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We characterized a layered community of cyanobactena and purple bacteria that developed within a gypsum crust on the bottom of a hypersaline saltern pond (salinity 280 to 290 g 1-0 in Eilat, Israel. The upper 1 to 2 cm of the 4 to 5 cm thick gypsum crust is inhabited by carotenoid-rich unicellular cyanobacteria (Aphanothece sp. and others), imparting an orange-brown color to the gypsum. Under the brown layer, a green layer dominated by unicellular cyanobacteria of the genus Synechococcus is found, with filanlentous Phormidjum-type cyanobacteria as a minor component. Below these layers of oxygenic phototrophs is a red layer of purple bacteria. We studied the optical properties of the gypsum crust, both by characterization of the pigments present in the different layers and by measuring spectral scalar irradiance at different depths in the crust, using fiber-optic microprobes. In the upper 2 mm of the crust, a maximum of scalar irradiance of up to 200% of incident light was measured. Light in the blue range of the spectrum (400 to 500 nm) was effectively absorbed by the protective caroteno~ds (myxoxanthophyll, echinenone, and others) in the upper brown layer. However, significant amounts of light in the red part of the spectrum penetrated down to the green layer to enable photosynthesis: about 1 % of the incident irradiance at 620 and 675 nm reached the green layer at a depth of 15 mm, and > l % of the incident light in the infrared part of the spectrum reached the purple bacteria located at a depth of 20 to 23 mm.

An endoevaporitic microbial mat within a gypsum crust:zonation of phototrophs, photopigments, and light penetration