Geoscience Reference
In-Depth Information
different carbon compounds, including hydrocarbons, tannins, lignin, rubber and sev-
eral synthetic plant protection chemicals.
Chapter 3
described the nitrogen fixing abil-
ity of
Frankia
in association with plant roots.
Myxobacteria represent another group of peculiar soil microorganisms with a
complicated life cycle. They consume other typical living soil bacteria and, therefore,
can be regarded as predators. They are found and can be isolated from sheep or rabbit
dung pellets. Myxobacteria can be grown on plate surface cultures of various ordinary
bacteria. In their life cycle, masses of myxobacterial cells come together in swarms
and gradually develop into fruiting bodies, which are large enough to be seen with
the naked eye and are often brightly coloured. Fruiting body formation seems to oc-
cur in response to declining nutrient supply. The swarms of vegetative cells, which
possess gliding motility, gradually differentiate into a fruiting body with a stalk and
a head. Most of the cells accumulate in the fruiting body head where they become
transformed into myxospores. In due course, germination of the myxospore signals a
fresh round of the life cycle.
Algae, which include both unicellular and filamentous species, are often found
in soil. In common with green plants, they are photosynthetic organisms which con-
tain chlorophyll and assimilate carbon dioxide in the presence of light. Yellow-green
forms belonging to the class Xanthophyceae are one of the most common groups. Be-
cause of their dependence on light, they grow on or near the soil surface. The blue-
green Cyanobacteria were formerly considered to be algae; their importance in rice
paddy fields has already been mentioned.
Under suitable conditions, extensive blooms of coloured algae may develop in
summer on the surface of waters, both inland and marine. Algal cells often contain
other pigments besides chlorophyll so that deep red coloured algal blooms may form
on the surface of waters, as for example, on Lake Tovel in the Dolomite area of Italy.
This brief account gives only a general impression of the better known members
of the very diverse microbial world and its wide range of activities. In spite of their
small size, microorganisms are metabolically very active and consequently play an
important role in the living soil. Their rapid rate of multiplication enables many
generations to be produced in a short time span and also for mutations and genet-
ic changes to occur with some frequency. Adaptations and metabolic changes result
from these genetic alterations. Great advances have been made in recent years in mi-
crobial genetics and the transfer of genes from one bacterial species to another has
become possible. Such transfers can also be mediated by viable fractions of the cell
known as plasmids which are motile and can be incorporated in other cells. Bacteria
or other microorganisms thus acquire new enzymes.
