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J.
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J
J__J_
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_J J_ __i i _i _i
iJ
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25
%
The“Nitr0g{èn Cycie
_
Î
-
Inî‘ariy enclosed container of mature water,*organic material suchas ' _
fish exCreta‘., uneaten food -or dead bodies is automatically conVerted
__
f
intô mineral formby heterotr0phic bacteria. These organiSms feed '
only on organic substances and develop quite easily-and naturally
'
‘
whereVer sufcient food of this kind is available. For practical pur—
*
poses the result of:their activity is the production from waste material
_
_
of substances such as ammonia, which in sufficient quantities are
toxic to fishes and other animals. This process of breaking organic
_
matter into simple cher’nicalsis a fundamental part of what is known
as
the nitrogen cycle (Figure4 and Table 3), and’when the cycle is
completed the wastes are' eventually converted into nitrogen and
nitrous oxide, both of which are harmless to the inhabitants of the
_
-'
'environment. Alternatively, the nitrogenous mineralS (nitrites and
nitrates) are absorbed by plants, and then by some animals, thereby‘
_
_
completingthe cycle (see Figure 4).
‘‘
‘
Organic Waste
_
‘
‘
‘
_
'
_
MINERALISATION
\
,
‘
_
(heterotrophic
Animal
_‘
_
bacteria)
'
.
,,
Growth
'
_
f.
.
_
_
Ammonia and
‘
Animal F°°d
_
_
"
‘
similar chemicals
_
,
.
,
'
_
‘
‘
Plant
,
,
‘
(autotrophic
‘
Growth
'
'
:"
__‘_
bacteria)
'
‘
'
'
.
_
,
_,
,
___
‘
‘
Food
"
‘
‘
‘
"
_‘
'
__
N—itriteand
‘
'
-‘
*.‘
.
'
.
.
Nitrate
_
‘
‘ _ ___:
,
DE- NI TRI FI CATI ON\
‘
‘
,
_
"
.
'
“
‘
—
Nitrogen & 'Nitrous Oxide
«
_'Î_—_____
‘
__
‘
‘
‘
Fig. 4. The nitrogen cycle
'
‘
‘
_
The problem, in completing the nitrogen cycle comes at the second
where by various means the highly toxic ammonia is
_:
to,the less harmful nitrates and nitrites by autotrophic
on _:inorganic substances and use Carbon d10_x1de
in the ,
conversmn
Whereàs the development of heterotrophrc
_