Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 42

is not the only route of entry of carbon dioxide into carbohydrate synthesis, the
other being the Hatch Slack Pathway. This subject is discussed in more detail
later in this chapter.
Returning to the Calvin Cycle, rearrangements of 3-phosphoglycerate produced
by rubisco then take place by similar steps to the reversible steps of glycoly-
sis and reactions of the pentose phosphate pathway. After completion of three
cycles, the net result is the fixation of three molecules of carbon dioxide into
a three carbon sugar, each cycle regenerating a ribulose phosphate molecule.
After phosphorylation of the trioses at the expense of ATP, they may enter into
glycolysis and be converted into glucose and then to starch to be stored until
required. The Calvin Cycle is so familiar that it is easy to overlook the fact
that not all reducing equivalents are channelled through rubisco to this cycle and
carbohydrate synthesis.
Some organisms may use additional pathways involving other electron
acceptors such as nitrate, nitrogen and hydrogen atoms, the reduction of
which obviously does not produce carbohydrate but different essential
nutrients which may then also be available to other organisms. These products
are summarised in Figure 2.11. For example, when nitrogen or nitrate is the
electron donor, the product is ammonia which becomes incorporated by amino
transfer into amino acids and thus forms part of the nitrogen cycle. Nitrogen
is a particularly noteworthy case in the context of this topic as it is the process
of nitrogen fixation. This is performed by a number of nitrogen fixing bacteria,
some of which are free living in the soil and some form symbiotic relationships
with certain leguminous plants, forming root nodules. Nitrogen fixation is, by
necessity, an anaerobic process, and so one essential role for the plant is to pro-
vide a suitable oxygen-free environment for these bacteria, the other, to provide
energy. Genetic manipulation of plants is discussed in Chapter 9 but it is relevant
to point out here that the suggestion is often mooted of introducing the genes
responsible for allowing nitrogen fixation to be transferred from the relevant
bacteria into suitable plants. The symbiotic relationship between plant and bac-
teria is very difficult to create artificially and has been a stumbling block in the
drive to increase the number of plant species able to host nitrogen fixation. The
complicated interaction between plant and bacterium involves intricacies of plant
physiology as well as genetic capability provided by the bacterium, and so it is
unlikely that a simple transfer of nitrogen fixation genes from bacterium to plant
will be successful. This, however, remains a research area of major importance.
The issue of nitrogenous material, particularly in respect of sewage and asso-
ciated effluents, is of considerable relevance to the environmental application
of biotechnology. In addition, there is great potential for phytotechnological
intervention to control nitrogen migration, most especially in the light of the
burgeoning expansion of nitrate sensitive areas within the context of agricultural
fertiliser usage. Hence, bio-engineering of the nitrogen cycle, at least at the local
level, provides an important avenue for the control of pollution and the mitigation
of possible eutrophication of aquatic environments. The cycle itself, and some
of the implications arising, are discussed later in this chapter.