Dataset for journal paper entitled "Variability in Arsenic Methylation Efficiency across Aerobic and Anaerobic Microorganisms" (DOI: 10.1021/acs.est.0c03908). Partial publication's abstract: "Microbially-mediated methylation of arsenic (As) plays an important role in the As biogeochemical cycle, particularly in rice paddy soils where methylated As, generated microbially, is translocated into rice grains. The presence of the arsenite (As(III)) methyltransferase gene (arsM) in soil microbes has been used as an indication of their capacity for As methylation. Here, we evaluate the ability of seven microorganisms encoding active ArsM enzymes to methylate As. Amongst those, only the aerobic species were efficient methylators. The anaerobic microorganisms presented high resistance to As exposure, presumably through their efficient As(III) efflux, but methylated As poorly. The only exception were methanogens, for which efficient As methylation was seemingly an artifact of membrane disruption." The files deposited include: the flow cytometry data and fluorescence microscopy pictures used to assess membrane disruption of the methanogen Methanosarcina mazei, for experimental details please refer to publication, and the supporting information of the publication. Files: Figure 3_flowcytometry files.zip: flow cytometry measurements reported in Figure 3 of publication. Measurements were performed with a 5-laser LSRII SORP flow cytometer. SYBR Green I (SG) (Invitrogen) was excited by the Blue laser (488 nm) and detected using a 530/30 band pass filter. propidium iodide (PI) (Sigma) was excited by the YG laser (561 nm) and detected using a 610/20 band pass filter. 30’000 events per sample were analyzed into four populations (no fluorescence, SG, SG/PI, or PI). Cells could be assigned to the membrane-compromised population, based on the gating of double-stained and single-stained controls of glutaraldehyde- fixed and ethanol-permeabilized cells. Cytometric data were acquired and analyzed using BD TM FACSDiva software v. 8.0.1 (BD Biosciences, CA, USA). The files consist of the reports generated by BD TM FACSDiva software in .jpg format. Figure S14_fluorescence microscopy files.zip: Fluorescence microscopy pictures in .lsm format of single-stained SG control (SG), single-stained PI control (PI), double-stained control (SG/PI), 16-days sample (16 days) and 20-day sample (20 days) of a Methanosarcina mazei culture grown with 10 μM As(III) as initial concentration. The pictures are published as Figure S14 of the publication. The pictures were taken using Zeiss LSM 700 in the upright configuration equipped with a Plan-Apochromat 63x/1.40 oil immersion objective. For more details please refer to supplementary information in publicaiton. Recommended software for .lsm format included in .zip file. SI_tables_Viacava_et_al_for_publication: file in .xlsx format including the tables: Accession numbers for As(III)-efflux and ArsM proteins and genes; primers used in preparing mutants of C. pasteurianum; growth curves and growth rates values for all sampled cultures; relative abundance of flow-cytometry populations; ICP-MS settings for As analysis; primers for arsM gene amplifications; primers for RT-qPCR of C. pasteurianum; HPLC-ICP-MS spectrum values ; values of arsM and acr3 expression in C. pasteurianum WT and Δacr3; and concentration of total soluble arsenic and soluble arsenic species in filtered medium from all sampled cultures. SI_Viacava_et_al_for_publication: file in .pdf format including: Materials and methods: total arsenic and arsenic speciation analysis; cloning the arsM genes and gene expression in E. coli AW3110 (DE3); growth conditions of C. pasteurianum H0D0R4, strain used for genetic modification; isolation of the Δacr3 and ΔpyrE::Δacr3 mutants; arsenic methylation by C. pasteurianum Δacr3; transcription of arsM in C. pasteurianum WT and Δacr3; and membrane-integrity assessment of M. mazei cells using flow cytometry. Figures: growth rate of each individual species; abiotic control growth curves; total soluble anaerobic bacterium culture; soluble arsenic species in filtered medium from anaerobic bacterial cultures grown with 50 μM As(III); soluble arsenic species in filtered medium and volatile arsenic species from an A. rosenii culture; soluble arsenic in filtered medium from S. vietnamensis, M. mazei and M. acetivorans cultures; soluble arsenic species in abiotic controls; spiked HPLC-ICP-MS spectra; growth and concentration of soluble arsenic species in ArsM-expressing E. coli AW3110 (DE3); fluorescence microscopy pictures of flow cytometry controls from the membrane-integrity assessment from a M. mazei culture grown with 50 μM As(III); expression of arsM and acr3 in C. pasteurianum WT and Δacr3 mutant; and alignment of ArsM proteins. README.txt: .txt file with this description text. The work was funded by the Swiss National Science Foundation (SNSF) grant 310030_176146, an SNSF professorship grant PP00P2_163661, the SNSF National Center of Competence in Research "Microbiomes", and the UK Biotechnology and Biological Sciences Research Council grants BB/J014508/1718 and BB/L013940/1.