Microbes and Metabolism 37the cycle. Eukaryotes capable of carrying out photosynthesis include higher green
plants, multicellular green, brown and red algae and various unicellular 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 noteworthy given the
current estimates that they are responsible for fixing 20 to 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
nonsulphur 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 nonsulphur
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 additional acces-
sory 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 fac-
tor into account. It is important to test the growth and performance 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
ofPhragmitesspecies 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 from potential damage by
oxidation. A rather unusual pigment which functions as a primary pigment, is
bacteriorhodopsin which causes the archaea which express it, to appear purple.
Returning to the principally eukaryotic process, the chlorophylls described above