Nutritional Categories
Nutritional categories
Microorganisms can be categorised according to how they obtain their carbon and
energy. As we have seen, carbon is the most abundant component of the microbial cell,
and most microorganisms obtain their carbon in the form of organic molecules, derived
directly or indirectly from other organisms. This mode of nutrition is the one that is
familiar to us as humans (and all other animals); all the food we eat is derived as complex
organic molecules from plants and other animals (and even some representatives of the
microbial world such as mushrooms!). Microorganisms which obtain their carbon in
82 MICROBIAL NUTRITION AND CULTIVATION
A heterotroph must use
one or more organic
compounds as its source
of carbon.
this way are described as heterotrophs, and include all
the fungi and protozoans as well as most types of bacteria.
Microorganisms as a group are able to incorporate
the carbon from an incredibly wide range of organic
compounds into cellular material. In fact there
is hardly any such compound occurring in nature that
cannot be metabolised by some microorganism or other,
explaining in part why microbial life is to be found thriving in the most unlikely habitats.
Many synthetic materials can also serve as carbon sources for some microorganisms,
which can have considerable economic significance.
An autotroph can derive
its carbon from carbon
dioxide.
A significant number of bacteria and all of the algae
do not, however, take up their carbon preformed as organic
molecules in this way, but derive it instead from
carbon dioxide. These organisms are called autotrophs,
and again we can draw a parallel with higher organisms,
where all members of the plant kingdom obtain
their carbon in a similar fashion.
A chemotroph obtains
its energy from chemical
compounds. A phototroph
uses light as its
source of energy.
We can also categorise microorganisms nutritionally
by the way they derive the energy they require to carry
out essential cellular reactions. Autotrophs thus fall into
two categories. Chemoautotrophs obtain their energy
as well as their carbon from inorganic sources; they
do this by the oxidation of inorganic molecules such
as sulphur or nitrite. Photoautotrophs have photosynthetic
pigments enabling them to convert light energy
into chemical energy. The mechanisms by which this is achieved will be discussed in
Chapter 6.
The great majority of heterotrophs obtain energy as well as carbon from the
same organic source. Such organisms release energy by the chemical oxidation of organic
nutrient molecules, and are therefore termed chemoheterotrophs. Those few heterotrophs
which do not follow this mode of nutrition include the green and purple
non-sulphur bacteria. These are able to carry out photosynthesis and are known as
photoheterotrophs.
There is one final subdivision of nutritional categories in microorganisms! Whether
organisms are chemotrophs or phototrophs, they need a molecule to act as a source
A lithotroph is an organism
that uses inorganic
molecules as a source
of electrons. An organotroph
uses organic
molecules for the same
purpose.
of electrons (reducing power) to drive their energygenerating
systems (see Chapter 6). Those able to use an
inorganic electron donor such as H2O, H2S or ammonia
are called lithotrophs, while those requiring an organic
molecule to fulfil the role are organotrophs. Most (but
not all) microorganisms are either photolithotrophic autotrophs
(algae, blue-greens) or chemo-organotrophic
heterotrophs (most bacteria). For the latter category, a
single organic compound can often act as the provider of
carbon, energy and reducing power. The substance used
by chemotrophs as an energy source may be organic (chemoorganotrophs) or inorganic
(chemolithotrophs).
Microorganisms can be categorised according to how they obtain their carbon and
energy. As we have seen, carbon is the most abundant component of the microbial cell,
and most microorganisms obtain their carbon in the form of organic molecules, derived
directly or indirectly from other organisms. This mode of nutrition is the one that is
familiar to us as humans (and all other animals); all the food we eat is derived as complex
organic molecules from plants and other animals (and even some representatives of the
microbial world such as mushrooms!). Microorganisms which obtain their carbon in
82 MICROBIAL NUTRITION AND CULTIVATION
A heterotroph must use
one or more organic
compounds as its source
of carbon.
this way are described as heterotrophs, and include all
the fungi and protozoans as well as most types of bacteria.
Microorganisms as a group are able to incorporate
the carbon from an incredibly wide range of organic
compounds into cellular material. In fact there
is hardly any such compound occurring in nature that
cannot be metabolised by some microorganism or other,
explaining in part why microbial life is to be found thriving in the most unlikely habitats.
Many synthetic materials can also serve as carbon sources for some microorganisms,
which can have considerable economic significance.
An autotroph can derive
its carbon from carbon
dioxide.
A significant number of bacteria and all of the algae
do not, however, take up their carbon preformed as organic
molecules in this way, but derive it instead from
carbon dioxide. These organisms are called autotrophs,
and again we can draw a parallel with higher organisms,
where all members of the plant kingdom obtain
their carbon in a similar fashion.
A chemotroph obtains
its energy from chemical
compounds. A phototroph
uses light as its
source of energy.
We can also categorise microorganisms nutritionally
by the way they derive the energy they require to carry
out essential cellular reactions. Autotrophs thus fall into
two categories. Chemoautotrophs obtain their energy
as well as their carbon from inorganic sources; they
do this by the oxidation of inorganic molecules such
as sulphur or nitrite. Photoautotrophs have photosynthetic
pigments enabling them to convert light energy
into chemical energy. The mechanisms by which this is achieved will be discussed in
Chapter 6.
The great majority of heterotrophs obtain energy as well as carbon from the
same organic source. Such organisms release energy by the chemical oxidation of organic
nutrient molecules, and are therefore termed chemoheterotrophs. Those few heterotrophs
which do not follow this mode of nutrition include the green and purple
non-sulphur bacteria. These are able to carry out photosynthesis and are known as
photoheterotrophs.
There is one final subdivision of nutritional categories in microorganisms! Whether
organisms are chemotrophs or phototrophs, they need a molecule to act as a source
A lithotroph is an organism
that uses inorganic
molecules as a source
of electrons. An organotroph
uses organic
molecules for the same
purpose.
of electrons (reducing power) to drive their energygenerating
systems (see Chapter 6). Those able to use an
inorganic electron donor such as H2O, H2S or ammonia
are called lithotrophs, while those requiring an organic
molecule to fulfil the role are organotrophs. Most (but
not all) microorganisms are either photolithotrophic autotrophs
(algae, blue-greens) or chemo-organotrophic
heterotrophs (most bacteria). For the latter category, a
single organic compound can often act as the provider of
carbon, energy and reducing power. The substance used
by chemotrophs as an energy source may be organic (chemoorganotrophs) or inorganic
(chemolithotrophs).