376 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



If we have a heterogeneous microbial population, the cycle can operate 

 without the intervention of animals, and even without that of plants, as 

 Fig. 102 demonstrates. 



III. THE SULPHUR CYCLE 



This is somewhat analogous to the nitrogen cycle. Like nitrates, sulphates 

 are utiHzed and reduced by plants. In its reduced form ( — SH groups), 

 like nitrogen in its reduced form as the amino groups of amino acids, 

 sulphur plays a part in the construction of vegetable proteins, from whence 

 it enters the animal organism. A part of this sulphur appears in the 

 oxidized form in the urine (derived from sulphuric acid). When the plant 



Sulphur 



Sulphides 



Beggiatoa 

 Thiobacillus spp.^ 

 Chromatium , 

 ChlorobiuTTi/ 



Beggiatoa 



^Thiobacillus spp. 

 , Chromatium 

 Chlorobium 



Thiobacillus spp. 



Desulfovibrio 

 Fig. 104 (Butlin and Postgate) — A microbial sulphur cycle. 



Sulphates 



or animal putrefies, the sulphur is liberated by many aerobic and 

 anaerobic bacteria as HgS (analogous to deamination with the production 

 of NHg). Part of naturally occurring RgS also comes from the removal of 

 sulphur from sulphates by bacterial processes. 



The opposite of the reduction of sulphates is a process corresponding to 

 nitrification, by which bacteria convert HgS and elementary sulphur to 

 H2SO4. Other bacteria can reduce elementary sulphur to HgS. 



As in the case of the nitrogen cj^cle, animals and plants are not indis- 

 pensable to the operation of the sulphur cycle. In nature one sometimes 

 encounters this cycle maintained by a microbiological association of auto- 

 trophic bacteria, algae and protozoa, the name sulphuretum is applied to 

 this community which can be found in clays, muds and in stagnant waters. 

 Figure 104 shows a bacterial sulphur cycle. 



