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iron oxides and algae in a Colorado stream with acid mine drainage

Close-up of iron oxides and periphyton (mats of stream algae) in St. Kevin Gulch, Colorado. These algal mats are comprised of a metal tolerant green alga, Ulothrix sp., that is able to survive under low pH (~3.5) and high metal concentrations (~10 mg/L of zinc, for example). Experiments using conservative and reactive tracers have shown that these algae assimilate more phosphate during the day when photoreduction of the iron oxides releases phosphate from the streambed. Photo: Diane McKnight (CEAE, ENVS, & INSTAAR).