Microorganisms require nutrients for growth and metabolism. There are two categories of essential nutrients: macro-nutrients which are needed in large amounts to maintain cell structure and metabolism, and micro-nutrients which are needed in trace amounts to help enzyme function and maintain protein structure. Microorganisms obtain carbon, nitrogen, and other macro-nutrients from both inorganic and organic sources, while micro-nutrients like metals serve as catalysts in enzymes. Microorganisms are also classified based on their energy and electron sources as phototrophs or chemotrophs, and lithotrophs or organotrophs.

1. NUTRITION
 Nutrients are materials that are acquired from the environment and are used for growth and metabolism.
 Microorganisms vary significantly in terms of the source, chemical form, and amount of essential
elements they need.
 Examples: carbon, oxygen, hydrogen, phosphorus, and sulfur.
TYPES OF NUTRITION:
There are two categories of essential nutrients:
1. MACRO-NUTRIENTS - Macro-nutrients usually help maintain the cell structure and metabolism.
(Which are needed in large amounts).
2. MICRO-NUTRIENTS - Micro-nutrients help enzyme function and maintain protein structure. (Which
are needed in trace or small amounts).
MACRO NUTRITION:
 The microbial cells contain 80-90% water for their total weight and, therefore, the water is always the
major essential nutrient.
 The solid matter of cells contain, in addition to oxygen and hydrogen, and other macro elements are
carbon(C), nitrogen(N), phosphorus(P), sulphur(S), potassium(K), magnesium(Mg), sodium(Na),
calcium(Ca) and iron.
 Carbon assumes great importance as the main constituent of all organic cell materials and represents
about 50% of cell’s dry weight. CO2 is the most oxidized form of carbon and the photo-synthetic
microorganisms reduce CO2 to organic cell constituents. On the other hand, all the non-photosynthetic
microorganisms obtain their carbon requirement mainly from organic nutrients which contain reduced
carbon compounds.
 Some microbes have the ability to synthesize their cellular components using a single organic carbon
source while others, in addition to this one major carbon source, also need other complex carbon
containing components which they cannot synthesize. These components are called growth factors and
include vitamins.
 Sulphur and nitrogen are taken up by most organisms and are subsequently reduced within the cell and
utilized in other biosynthetic processes. The sulphur and nitrogen requirements of most organisms can
also be met with organic nutrients that contain these two elements in reduced organic combinations such
as amino acids.

2.  A few microorganisms are capable of reducing elemental nitrogen to ammonia and this process of
nitrogen assimilation is known as biological nitrogen fixation.
 Most of the microorganisms need molecular oxygen for respiration. In these, the oxygen serves as
terminal electron acceptor, and such organisms are referred to as ‘obligate aerobes’.
MICRO NUTRITION:
 The microorganisms, in general do not use only macro or major elements but also others like cobalt,
copper, manganese, molybdenum, nickel, selenium, tungsten, vanadium and zinc which are required in
residual fraction by nearly all microorganisms.
 These elements are often referred to as minor or micro nutrients or trace elements. The micronutrients or
trace elements are nevertheless just as critical to cell function as are the macronutrients.
 They are metals playing the role of cell’s catalysts and many of them are play a structural role in various
enzymes.
 Some microorganisms, however, need additional specific mineral nutrients, for example, diatoms and
some microalgae require silica, supplied as silicate, to impregnate their cell walls.
CLASSIFICATION MICROORGANISM BASED ON NUTRITION UTILIZATIONS:
1. Based on energy source
I. Phototrophs
II. Chemotrophs
2. Based on electron source
I. Lithotrophs
II. Organotrophs
PHOTOTROPHS:
The organisms which can utilize light as an energy source are known as phototrophs. These bacteria
gain energy from light.
CHEMOTROPHS:
These bacteria gain energy from chemical compounds. They cannot carry out photosynthesis.
LITHOTROPHS:
Some organisms can use reduced organic compounds as electron donors and are termed as Lithotrophs.
They can be Chemolithotrophs and Photolithotrophs

3.  Photo-lithotrops: These bacteria gain energy from light and use reduced inorganic compounds
such as H2S as a source of electrons.
eg: Chromatium okeinii.
 Chemo-lithotrophs: These bacteria gain energy from reduced inorganic compounds such as
NH3 as a source of electron
eg; Nitrosomonas.
ORGANOTROPHS:
Some organisms can use organic compounds as electron donors and are termed as organotrophs.
Some can be Chemoorganotrophs and Photoorganotrophs.
 Photo-organotrophs: These bacteria gain energy from light an d use organic compounds such
as Succinate as a source of electrons.
eg; Rhodospirillum.
 Chemo-organotrophs: These bacteria gain energy from organic compounds such as glucose
and ammino acids as a source of electrons.
eg; Pseudomonas pseudoflora.
Some bacteria can live ether chemo-lithotrophs or chemo-
organotrophs like Pseudomonas pseudoflora as they can use either glucose or H2S as electron source.