(2) Oxidation by nitrite formed by bacterial nitrate reduction (lya & Screenivasaya 

 1944,) may have occurred, but the low nitrate content of the water would not favour it. 



(3) Oxidation by Th. thioparus, which is an obligate aerobe, could have yield- 

 ed sulphur at the air- water interface, but would not have occurred at lower (anaero- 

 bic) levels. Senez (1951) attributed sulphur formation in air by impure cultures of D. 

 de sulphur icons to this organism. Th. thioparus was not found in t"he samples 

 examined for its presence, but it is a fragile organism and may not have survived the 

 journey to England. 



(4) Oxidation by Chromatium undoubtedly occurred in the lakes and was repro- 

 duced with pure cultures. However, Chromatium stores sulphur granules inside the 

 cell and would therefore not produce free sulphur unless lysis of the cell occurred. 



(5) Oxidation by Chlorobium also occurred and was demonstrated with pure cul- 

 tures. As Chlorobium deposits sulphur outside the cell, oxidation by this organism 

 could account for much of the sulphur formed in the lakes. 



The importance of Chromatium and Chlorobium in sulphur formation is supported 

 by three facts :- 



(i) The insignificant production of sulphur in Ain-umm- el-Gelud corresponded 

 with an absence of the coloured gelatinous material. This suggests that the 

 coloured material, which consisted largely of Chlorobium and Chromatium, 

 played a key part in sulphur formation. 



(ii) Chlorobium and Chromatium are obligate anaerobes and, subject to light being 

 available, would be active throughout the lake water. Aerobic sulphide oxi- 

 disers could only function at the surface. 



(iii) Chlorobium and Chromatium synthesised organic matter from CO2 and sunlight 

 which would support growth and sulphate reduction by D. de sulphuric ans. 



Thus there was probably a symbiosis between D. de sulphuric ans and the coloured 

 sulphide oxidisers, in which the sulphate reducers formed sulphide for growth of the 

 sulphide oxidisers, which in turn made organic matter photosynthetically for the sul- 

 phate reducers. 



The formation of sulphur by this symbiosis is of considerable intrinsic scientific 

 interest. It also suggests that the larger sulphur deposits in nature may have been 

 laid down by a similar process. Its economic aspects are mostly obvious. No lakes 

 other than those in Cyrenaica are known to produce sulphur on a scale sufficient to 

 justify commercial exploitation. The production of about 200 tons annually is insig- 

 nificant in relation to the prevailing shortage of sulphur. Nevertheless it suggests a 

 method of augmenting sulphur supplies. Sufficient is now known of the physiology of 

 sulphate -reducing bacteria and the coloured sulphide- oxidising organisms to make it 

 clear that conditions in Ain-ez- Zauia are by no means optimal for the separate activi- 

 ties of these bacteria, though much more research is required before the best conditions 

 for their symbiosis and for maximum sulphur production are established. Even so, it is 

 possible that the addition of organic matter (e.g. vegetable waste) and phosphate might 

 increase the yield of sulphur. Unproductive lakes such as Ain- umm- el - Gelud might 

 be made productive by addition of necessary nutrients, which may be simple. More- 



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