A strain of Calothrix viguieri, isolated originally from the surface of a mangrove root, showed marked morphological and physiological responses to the salinity of the medium. When grown to P limitation in a freshwater medium, each trichome ended in a long hair and the culture showed marked cell-bound phosphomonoesterase and phosphodiesterase activities. When grown with various P sources (inorganic and organic) to P limitation in medium including 5'6g 1-~ NaCI, hairs were absent, phosphomonoesterase activity was much lower and phosphodiesterase activity absent. With five of the seven organic P sources tested, the yield was also lower in saline medium. Reduction of NaCI concentration led to development of hairs over a 24 h period, whereas increase in NaC1 led to the hairs being shed. Neither mannitol nor sorbitol led to similar effects to those brought about by NaC1. Use of an azo-dye staining technique (napthol AS-MX phosphate as P source and Fast Blue RR diazonium salt as coupling agent) showed that cell-bound phosphomonoesterase activity of cultures with hairs was localized on the hairs, with a sharp transition between the part of the trichome showing activity and that not doing so. In comparison with the

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Abstract. Sabkhat are widely distributed throughout the Qatar Peninsula, and the aim of this study is to give a brief overview of their geographical distribution, geomorphology factors influencing their formation and biological characteristics within the area, with particular reference to their microorganisms. Although there are a large number of individual areas of sabkha, they tend to be small in size. The formation of sabkhat is shown to be a multifactorial process that is affected by several dynamic factors, such as geography, topography, environment and oceanography, as well as various anthropogenic ones.The microbial communities of the sabkhat in Qatar display a conspicuous horizontal and vertical zonation. Whereas the upper-intertidal zone is composed of cyanobacterial mats, the middle-intertidal zone is dominated by the archaebacterium Halobacterium salinarium and the micro-green alga Dunaliella salina. The lower-intertidal zone is characterized by a black muddy substrate in which large quantities of sulphur and methane are produced due to the presence of certain bacteria. Analysis of the vertical zonation of the microbial profile has revealed the presence of various characteristic species of bacteria, cyanobacteria and diatoms at specific depths.

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