soil PNR increased with the increasing Cd concentration from 0.5 to one mg kg^-1, and continuously declined from 1 to 10 mg kg^-1. Moreover, we predicted the bacterial functions of samples with hormetic Cd dose (one mg kg^-1) by PICURSt (Phylogenetic Investigation of Communities Reconstruction of Unobserved States), and found that the expression of protein disulfide isomerase (PDI) increased with the hormetic Cd dose. PDI is known to enhance the activity of compounds containing -SH or -S-S which can help prevent oxidative damage to membranes. The soil PNR was significantly correlated with AOA abundance rather than AOB, even the abundance of AOB was higher than that of AOA, indicating that AOA functionally predominated over AOB. Our study effectively evaluated the Cd toxicity on soil microbial community and clearly illustrated the ecological niches of AOA and AOB in the agricultural soil system studied, which will be instructive for the sustainable development of agriculture.