Microelectrode studies of the photosynthesis and O 2 , H 2 S, and pH profiles of a microbial mat1

The profiles of O 2 , H 2 S, and pH within a microbial mat of the hypersaline pond Solar Lake, Sinai, were measured by 2–200‐ µ m‐thick microelectrodes during diurnal and artificial light cycles. The oxygen concentration in the photic layer varied from a maximum of 1,400 µ M during the day to 0 during the night. The pH in the same layer varied between 9.6 in the early afternoon and 7.7 in the early morning. Sulfide was not present in the photic zone during the day, but built up to about 50 µ M during the night. The diffusion gradients of sulfide and oxygen were very steep and the two compounds coexisted in a layer only 0.25 mm thick during the day. Diffusion flux calculations showed that the average turnover time of sulfide within this layer was 21 s. The rapid turnover indicated that the oxidation of sulfide must be biologically mediated. Oxygenic photosynthesis was measured by a new oxygen microprofile method which accurately determines the vertical distribution of photosynthetic activity. There was no difference in the efficiency of photosynthesis between morning and afternoon. The photosynthetic efficiency of the whole mat was about fourfold higher at low light intensities, <120 µ Einst· m ‒2 ·s ‒1 , than at high light intensities, 120–1,600 µ Einst·m ‒ 2 ·s ‒ 1 . Anoxygenic photosynthesis within the mat was not quantitatively important.