364 UNITY AND DIVERSITY IN BIOCHEMISTRY 



to be formed from the nitrogen of the air. The essential fact is the pro- 

 vision of oxaloacetic acid by the host plant. 



The nodules contain a haemoglobin (leghaemoglobin) of low molecular 

 weight (17,000), but is role in nitrogen fixation is unknown. 



B. Fixation by Autonomous Organisms 



In 1883, Berthelot showed that earth became enriched in nitrogen just 

 by contact with the air, even if it was sterilized ; he concluded that micro- 

 organisms capable of fixing nitrogen were present in the soil. 



(a) Clostridium 



The first of these micro-organisms was isolated by Winogradsky in 1895, 

 it is Clostridium pasteurianum, sometimes called Amylobacter. In this 

 anaerobe, unlike Rhizohium, fixation is not inhibited in the presence of 

 carbon dioxide. The ability to fix nitrogen is widespread in the Clostridia 

 and it has also been found in another anaerobe, Desulphovibrio, which 

 reduces sulphates. 



(b) Myxophycae {Blue-green Algae) 



These algae and especially those of the genus Nostoc and the genus 

 Anabaena, fix nitrogen, but rather slowly. The fixation is inhibited by 

 ammonia and nitrates and is not quantitatively important in soils. 



(c) The Purple Bacteria 



Many of the bacteria in the Athiorhodaceae group (non-sulphur purple 

 bacteria) fiix nitrogen. This fixation is coupled to growth and only occurs 

 in the presence of light and oxygen. 



(d) Azotobacter 



These bacteria are exceptionally large and resemble the blue-green 

 algae. They easily form gums and they are obligatory aerobes. Fixation 

 of nitrogen only occurs in the presence of a source of carbon and is probably 

 coupled to oxidation. A part of the nitrogen fixed is excreted into the 

 medium in the form of amino acids. Again, ammonia itself or a precursor 

 of it is combined with the dicarboxylic acid. As in the other cases of 

 fixation, the mechanism of the process is shrouded in mystery. 



IV. SULPHUR 



Plants absorb and assimilate sulphates. Soil contains a great deal of 

 calcium sulphate. Bacteria exist which reduce sulphates to HgS. Such 

 bacteria live in muds, animal intestines, etc., and everywhere where 

 organic materials are putrefying. 



SO4- + 4H2 -^ HaS + 2H2O + 20H- 



