Diurnal Cycle of Oxygen and Sulfide Microgradients and Microbial Photosynthesis in a Cyanobacterial Mat Sediment

Abstract

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 14 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.