66 AQUATIC CONDITIONS 
fixing bacteria, such as Azotobacter, an aerobe, and Clostridium, an 
anaerobe. These bacteria occur on the outside of plants and animals, 
in the mud of the bottom, etc. Plants and animals provide carbon for 
the bacteria; bacteria provide the nitrites or nitrates for the plants. 
Ammonia, resulting from the decomposition of proteid of the dead 
bodies of plants and animals, is oxidized to nitrous acid; nitrous acid is 
oxidized to nitric acid by the bacteria (Nitrosomonas, Nitrobacter, Nitro- 
coccus). This acid unites with bases to form nitrates and nitrites. 
There are accordingly two sources of nitrate and nitrite. Working 
against these are the denitrifying bacteria (Bacterium actinopelte [Baur)) 
which reduce nitrogen compounds to free nitrogen. Their work is 
influenced by temperature. Baur placed a standard quantity of nitrate 
infected with Bacterium actinopelte at several temperatures (47, p. 271) 
with results as follows: 
1. Temperature 25° C.: Denitrification began 24 hours after inocu- 
lation; in 7 to 11 days later the solution was nitrate-free. 
2. Temperature 15° C.: Denitrification began 4 days after inoculation; 
in 27 days the solution was nitrate-free. 
3. Temperature 4-5° C.: Denitrification began 20 days after inocula- 
tion; process incomplete 112 days after. 
4. Temperature 0° C.: Denitrification not initiated. 
The quantity of life in water has been held by some to be in propor- 
tion to the available nitrogen. The amount of plankton in the sea is 
greatest in the polar regions in summer. It has been suggested that 
the greater retarding effect of low temperature on the denitrifiers, as 
compared with the producers of nitrates, is a cause of the greater quantity 
of life in colder waters. Atmospheric nitrogen in solution is important 
in the building of nitrogen compounds by nitrogen-fixing bacteria. 
Oxygen is necessary for the life of most organisms, though a few can 
live for considerable periods in its absence. Carbon dioxide is necessary 
for starch building by chlorophyll-containing plants and animals. 
These organisms form the principal (food) basis of all other organisms. 
Complex foodstuffs, such as proteids, are necessary for most animals. 
It is only animals which contain chlorophyll in the form of algae living 
symbiotically in their bodies, or otherwise, that can live without taking 
in proteid from the outside. Proteids are made only when light for the 
production of starch, nitrates, and several other inorganic foods are 
present. Light is then indirectly necessary to animals which can live 
in darkness. 
The smaller aquatic animals are commonly either alga-eaters or 
predatory. The larger aquatic animals are commonly predatory or 
