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Summary of Nutritional Requirement :-

Mineral Nutrients - The microbial nutrients can be classified as macro (major) nutrients, and micro (minor) nutrients or trace elements on the basis of their amount required.

1. Macro or Major Mineral Nutrients :-
The microbial cells contain water accounting for some 80-90% of their total weight and, therefore, the water is always the major essential nutrient in quantitative terms.

The solid matter of cells contain, in addition to oxygen and hydrogen (derivable metabolically from water), the other macro (major) elements, namely, carbon, nitrogen, phosphorus, sulphur, potassium, magnesium, sodium, calcium and iron in order of decreasing abundance.

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.

2. Micro or Minor Mineral Nutrients or Trace Elements :-
The microorganisms, in general do not use only macro (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 (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. Table 18.2 summarizes the major micronutrients of living systems and gives examples of enzymes in which each plays a role. Some microorganisms, however, need additional specific mineral nutrients, for example, diatoms and some microalgae require silica, supplied as silicate, to impregnate their cell walls.

Growth Factors :-
Besides the mineral nutrients, the microorganisms need some organic compounds. Most of the microorganisms are capable of synthesizing these organic compounds from simpler carbon resources, others cannot and need their supply from outside for their proper growth and development.

Organic nutrients of this type are known collectively as growth factors (essential metabolites) and can be categorized into three groups (amino acids, purines and pyrimidines and vitamins) on the basis of their chemical structure and metabolic function.

Amino acids and purines and pyrimidines are the constituents of proteins and nucleic acids, respectively. Vitamins, however, are the most commonly needed growth factor and form parts of the prosthetic groups or active centres of certain enzymes.