Regulation of arsenic methylation and demethylation dynamics by organic carbon in rice paddy soil

Authors: VEGA, MICHAIE - Cornell University; DIGNAM, WILLIAM - Cornell University; SLADE, AVA - Cornell University; KARANIKAS, ALLY - Cornell University; YOON, HYUN - University Of California Berkeley; Rohila, Jai; REID, MATTHEW - Cornell University

Submitted to: Annual Goldschmidt Conference
Publication Type: Abstract Only
Publication Acceptance Date: 5/1/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: Microbial methylation and demethylation reactions influence the toxicity and mobility of arsenic (As) in soil environment but the impact of quantity and quality of available organic carbon on these reactions remains poorly understood. A general convention is that greater organic carbon prefers greater microbial methylation. Moreover, if an As pool is enriched in methylated species, studies have identified upper bounds on the stimulatory effect of organic carbon on As methylation. Further, recent pure culture studies have shown that upper glycolytic substrates (e.g., glucose, xylose) can decrease methylation rates by repressing arsenite uptake by microbes. Stimulation of methylotrophic methanogenesis using methanol has also been linked to As-demethylation, preferring inorganic arsenic (iAs) production under carbon-rich environment. In this contribution, we present a revised conceptual model integrating glycolytic and methylotrophic carbon substrates as key exceptions to the convention of greater organic carbon preferring methylated-As species – arguing that in some cases, greater organic carbon can favor net demethylation and predominantly iAs. We interrogated this model using greenhouse-scale rice paddy soil mesocosms amended with variable amounts of organic matter (e.g., dried leaves). Porewater monitoring showed that organic matter amendment resulted in less methylated-As during key stages of rice grain development (i.e. heading, grain filling, and maturation). Analysis of As-speciation in rice grains is underway to link soil-As dynamics to rice grain accumulations. Additionally, porewater dissolved methane concentrations were inversely correlated with As methylation efficiency, suggesting that As demethylation may have a link with methanogenesis. This finding was supported by complementary soil slurry incubations, also amended with organic matter, showing that As was actively demethylated by methanogens. Interestingly, methanogens demethylated As in equal amounts, albeit at slower rates, to incubations amended with a model methylotrophic methanogenesis substrate (i.e., trimethylamine), indicating that leaf-derived carbon could promote methylotrophic methanogenesis and associated demethylation of dimethyl-As. Collectively, these results suggest that organic matter addition can result in iAs species predominance due to enhanced demethylation and repressed methylation.