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

Microbial mats and microbialites are essential tools for reconstructing early life and its
environments. To better understand microbial trace element cycling, a microbial mat was collected from the
sinkhole systems of the western shores of the Dead Sea, a dynamic environment exhibiting diverse extreme
environments. Intense arsenic enrichment was measured (up to 6.5 million times higher than current
concentrations in water, and 400 times the bulk concentration in the mat). Arsenic was found predominantly as
As(V) in organic molecules, as shown by XANES spectra and high‐resolution elemental mapping. Arsenic
cycling genes obtained from metagenomic analysis were associated with arsenic detoxification, supporting an
active mechanism of As(V) uptake, As(III) efflux and organoarsenic accumulation in the extracellular
polymeric substances (EPS) of the mat. Thus, we propose that such localized As enrichment can be attributed to
a transient increase in As(V) concentrations in the circulating subsurface water of the Dead Sea shore and its
subsequent incorporation into organoarsenic molecules through microbial detoxification processes. Our data set
supports the possibility of metalloid enrichment recorded in very localized facies due to rapid geogenic
fluctuations in the chemistry of the water flowing over a biofilm. In this context, this example calls for caution in
interpreting metal(loid) enrichment in organic matter‐rich layers and microbialites of Paleoproterozoic origin.
Arsenic signatures in Precambrian organic matter and carbonate rocks may host biosignatures, including
evidence for extracellular polymeric substances, As‐binding and detoxification processes, without supporting
arsenotrophy. However, they provide clues to better assess the paleoenvironmental conditions at the time of
microbial mat formation.