A novel oxygen microelectrode with a tip diameter of 2 to 20 mum was constructed which could function satisfactorily under a variety of environmental conditions and in a variety of media, including human blood serum, citric acid at pH 2.5, moist air, and paraffin oil. Measurement of oxygen by this electrode does not require stirring of the medium. Electrodes could be made so that the 90% response time necessary to detect changes in oxygen concentration was less than 0.2 s, and response was linear with oxygen concentration. To demonstrate the performance of the electrode, oxygen and photosynthesis profiles of an acid microbial mat (pH 2.8) dominated by the eucaryotic alga Cyanidium caldarium were made. Photosynthetic rates as high as 95 mmol of O(2) dm h were measured within the most active 0.1-mm layer, which was ca. 0.2 mm below the surface of the microbial mat. The total photosynthetic activity was 47 mmol of O(2) m h. Vertical profiles of photosynthesis at different light intensities showed that the microalgae within the mat were not photoinhibited at bright sunlight (2,090 muEinsteins m s).