Phototrophs in High-Iron-Concentration Microbial Mats: Physiological Ecology of Phototrophs in an Iron-Depositing Hot Spring
Applied and Environmental Microbiology, 1999
At Chocolate Pots Hot Springs in Yellowstone National Park the source waters 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 [14C]bicarbonate was used to assess the impact of
ferrous iron on photosynthesis in this environment. Photoautotrophy in some of the mats was stimulated by
ferrous iron (1.0 mM). Microelectrodes were used to determine the impact of photosynthetic activity on the
oxygen content and the pH in the mat and sediment microenvironments. Photosynthesis increased the oxygen
concentration to 200% of air saturation levels in the top millimeter of the mats. The oxygen concentration
decreased with depth and in the dark. Light-dependent increases in pH were observed. The penetration of light
in the mats and in the sediments was determined. Visible radiation was rapidly attenuated in the top 2 mm of
the iron-rich mats. Near-infrared radiation penetrated deeper. Iron was totally oxidized in the top few
millimeters, but reduced iron was detected at greater depths. By increasing the pH and the oxygen concentration
in the surface sediments, the cyanobacteria could potentially increase the rate of iron oxidation in situ.
This high-iron-content hot spring provides a suitable model for studying the interactions of microbial photosynthesis
and iron deposition and the role of photosynthesis in microbial iron cycling. This model may help
clarify the potential role of photosynthesis in the deposition of Precambrian banded iron formations.
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