pH dependence of arsenic speciation in paddy soils: The role of distinct methanotrophs

Arsenic (As) is a priority environmental pollutant in paddy field. The coupling of arsenate (As(V)) reduction with anaerobic methane (CH₄) oxidation was recently demonstrated in paddy soils and has been suggested to serve as a critical driver for As transformation and mobilization. However, whether As(V)-dependent CH₄ oxidation is driven by distinct methanotrophs under different pH conditions remains unclear. Here, we investigated the response of As(V)-dependent CH₄ oxidation to pH shifts (pH 5.5–8.0) by employing isotopically labelled CH₄. Furthermore, the underlying mechanisms were also investigated in well-controlled anoxic soil suspension incubations. Our results showed that As(V)-dependent CH₄ oxidation is highly sensitive to pH changes (1.6–6.8 times variation of arsenite formation). A short-term (0–10 d) pH shift from near-neutral pH to acidic conditions (i.e., pH 5.5, −85% arsenite formation) had an inhibitory effect on As(V)-dependent CH₄ oxidation. However, prolonged acidic conditions (i.e., >15 d) had no significant influence on As(V)-dependent CH₄ oxidation. The microbial analyses indicated that As reduction in paddies can be driven by anaerobic CH₄ oxidation archaea (ANME) and methanotrophs. And, methanotrophs may serve as a critical driver for As(V)-dependent CH₄ oxidation. Moreover, type I methanotrophs Methylobacter were more active in oxidizing CH₄ than type II methanotrophs Methylocystis when the pH ≥ 6.5. However, Methylocystis had a higher tolerance to soil acidification than Methylobacter. This study illustrates that As(V)-dependent CH₄ oxidation could be dominated by distinct methanotrophs along with pH shifts, which eventually enhances As release in paddy soils.