Fig. 1.

Cyanidiales mats dominate the microbial biomass in cooler reaches of acidic geothermal outflow channels in YNP. The images were taken on September 15, 2008 (A and B) and October 30, 2008 (C). (A) East Fork, Tantalus Creek drains the Norris Geyser Basin. The algal mat is recovering from UV-associated mat decline (15). (B) Nymph Creek, located in the Norris-Mammoth corridor, is heavily shaded and does not experience mat decline. (C) Cyanidiales algae associated with realgar-like mineral phases and geothermal-derived waters containing 0.76 mM total arsenic. The orange-yellow deposits were determined by scanning electron microscope energy-dispersive X-ray spectroscopy to be realgar-like, with an As/S ratio range of 1.1 to 1.3 (Fig. S1).