SOIL ORGANISMS - Soil Ecology - page 207

2.1.5 ALGAE

The soil algae are a rather heterogenous assemblage of uni- and multicellular, motile
and immotile eukaryote organisms. They comprise several hundreds of different taxa.
They appear to become important only in extreme conditions such as dry deserts or
extremely nutrient poor systems. Nonetheless, the roles they play in, e.g., humid and
temperate ecosystems, may well have been underestimated since they have been little
studied to date. On the other hand, they are highly sensitive to all the relevant parameters
of soil fertility and pollutants e.g., pH, nutrients and pesticides (Pipe and Shubert, 1984).

2.2 Quantitative estimates

The satisfactory estimation of microbial abundance in soils has long been limited by
the lack of reliable techniques. Improved methods are presently available and a better
appreciation of the distribution patterns and biology of micro-organisms is helping to
define appropriate sampling designs.
Bacteria and fungi are the major components of microbial communities and generally
account for most of their biomass. Actinobacteria are often included with the bacteria
and may occasionally comprise 30 to 50 % of overall abundance.

2.2.1 TECHNIQUES FOR ESTIMATING ABUNDANCE

Micro-organism abundances may be assessed by direct methods such as counting cells
or analysing soils for specific components of their cells. Indirect or “physiological”
methods may be preferred since they are less time-consuming to perform and provide
a valuable index assessment of microbial functions in soils (see reviews of Jenkinson
and Ladd, 1981; Nicolardot et al., 1982; Parkinson, 1982).
(a) Indirect counting by the soil dilution plate method has long been the most com-
monly used method. Dilution series are prepared from a soil suspension and inoculated
on to an appropriate solid or liquid culture media. The number of colonies that develop
gives an estimate of the number of colony forming units (CFU). Despite the use of a
variety of culture media, this method generally results in underestimations of 2 to 4 orders
of magnitude and its use should be restricted to semi-quantitative estimates in compara-
tive studies (Brierley et al., 1928 and detailed procedures in Pochon and Tardieux, 1962).
The major drawbacks of this method are (i) that culture media are selective towards
particular groups of micro-organisms and are exaggeratedly nutrient-rich in comparison
to soil conditions and (ii) that a single fungal CPU may be derived from either a spore
or from a large piece of mycelium, which are not comparable in either biomass or
associated activity.
(b) Direct counts of bacterial cells and measurements of fungal hyphae may be
conducted on thin soil sections or, rather, thin agar films prepared from a soil suspension
(Jones and Mollison, 1948 modified by Thomas et al., 1965) or through filtration of
determined amounts of the suspension and direct observation of the filtrate (Sundman
and Sivelä, 1978; Bingle and Paul, 1986).