(7) Examination of Other Lakes. Ain-el- Rabaia (roughly circular, 100 yards 

 diameter) and Ain- el- Braghi (80 x 50 yards) were similar to Ain- ez- Zauia in smell 

 (H2S), colour (milky blue with coloured gelatinous material stretching 2-3 yards from 

 the banks) and in producing sulphur. Both were fed by warm springs, but we were un- 

 able to take the temperatures because both our thermometers were broken. According 

 to Mancuso, the temperature of Ain-el- Braghi varies between 32° and 34°C during the 

 year. Ain- el -Rabaia appeared to be cooler. 



Ain- umm-el-Gelud was considerably larger (approx. lx% mile) and was differ- 

 ent in two important respects. It contained very little free sulphur, though there was 

 a pronounced smell of hydrogen sulphide. There was also none of the coloured gela- 

 tinous material round its borders, i.e. there had been no mass development of sulphide- 

 oxidising bacteria. It appeared likely that the non - production of sulphur was related 

 to the absence of gelatinous material. 



Sulphate -reducing bacteria and sulphide -oxidisers were isolated from all three 

 lakes. 



(8) Recovery of Sulphur. In the dry season the sulphur is scooped out by 

 hand dredges made of sacking into shallow earthen pans at the water edge. The sul- 

 phur is left to drain and dry for a week. It is afterwards transferred to higher ground 

 for further drying and is then collected into heaps for transport by lorry. The crude 

 product contains about 50% sulphur, most of the remainder being sodium chloride (20?0 

 and silica (12%). The total quantity recovered annually from three lakes is about 200 

 tons ; the total amount formed would be larger. 



Laboratory Experiments 



Our observations in Cyrenaica suggested strongly that the formation of sulphur in 

 the lakes was a microbiological process in which sulphate - reducing bacteria were as- 

 sociated with the coloured gelatinous material developing so prolifically in the shal- 

 low waters. We tested this hypothesis by experiments in an 'artificial lake*. 



^Artificial Lake' Experiment. A 40- litre tank containing 'artificial lake medium' 

 (Table 1 supplemented with sodium lactate as organic source) was inoculated with 

 both the coloured gelatinous material and crude cultures of sulphate reducers from Ain- 

 ez- Zauia. The whole was incubated at 32°C with continuous illimiination from a 200- 

 watt bulb 18" above the water surface. At intervals the sulphide concentration was 

 renewed with saturated HjS- water. The pH was maintained at about 7.5. After 5 days 

 a red colouration appeared and sulphur deposition began. After a month a thick layer 

 of gelatinous material containing sulphur covered the bottom of the tank, closely resem- 

 bling that taken from the lake. 



This experiment and others like it showed that sulphur formation could be induced 

 in an artificial lake medium (with sulphate as the sole sulphur source) by inoculating 

 with crude cultures of sulphate reducers and gelatinous material from the lake. Micro- 

 scopical and bacteriological examination of the gelatinous material had shown that it 

 consisted chiefly of the coloured photosynthetic sulphide- oxidising bacteria Chroma- 

 tium and Chlorobium. It was therefore very probable that sulphur formation in the 



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