formed zoogleal masses closely resembling the gelatinous material of the lake. It is 

 therefore reasonable to conclude that the latter consisted chiefly of masses of the two 

 photosynthetic sulphide- oxidising organisms. Both are obligate anaerobes. Their 

 significance in the production of sulphur in sulphide- containing waters, which are 

 essentially anaerobic, is discussed below. 



(4) Microbial Population. In addition to the bacteria mentioned above and 

 despite the anaerobic environment, aerobic organisms were present in the lake, pre- 

 dominantly at the surface (Table 2). Fourteen morphologically distinct types of 

 aerobe, including a fungus, were isolated from the gelatinous material. Eight types 

 were isolated from a bottom sample (9 ft.) of water, three from half way and four from 

 a surface sample. 



Thiobacillus thiooxidans, an aerobic sulphur- oxidising organism, was not detect- 

 ted in water and mud samples, but it was present in samples taken above water level. 

 Thiobacillus thioparus was only sought in the gelatinous material and was not found. 

 Cellulose- decomposing bacteria were found only in one bottom mud sample, so were 

 probably not plentiful. No algae were isolated with the exception of a blue -green 

 alga from another lake, Ain-ei-Braghi. These Mycophyceae are often observed in sul- 

 phide-containing waters (Allen, 1952). 



(5) Fish. Despite the high concentration of salts and sulphide, shoals of 

 small fish (1" - 2" long) were observed in the two streams leaving the smaller section 

 of the lake. They were later identified as belonging to the genus Cyprinodon, (Smithy 

 1952). Mancuso (1939) reports that Desio (1935) collected Cyprinodon from Ain-el-Braghi 

 in 1930. 



(6) Sulphur. The vivid milky- blue appearance of the water by reflected light 

 was undoubtedly due to suspended colloidal sulphur particles, which could be seen as 

 highly refractive bodies under the microscope. A deposit of finely divided sulphur, 



6" or more in depth, covered the bottom of the lake. This was removed annually by 

 local Arabs. The possible processes responsible for this sulphur formation are dis- 

 cussed below. 



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