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We used microsensors to characterize physicochemical microenvironments and photosynthesis occurring immediately after water saturation in two desert soil crusts from southeastern Utah, which were formed by the cyanobacteria Microcoleus vaginatus Gomont, Nostoc spp., and Scytonema sp. The light fields within the crusts presented steep vertical gradients in magnitude and spectral compostion. Near-surface light-trapping zones were formed due to the scattering nature of the sand particles, but strong light attenuation resulted in euphotic zones only ca. 1 mm deep, which were progressively enriched in longer wavelengths with depth. Rates of gross photosynthesis (3.4-9.4 mmol O[sub2; &middot[sup-2] h[sup-1]) and dark respiration (0.81-3.1 mmol &O[sub2] &middot[sup-2] &middoth[sup-1]) occurring within 1 to several mm from the surface were high enough to drive the formation of marked oxygen microenvironments that ranged from oxygen supersturation to anoxia. The photosynthetic activity also resulted in localized pH values in excess of 10, 2-3 units above the soil pH. Differences in metabolic parameters and community structure between two types of crusts were consistent with a successional pattern, which could be partially explained on the basis of the microenvironments. We discuss the significance of high metabolic rates and the formation of microenvironments for the ecology of desert crusts, as well as the advantages and limitations of microsensor-based methods for crust investigation. [ABSTRACT FROM AUTHOR]