Combined Genomic and Imaging Techniques Show Intense Arsenic Enrichment Caused by Detoxification in a Microbial Mat of the Dead Sea Shore

27 Microbial mats and microbialites are essential tools for reconstructing early life and its 28 environments. To better understand microbial trace element cycling, a microbial mat was 29 collected from the sinkhole systems of the western shores of the Dead Sea, a dynamic 30 environment exhibiting diverse extreme environments. Intense arsenic enrichment was measured 31 (up to 6.5 million times higher than current concentrations in water, and 400 times the bulk 32 concentration in the mat). Arsenic was found predominantly as As(V) in organic molecules, as 33 shown by XANES spectra and high-resolution elemental mapping. Arsenic cycling genes 34 obtained from metagenomic analysis were associated with arsenic detoxification, supporting an 35 active mechanism of As(V) uptake, As(III) efflux and organoarsenic accumulation in the 36 extracellular polymeric substances of the mat. Thus, we propose that such localized As 37 enrichment can be attributed to a transient increase in As(V) concentrations in the circulating 38 subsurface water of the Dead Sea shore and its subsequent incorporation into organoarsenic 39 molecules through microbial detoxification processes. 40