42 Environmental Biotechnology
to form two molecules of 3-phosphoglycerate, an intermediate of glycolysis. This
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 accep-
tors such as nitrate, nitrogen and hydrogen atoms, the reduction of which obvi-
ously 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 note-
worthy case in the context of this book 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 pro-
cess, and so one essential role for the plant is to provide a suitable oxygen-free
environment for these bacteria, the other, to provide energy. Genetic manipula-
tion 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 bacteria 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 trans-
fer 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, bioengineering of the nitrogen cycle, at least at the local