In this article we will discuss about the nutritional requirements of microorganisms.
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.
About 95% of cellular dry weight of microbial cells is accounted for only six macro (major) elements (O, H, C, N, P and S). However, approximate percentage of dry weight and general physiological functions of major mineral nutrients are given in Table 18.1.
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.
These organic compounds not only provide the carbon for synthesis but also meet the energy requirement by entering into energy yielding metabolic pathways and are eventually oxidised to CO2.
Some microbes have the ability to synthesize all 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. Some microbes can utilize more than one carbon compound and exhibit a great degree of versatility. The others, however, are specialized in this regard.
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. 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’.
As opposed to this there are a few organisms which do not use molecular oxygen as terminal electron acceptor. We recall that oxygen is a component of the cellular material of all the microorganisms. These microbes are called ‘obligate anaerobes’.
In fact, molecular oxygen is toxic to these organisms. Aerobes which can grow in the absence of oxygen are called ‘facultative anaerobes’ and the anaerobes which can grow in the presence of oxygen are referred to as ‘facultative aerobes’. In addition to these major classes, there are organisms which grow best at reduced oxygen pressure but are obligate aerobes and these are called ‘Microaerophilic’.
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.
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. Some important vitamins and their functions are summarized in Table 18.3.
Since the growth factors fulfill specific needs in biosynthesis of certain molecules, they are needed in very small amounts; the vitamins even in less smaller quantities, because of the various coenzymes of which they are precursors, have catalytic roles and consequently are present at levels of a few parts per million in the microbial cell.