Fig. 2: Biogeochemistry of Laguna La Brava Channel mats.

Geochemical and microbial metabolic observations in the channel mats; from left to right: a μXRF (top) and μXANES (bottom) images showing a heat map of As distribution (top) and point measurements indicating the presence of two As oxidation states (bottom; red lines depict As spectra at pt 1 and pt 2; black lines are As(III) and As(V) standards) in the surface of the mat (see white frame, b); b Hand sample in cross section showing the different layers and mineral precipitates in pink layer; dashed line indicates mat surface. Arrow indicates precipitated carbonate mineral. White frame shows location of measurements in (a); c Depth profiles of sulfide (black), pH (red), and oxygen (blue; minute concentration only detected near the surface of the water indicating anoxia in the mat and overlying water) solid lines represent daytime measurements, dashed lines are night-time observations; bold black horizontal line indicates mat-water interface (depth = zero mm; water depth indicated as negative integer); d 2D distribution of sulfate-reducing activity in upper 10 mm of the mat (pixels showing locations of SR; darker pixels indicate higher rates). Measurements were carried out in the Chilean summer of 2015. Please note that b–d have corresponding vertical (depth) scales.