Geoscience Reference
In-Depth Information
F
IG. 47
Azotobacter
cells, showing capsules. (Photomicrograph of a negatively stained preparation, by courtesy of
Christine Hepper, Rothamsted Experimental Station, magnification x 2000.)
Of course, the turnover of carbon and nitrogen in their respective cycles is more
complicated than so far described. We tend to separate our knowledge into distinct
compartments, perhaps for ease of classification. It is worth pointing out, therefore,
that the apparently simple autotrophic nitrifying bacteria also play their part in car-
bon cycling because they use carbon dioxide as a source of carbon for building up
the cell substance. There are many other examples of microorganisms performing a
variety of roles in element cycling in processes that occur constantly in the soil. Some
nitrogen-fixing bacteria, like the free-living
Azotobacter
species (
Fig. 47
) or the sym-
biotic
Rhizobium
species do other things besides fixing nitrogen. There are
Rhizobi-
um
species which are capable of degrading certain synthetic pesticides; likewise some
Azotobacter
species degrade a variety of synthetic chemicals, and both types of mi-
croorganisms possess capsules filled with gum-like polysaccharides which may take
part in cementing soil particles together and so affect soil structure.
Other autotrophic bacteria found occasionally in soils include sulphur-oxidizing
bacteria of the genus
Thiobacillus
and iron-oxidizing species, such as
Thiobacillus
ferrooxidans
, which obtain their energy for growth by the oxidation of ferrous to fer-
ric iron compounds; thiobacilli also oxidize sulphur to sulphuric acid. On occasions
when sulphur is added to soil, it can be oxidized by these bacteria to sulphuric acid
and so alkaline or neutral soils are made more acid. There are also hydrogen-oxid-
