ENTRY INTO THE BIOSPHERE 353 



Certain other autotrophes which oxidize sulphur are anaerobic. This is 

 the case with Thiobacillm denitrifians which oxidizes sulphur, HgS, 

 NagSgOg (with formation of H2SO4), at the expense of oxygen derived 

 from nitrates. It is widely distributed, being present in soils, water and 

 muds. 



SNagSgOg + 8KNO3 + 2NaHC0g ^ eNagSO^ + 4K2SO4 + 



4N2 + 2CO2 + H2O 



In addition to the simple bacteria we have just considered, there are 

 some morphologically more complex, such as Thiothrix and Beggiatoa 

 who oxidize H2S to H2SO4 using part of the energy from this reaction to 

 synthesize carbon chains. They are aerobes containing particles of 

 sulphur in their cells. In addition to COg and oxygen they require the 

 presence of H2S. As long as H2S is present, the sulphur particles remain 

 in the cells. In the absence of HgS, these bacteria can use their reserve of 

 sulphur granules. When this reserve is exhausted, they die. The energy- 

 yielding reactions are as follows: 



H2S + O -> H2O + S 



2S + 3O2 + 2H2O -^ 2H2SO4 



{d) The Mechanism of Autotrophy 



The autotrophic bacteria pose the major problem of how the energy 

 produced by an oxidation reaction is transferred and used for the synthesis 

 of a sugar. In a general way the process can be represented as occurring 

 in the following stages: 



1. Activation by an enzyme of the oxidizable substrate (a thiosulphate 

 dehydrogenase in Thiobacillus, an ammonium dehydrogenase in Nitro- 

 somonas). 



2. Oxidation by the cytochrome-cytochrome oxidase system. 



3. Formation, during this oxidation, of energy-rich phosphate bonds. 



4. Fixation of COg by conversion to a carboxyl group, in the usual way. 



5. Formation of a carboxyl phosphate. 



6. Reduction of the latter by the enzyme of reaction 1, liberation of 

 phosphate. 



XCO— O - H2PO3 + RHo ^ XCHO + H3PO4 + R 



However the above is almost entirely hypothetical. 



B. Photosynthesis in the Green Algae and Green Plants 



Photosynthesis by green algae and green plants is the major means of 

 introducing carbon into the biosphere. The annual production of organic 

 substances by photosynthesis in the biosphere is 2000 times greater than 



