Phototrophs in high-iron-concentration microbial mats: Physiological ecology of phototrophs in an iron-depositing hot spring

At Chocolate Pots Hot Springs in Yellowstone National Park the source water s have a pH near neutral, contain high concentrations of reduced iron, and lack sulfide. An iron formation that is associated with cyanobacterial mats is actively deposited, The uptake of [C-14]bicarbonate was used to assess the impact of ferrous iron on photosynthesis in this environment. Photoauto trophy in some of the mats was stimulated by ferrous iron (1.0 mM). Microel ectrodes were used to determine the impact of photosynthetic activity on th e oxygen content and the pH in the mat and sediment microenvironments. Phot osynthesis increased the oxygen concentration to 200% of air saturation lev els in the top millimeter of the mats. The oxygen concentration decreased w ith depth and in the dark Light-dependent increases in pH were observed. Th e penetration of light in the mats and in the sediments was determined. Vis ible radiation was rapidly attenuated in the top 2 mm of the iron-rich mats , Near-infrared radiation penetrated deeper, Iron was totally oxidized in t he top few millimeters, but reduced iron was detected at greater depths. By increasing the pH and the oxygen concentration in the surface sediments, t he cyanobacteria could potentially increase the rate of iron oxidation in s itu, This high-iron-content hot spring provides a suitable model for studyi ng the interactions of microbial photosynthesis and iron deposition and the role of photosynthesis in microbial iron cycling. This model may help clar ify the potential role of photosynthesis in the deposition of Precambrian b anded iron formations.