Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 14

origins 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 envi-
ronmental 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 deliber-
ate introduction.
Microbes
Microbes are referred to as such, simply because they can not 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. Individual cells of a eukaryotic multicellular organism like a higher
plant or animal, are approximately 20 μ m in diameter, while a yeast cell, also
eukaryotic but unicellular, is about 5 μ m 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 1 μ m in width and 2 μ m
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' represents a complicated infrastructure
differing distinctly between prokaryotes and eukaryotes (Cavalier-Smith, 2002),
but a discussion of this is beyond the scope of this topic.
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,
a flexible tail, giving mobility through the surrounding environment. Survival
requires cell growth, replication of the deoxyribonucleic acid (DNA) and then