Figure 1.

Oxygen profile measurements in the snottite. (A) Location of oxygen profiles across a snottite biofilm, taken at distances of 1, 3, 4.5 and 6 cm from the ceiling. (B) Oxygen profiles. (a) The profile taken closest to the rock surface shows the fastest oxygen decrease: at a depth of 700 μm, oxygen was no longer detectable. (b and c) show anoxic signals at 1300 μm. Profile (d) from spot 4 shows no anoxic zone anymore. (C). Model of the oxygen distribution in the snottite biofilm. Indicated are the outer oxic part and the inner anoxic part.