Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 37

and water releasing the energy consumed originally to synthesise the molecule.
Here is another example of a natural cycle, where carbon is introduced, as car-
bon dioxide, into the synthesis of a sugar which is then interconverted through
the various metabolic pathways until finally it is released as carbon dioxide thus
completing the cycle. Eukaryotes capable of carrying out photosynthesis include
higher green plants, multicellular green, brown and red algae and various uni-
cellular organisms such as the euglenoids and dinoflagellates, both of which are
commonly found in fresh water environments, and diatoms which are also found
in salt water. The diatoms which are unicellular algae, are particularly notewor-
thy given estimates that they are responsible for fixing 20-25% of the world's
carbon through photosynthesis (Round, Crawford and Mann, 1990). Prokaryotes
capable of photosynthesis include blue-green algae, and both the sulphur and
non-sulphur purple and green bacteria. The blue-green algae which are oxygenic
bacteria and are alternatively named cyanobacteria, operate light reactions very
similar to those of eukaryotes. Conversely, the green and the purple non-sulphur
bacteria which are both facultative aerobes and the strictly anaerobic green and
the purple sulphur bacteria utilise a rather different set of light reactions as a con-
sequence of their possessing a 'simpler' photosystem. Eukaryotic and bacterial
systems are both described in the following sections.
The light reactions
Visible light is the outcome of the nuclear fusion of hydrogen atoms, resulting in
the production of helium atoms, gamma radiation and two electrons. This fusion
occurs in the sun at a temperature of approximately 20 000 000 K. The gamma
radiation and electrons combine to produce quanta of visible light. The entrap-
ment of light is performed in photosynthetic cells by pigments; the most important
of which are the chlorophylls. These are flat ring structures, with regions of con-
jugated double and single bonds, and a long hydrophobic tail well designed for
anchoring the pigments into membrane. Only red and blue light is absorbed by
the chlorophylls in most organisms, consequently, when white light from the
sun shines upon them, they reflect green light, thus making these organisms
appear green. Variation in the types of chlorophylls and the presence of addi-
tional accessory pigments all contribute to the observed colour of the organism
and are the result of evolution which has developed the 'best fit' of light trapping
molecules to suit the ecological niche of the organism. It is worth pointing out
that wholesale transport of the plant or bacterium for biotechnology purposes
has to take this factor into account. It is important to test the growth and perfor-
mance characteristics of any translocated plant or bacterium to ensure that the new
environment does not produce disappointing results. This problem is addressed
with respect to choice of Phragmites species in the case study on reed beds
in Chapter 7. The purpose of the accessory pigments referred to above, which
include the carotenoids and phycobiliproteins, the latter found in red algae and
cyanobacteria, is to extend the range of absorbed wavelengths thus maximising
the amount of energy trapped from light and protecting the photosynthetic system