EESS seminar talk on "Biogeochemical Controls on Arsenic Methylation and Demethylation in Rice Paddy Soils"

Arsenic methylation and demethylation reactions influence arsenic speciation and fate in rice paddy soils, and understanding the balance between methylation and demethylation is important for assessing risks of arsenic toxicity in rice. Rice paddy soils are rich in organic carbon, and the conventional understanding has been that organic carbon enhances microbial arsenic methylation by fueling microbial activity.  This presentation will describe a series of recent investigations — spanning pure culture to greenhouse-scale experiments — that challenge this conceptual model.  First, the presentation will describe the role of carbon catabolite repression (CCR) in regulating cellular uptake and subsequent enzymatic methylation of arsenite.  A combination of biosensor, gene expression, and arsenic speciation analyses demonstrate that sugars lead to CCR and a decrease in arsenic methylation, while low molecular weight organic acids do not impact arsenite uptake.  The presentation will then examine couplings between methanogenesis and arsenic demethylation, wherein a greater abundance of methylotrophic carbon substrates (methanol, methylamines) promote arsenic demethylation by methanogens and limit the accumulation of methylated arsenic compounds.  A genome-resolved metatranscriptomic analysis of anaerobic soil slurry experiments resolved links between methylotrophic substrate utilization and arsenic demethylation, while greenhouse experiments showed that higher levels of methanogenesis in soils were associated with lower concentrations of methylated arsenic in rice grains. Taken together, these results provide a more nuanced understanding of how utilization of distinct carbon substrates impacts arsenic methylation-demethylation dynamics, and provides mechanistic insights into how paddy soil management practices influence arsenic speciation in rice.

Dr. Matthew Reid is an associate professor in the School of Civil and Environmental Engineering at Cornell University and directs the Biogeochemistry and Ecosystem Engineering Research Group. His research program focuses on biogeochemical element cycling in natural and nature-based systems with applications to water quality.  Research in the Reid Group spans physical scales and integrates mechanistic and molecular-level investigation with field-scale observation to build quantitative models for contaminant dynamics in complex biogeochemical systems. Dr. Reid's research has been recognized with early career awards from NSF and DOE. Dr. Reid was a postdoctoral scientist in the Environmental Microbiology Laboratory at EPFL, and completed his Ph.D. in Civil and Environmental Engineering at Princeton University.