Composition changes of phototrophic microbial communities along the salinity gradient in the solar Saltern evaporation ponds of Eilat, Israel

The planktonic and benthic microbial communities in eight hypersaline evaporation ponds of Eilat, Israel, ranging in salinity between 58 and 329 g l-1, were studied by microscopic methods. Marked shifts in the species composition of phototrophic microbial communities were found along the salinity gradient. Planktonic communities consisted mainly of cyanobacteria and diatoms, except for the pond with the highest salinity, which was dominated by green alga Dunaliella salina. Species composition of benthic communities was markedly different in each pond. At the lowest salinities, mats of oscillatoriacean cyanobacteria, accompanied with diatoms, covered the sediment. The cyanobacterium Halothece sp. dominated the ponds of intermediate salinity where gypsum precipitated. However, less abundant Halothece sp. also occurred in lower salinities. At the highest salinities the bottom was covered by salt crystals and was not colonized by phototrophic microorganisms. Photosynthetically active oxygenic microorganisms (cyanobacteria, diatoms) were found to abound even in deep layers of sediments (up to 5 cm) where light and oxygen are supposedly unavailable.