Microbes and Metabolism 13origins of the eukaryotic cell. There are many theories but the one which appears
to have the most adherents is the endosymbiotic theory. It suggests that the ‘proto’
eukaryotic cell lost its cell wall, leaving only a membrane, and phagocytosed or
subsumed various other bacteria with which it developed a symbiotic relationship.
These included an aerobic bacterium, which became a mitochondrion, endowing
the cell with the ability to carry out oxidative phosphorylation, a method of pro-
ducing chemical energy able to be transferred to the location in the cell where it is
required. Similarly, the chloroplast, the site of photosynthesis in higher plants, is
thought to have been derived from cyanobacteria, the so-called blue-green algae.
Chloroplasts are a type of plastid. These are membrane-bound structures found
in vascular plants. Far from being isolated cellular organelles, the plastids com-
municate with each other through interconnecting tubules (Kohler ̈ et al. 1997).
Various other cellular appendages are also thought to have prokaryotic origins
such as cilia or the flagellum on a motile eukaryotic cell which may have formed
from the fusion of a spirochete bacterium to this ‘proto’ eukaryote. Nuclei may
well have similar origins but the evidence is still awaited.
No form of life should be overlooked as having a potential part to play in
environmental biotechnology. However, the organisms most commonly discussed
in this context are microbes and certain plants. They are implicated either because
they are present by virtue of being in their natural environment or by deliberate
introduction.
Microbes
Microbes are referred to as such, simply because they cannot be seen by the
naked eye. Many are bacteria or archaea, all of which are prokaryotes, but the
term ‘microbe’ also encompasses some eukaryotes, including yeasts, which are
unicellular fungi, as well as protozoa and unicellular plants. In addition, there
are some microscopic multicellular organisms, such as rotifers, which have an
essential role to play in the microsystem ecology of places such as sewage treat-
ment plants. An individual cell of a eukaryotic multicellular organism like a
higher plant or animal, is approximately 20 microns in diameter, while a yeast
cell, also eukaryotic but unicellular, is about five microns in diameter. Although
bacterial cells occur in a variety of shapes and sizes, depending on the species,
typically a bacterial cell is rod shaped, measuring approximately one micron in
width and two microns in length. At its simplest visualisation, a cell, be it a
unicellular organism, or one cell in a multicellular organism, is a bag, bounded
by a membrane, containing an aqueous solution in which are all the molecules
and structures required to enable its continued survival. In fact, this ‘bag’ rep-
resents a complicated infrastructure differing distinctly between prokaryotes and
eukaryotes (Cavalier-Smith 2002), but a discussion of this is beyond the scope
of this book.
Depending on the microbe, a variety of other structures may be present, for
instance, a cell wall providing additional protection or support, or a flagellum,