AN ENDOEVAPORITIC MICROBIAL MAT WITHIN A GYPSUM CRUST - ZONATION OF PHOTOTROPHS, PHOTOPIGMENTS, AND LIGHT PENETRATION

We characterized a layered community of cyanobacteria and purple bacte ria that developed within a gypsum crust on the bottom of a hypersalin e saltern pond (salinity 280 to 290 g l(-1)) in Eilat, Israel. The upp er 1 to 2 cm of the 4 to 5 cm thick gypsum crust is inhabited by carot enoid-rich unicellular cyanobacteria (Aphanothece sp. and others), imp arting an orange-brown color to the gypsum. Under the brown layer, a g reen layer dominated by unicellular cyanobacteria of the genus Synecho coccus is found, with filamentous Phormidium-type cyanobacteria as a m inor component. Below these layers of oxygenic phototrophs is a red la yer of purple bacteria. We studied the optical properties of the gypsu m crust, both by characterization of the pigments present in the diffe rent layers and by measuring spectral scalar irradiance at different d epths in the crust, using fiber-optic microprobes. In the upper 2 mm o f 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 50 0 nm) was effectively absorbed by the protective carotenoids (myxoxant hophyll, echinenone, and others) in the upper brown layer. However, si gnificant amounts of Light in the red part of the spectrum penetrated down to the green layer to enable photosynthesis: about 1% of the inci dent irradiance at 620 and 675 nm reached the green layer at a depth o f 15 mm, and >1% of the incident light in the infrared part of the spe ctrum reached the purple bacteria located at a depth of 20 to 23 mm.