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Microelectrodes were used to measure oxygen, pH, and oxygenic photosynthetic activity in a hot spring microbial mat (Octopus Spring, Yellowstone National Park), where the cyanobacteriumSynechococcus lividus and the filamentous bacteriumChloroflexus aurantiacus are the only known phototrophs. The data showed very high biological activities in the topmost layers of the microbial mat, resulting in extreme values for oxygen and pH. At a 1-mm depth at a 55°C site, oxygen and pH reached 900 μM and 9.4, respectively, just after solar noon, whereas anoxic conditions with a pH of 7.2 were measured before sunrise. Although diurnal changes between these extremes occurred over hours during a diurnal cycle, microbial activity was great enough to give the same response in 1 to 2 min after artificial shading. Oxygenic photosynthesis was confined to a 0.5- to 1.1-mm layer at sites with temperatures at or above about 50°C, with maximum activities in the 55 to 60°C region. The data suggest thatS. lividus is the dominant primary producer of the mat.

Microelectrode Studies of Interstitial Water Chemistry and Photosynthetic Activity in a Hot Spring Microbial Mat