Selective utilization of organic carbon molecules promotes arsenic methylation by increasing methyltransferase activity in arsM-harboring microbes of paddy soils

The composition of organic carbon molecules (OCMs) impacts microbial diversity and in turn regulates the microbial transformation of elements in soils. However, the potential association of OCMs characteristics with arsenic methylation mediated by arsenite S-adenosylmethionine (SAM) methyltransferase gene (arsM)-harboring microbes in paddy soils remains unclear. Here, we investigated the impacts of OCMs on microbial methylators and arsenic methylation by identifying OCMs with Fourier transform ion cyclotron resonance mass spectrometry. The results showed that arsenic methylation efficiency increased by 57.4-260.1% in pig manure- or rice strawtreated paddy soils compared to the control. Molecules containing carbon, hydrogen, oxygen, and nitrogen (CHONs) with the atomic oxygen to carbon ratio (O/C) ranging from 0.5 to 0.8 had a more significant correlation (p < 0.01) with the abundance of the arsM gene than with that of 16S rRNA. This was confirmed by a significant positive correlation (p < 0.001) between consumed CHONs with O/C of 0.5-0.8 and increased methylated arsenic. CHONs (with O/C of 0.5-0.8) increased the intracellular SAM concentration and methyltransferase activity, stimulating arsenic methylation in vivo. The specialization of arsM-harboring taxa in CHONs with different O/C were identified. These observations provide a basis for predicting the risk of straight-head disease in paddy soils.