NUTRITION IN THE LOWER PLANTS 83 



which makes it more than eight times as efficient in heating as 

 starch. These hydrogen bacteria (Hydrogenomonas) are found 

 in the soil and can oxidize hydrogen by means of carbon dioxide 

 in the presence of water into formaldehyde and water: 



C0 2 +2H 2 = CH 2 0+H 2 0. 



The formaldehyde is then presumably condensed into carbohy- 

 drates as in the higher plants. If these bacteria are grown upon a 

 medium containing organic food, they cease to assimilate the 

 hydrogen and become ordinary saprophytes. 



Leptothrix and Crenothrix, which are found in conjunction with 

 iron deposits (sometimes as a result but more often as a cause), 

 can oxidize ferrous compounds, especially ferrous bicarbonate, 

 into ferric hydroxide and carbon dioxide: 



2FeHC0 3 +2H 2 0+0 2 = 2Fe(OH) 3 +2C0 2 . 



The ferric compound is generally deposited in the mucilaginous 

 sheath which surrounds the bacteria, and it is this deposition which 

 in large numbers has caused 90% of the world's bog iron deposits, 

 as estimated by Harder (1919). The carbon dioxide produced 

 by the bacteria is used in building up carbon compounds. Other 

 forms can use ferrous sulphate, which in the presence of lime is 

 converted into ferric oxide and gypsum. The iron oxide remains 

 within the cell of the bacterium and is deposited upon its death, 

 as mentioned above. Thus the Cuyuna and Mesaba Ranges of 

 Minnesota contain an estimate of 4,200,000,000,000 tons of iron 

 ore, all (or nearly all) of which was laid down by iron bacteria. 

 Molisch and Ellis have stated that the iron bacteria can thrive 

 without iron and attribute the deposits to physicochemical proc- 

 esses rather than to vital ones, but the majority of workers in 

 this field agree with Winogradsky that we have to deal here with 

 a group of autotrophic bacteria. 



According to Sohngen, there are bacteria which can oxidize 

 methane to carbon dioxide and water, using the resultant energy 

 in a similar manner: 



CH 4 +20 2 = C0 2 +2H 2 0. 



Others have been reported which can oxidize carbon monoxide, 

 charcoal, zinc, manganese, etc.; and it may be expected that still 



