Figure 1.

The GN microbial mat (a) photograph and (b) taxonomic composition by depth. Taxonomic assignments were made with tax2tree (Materials and methods). (a) and (b) are consistent in scale. Numeric values on top and bottom of (b) correspond to the phylum legend to aid orientation. As discussed in the text, the dashed white lines indicate approximate depths of the photic/oxic (0–3 mm depth, zone A), low sulfide (3–6 mm depth, zone B) and high sulfide (6 mm and deeper, zone C) zones seen with light intensity and microelectrode measurements (Jorgensen and Des Marais, 1986; Jorgensen, 1992; Ley et al., 2006).