258 Environmental Biotechnology
printers and a range of other sources including furnishing manufactured using
synthetic materials. Bacteria resident in the soil of potted plants in the office are
able to degrade many of these volatiles, which include phenolics, formaldehyde
and trichloroethylene, thus improving the air quality. The plants themselves con-
tribute to this improvement, not only by supporting the rhizosphere microbial
community, but also by producing oxygen during photosynthesis described in
Chapter 2. In essence, this is root zone phytotechnology on a small scale.
Although straying away slightly from the remit of this section, it is interesting
to point out that soil microbial activity has a major influence in the balance of
stable atmospheric gases. These include the greenhouse gases, carbon dioxide,
nitrous and nitric oxide and methane, so called because they trap heat re-emitted
by earth from energy radiated by the sun. The atmospheric balance of less stable
gases which include ammonia, hydrogen sulphide and dimethylsulphide are also
subject to microbial activity, as will be apparent from an understanding of the
foregoing chapters on metabolism. A final word on soil microbes concerns the
degradation of lignin. This is a major constituent of woody plant material and
is recalcitrant to degradation. However, filamentous fungi are responsible for its
degradation worldwide, augmented in the tropics by bacteria living in the gut of
termites. This degradation requires the presence of oxygen, hence wood residing
in anaerobic conditions is somewhat protected. Clearly, should the water table
drop, exposing to air the wood pilings supporting buildings, for example, there is
a danger of invasion by filamentous fungi able to degrade lignin and thus weaken
the building structure. This also explains in part the necessity to aerate a compost
heap containing any woody plant material to allow invasion by filamentous fungi
capable of degrading lignin.
Microbes internal to the plant
Two categories fulfil this description. The first are the internal fungi or endomy-
corrhizae, referred to in the earlier section, together with the endophytic bacteria
and the second comprises plant pathogens, which may be bacterial or viral in
form. Although the term ‘endophytic’ seems unambiguous, it is sometimes used
to describe only bacteria which may be isolated from plants which have been
superficially cleansed with disinfectant, or isolated from within plant tissue and
which cause no discernible harm to the plant. Thus defined, plant pathogens are
excluded from this description. There are endophytic bacteria, called commen-
sals, which neither benefit nor harm the plant, but there are also those which are
beneficial to plant growth. These are symbiots which achieve this status either
by promoting plant growth or by protection against plant pathogens.
Symbiotic nitrogen fixation
The classic example of plant growth stimulation by plant/microbe symbiosis is
nitrogen fixation byRhizobiumbacteria within plant root tissue. A full expo-
sition of nitrogen fixation may be found in many classical textbooks and so