Diurnal cycle of oxygen and sulfide microgradients and microbial photosynthesis in a cyanobacterial mat sediment

The diurnal variation in the microgradients of O(2), H(2)S, and Eh were studied in the benthic cyanobacterial mats of a hypersaline desert lake (Solar Lake, Sinai). The results were related to light intensity, light penetration into the mat, temperature, pH, NH(4), photosynthetic activity, pigments, and the zonation of the microbial community. Extreme diurnal variation was found, with an O(2) peak of 0.5 mM at 1 to 2 mm of depth below the mat surface during day and a H(2)S peak of 2.5 mM at 2 to 3 mm of depth at night. At the O(2)-H(2)S interface, the two compounds coexisted over a depth interval of 0.2 to 1 mm and with a turnover time of a few minutes. The photic zone reached 2.5 mm into the mat in summer, and the main CO(2) light fixation took place at 1 to 2 mm of depth. During winter, light and photosynthesis were restricted to the uppermost 1 mm. The quantitative dynamics of O(2) and H(2)S were calculated from the chemical gradients and from the measured diffusion coefficients.