Encyclopedia of Environmental Science and Engineering, Volume I and II

698 MICROBIOLOGY

aerobic fixation by

bacteria (Azotobacter),

blue green algae,

fungi

bacterial autotroph

bacterial autotroph

NH 4 +

NH 4 +

NO 2 –

NO 3

Nitrosomonas

Nitrobacter

algal autotroph

Ankistromonas

organic compounds

N-containing

(atmospheric)

leghemoglobin and other N-containing

organic compounds of higher plants

Animal protoplasm

N-containing animal wastes

[NO 2 ]

[N 2 O+]

anaerobic bacterial denitrification

by several organisms

Clostridium

Anaerobic fixation

bacterial bacterial symbiotic fixation

Rhizobium

N 2

FIGURE 12 The nitrogen cycle presented in a generalized way so that the role of various microorganisms are

indicated as well as the relationship to higher plants and animals. The precise arrangement will vary according to

several physical parameters of the natural environment.

since the degradation products may themselves be noxious.

Thus, the only degradation products which are acceptable

are those which may enter normal metabolism or are labile

enough to be further degraded to metabolizable compounds

by the physical conditions in the body in question.

Determination of Gross Populations of

Microorganisms

Quantitative sampling techniques are required for the deter-

mination of the microbial population in air, water, or soil

samples. The method used for sample collection must ensure

against (a) loss of more than a trivial number of microorgan-

isms and (b) cross-contamination from other sources during

sample transport and laboratory manipulation. Once the

sample has arrived in the laboratory, selective enrichment

techniques can be used to reveal the diversity of microorgan-

isms or mixed populations can be counted by using some

variation of the pour plate technique (see Figure 8).

A. Air sampling techniques Non-spore forming organ-

isms are rarely found in air samples because they

are too delicate to survive for long in the gener-

ally dehydrating conditions of atmospheric trans-

port. A convenient sampler consists of a sterile

membrane filter connected to a metering vacuum

pump. At the beginning of the sampling period, the

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