In this article we will discuss about the nutrition needed by autotrophic and heterotrophic bacteria.
Most of the bacteria lack chlorophyll. They are thus unable to synthesize organic compounds which they need as food from simple inorganic substances. They obtain these from the external sources readymade.
For this reason these organisms are put in the category of heterotrophic plants. However, there are some bacteria which are able to synthesize organic compounds necessary for their structure and metabolism from simple inorganic compounds.
These are called Autotrophic bacteria. The latter live on a strictly inorganic diet. The autotrophic bacteria resemble green plants in their mode of nutrition but with a difference.
The green plants utilize water as one of the raw materials in photosynthesis (food manufacturing process). The autotrophic bacteria, in contrast use, other compounds of hydrogen such as hydrogen sulphide (H2S) and methane (CH4) and not water with the result that no oxygen is evolved as a by-product in bacterial food manufacturing process.
The energy used is this case in either obtained from the sunlight (photosynthetic bacteria) or chemically by the oxidation of some inorganic substances present in the environment such as iron, sulphur, nitrogen compounds (Chemosynthetic bacteria).
1. Autophytic or Autotrophic Bacteria:
The autophytic bacteria are thus of two types, photosynthetic and chemosythetic. The former are also called photosynthetic autotrophs and the latter non-photosythetic autotrophs.
(a) Photosynthetic Bacteria:
These include all the three, coccal, bacillary and spirillum forms. All are anaerobes which grow in light and are usually found in sulphur springs where hydrogen sulphide is normally present.
The two common examples of photosynthetic bacteria are the purple sulphur bacteria and green sulphur bacteria. The former contain bacteriochlorophyll and carotenoids ad the photosynthetic pigments as the latter chlorobium chlorophyll.
The pigments under the light microscope appear to be uniformly distributed in the cytoplasm. Electron microscopy reveals that the pigments are associated with membranes which arise as invaginations from the plasma membrane and form a network of small spherical vesicles connected by narrow constrictions.
These bacteria are unique because they are the only organisms which are capable of synthesizing carbohydrate food without chlorophyll a. Moreover, photosynthesis in photosynthetic bacteria like Rhodospirillum is different from photosynthesis in green plants.
No free oxygen is released as a byproduct in bacterial photosynthesis. Hydrogen is provided by donor substances other than water. Hydrogen sulphide is the source of hydrogen. The by-product is sulphur and not oxygen.
The process takes place at low expenditure of energy. Chlorobium limicola is another example of photosynthetic autotrophs. It is a green sulfur bacterium. The hydrogen sulfide is the donor of hydrogen in this case also.
Light splits hydrogen sulphide in both cases. Hydrogen combines with carbon dioxide to form CH2O. The sulphur which is a by-product is stored as globules in the cells of sulphur bacteria but excreted in the green sulphur bacteria.
The reaction is:
There are non-sulphur purple and brown bacteria found in the mud and stagnant water. They are photosynthetic and contain bacteriochlorophyll pigment. They use organic hydrogen donors and sulphur is not the by-product in their case. Light is still the source of energy.
(b) Chemosynthetic Bacteria:
These are non-photosynthetic autotrophs (nitrifying bacteria) which lack pigments. They get energy for food synthesis by the oxidation of certain inorganic substances such as ammonia, nitrites, nitrates, ferrous iron, hydrogen sulphides and a number of other metallic and non-metallic materials available in the environment.
The bacteria absorb inorganic molecules of the substances into the body where a chemical reaction takes place. In this reaction the chemical bonds are broken and energy is released.
This energy is used up by the bacterial cell to combine CO2 and water into food molecules. This process of manufacturing food is called chemosynthesis. It is a chemical process and is evidently a form of respiration.
No light is involved in chemosynthesis. The reactions are all exothermic. These bacteria are thus independent both of light and of organic materials. They are unique because they can live even under conditions unfavourable for plant growth.
Examples of chemosynthetic bacteria are sulphur bacteria, iron bacteria, nitrifying bacteria and hydrogen bacteria.
Sulphur bacteria oxidise sulphur compounds present in water in which they live. The energy released in the reaction is utilized for food synthesis. Sulphur remains behind as a residue in the cytoplasm of the cell.
The reaction is represented by the following equation:
So is the case with iron bacteria. They inhabit water containing iron compounds. The iron bacteria oxidise the ferrous compounds to ferric form liberating energy.
The energy yielding reaction is:
The ferric iron is deposited as insoluble ferric hydroxide.
Nitrosomonas europaea and Thiobacillus denitrificans are the other examples of chemosynthetic autotrophs. The former obtains energy for food synthesis by the oxidation of ammonia.
The reaction is as follows:
In other nitrifying bacterium Nitrobacter, the only source of energy is the oxidation of nitrite.
It is represented by the following equation:
The oxidation of hydrogen gas by hydrogen bacteria is represented by the following equation:
The chemical energy released in the form of ATP in the above-mentioned examples is used by the bacterial cell to synthesize food molecules from the inorganic substances.
In all chemosynthetic bacteria the energy required for the food making process is thus obtained from their oxidative activities. The two processes of food synthesis in bacteria may be contrasted as under.
Chemosynthesis:
1. The amount of energy available in chemosynthesis is much less as compared with photosynthesis.
2. There is no gain of energy from outside the planet.
3. No light is involved in this process.
4. Reactions are all exothermic.
5. Energy required for the process is obtained by the oxidation of certain inorganic substances available in the environment.
Photosynthesis:
1. The amount of energy available in photosynthesis is much more than in chemosynthesis.
2. There is distinct gain of energy from outside the planet.
3. It takes place in the presence of light.
4. The reactions are endothermic.
5. Solar energy trapped by the pigments is used in the process.
2. Heterotrophic or Heterophytic Bacteria:
The colourless heterotrophic bacteria, which form the majority, cannot synthesize organic compounds from simple inorganic substances. Lacking the pigment, they cannot capture solar energy which is essential to synthesize the substances they need as food.
Perforce they secure their food ready made from external sources. Thus, the heterotrophic bacteria live in places where organic food is readily available either from living organism or their dead remaings and waste products. The former are called parasites and the latter saprophytes or saprobes.
(a) Saprophytic bacteria:
They grow in dead, decaying organic material, and live by digesting and absorbing them. In the course of obtaining food for themselves, the saprophytic bacteria gradually break down complex organic compounds into simpler products.
This they do by secreting enzymes. The breakdown of carbohydrates is called fermentation. The breakdown of protein materials is called putrefaction.
These simpler products are rendered into a soluble form and are absorbed as food. These two processes constitute the useful chemical activities of saprophytic bacteria.
Fermentation:
The word fermentation comes from a Latin word meaning “to boil”. It refers to bubbling due to the carbon dioxide given off. The fermentation of sugar by yeast is the classic example.
Certain species of bacteria are able to ferment glucose and galactose. Lactic acid fermentation is principally carried out by bacteria. It takes place without the evolution of carbon dioxide. In its place an organic acid (Lactic acid) is formed.
This acid causes the souring and curding of milk. It is carried out by lactic acid bacteria. The process takes place in two steps. First the lactic sugar in the milk is changed into glucose by the enzyme lactose.
It is secreted by milk souring bacteria. The glucose is later changed to lactic acid by the lactic acid bacteria. The formation of lactic acid causes the souring and curding of milk. Curding is the result of coagulation of the milk protein (casein).
Putrefaction:
Various species of bacteria are principally concerned in the decomposition of proteins into simpler compounds. The earlier stages in protein putrefaction take place in the absence of oxygen.
They are brought about by an enzyme produced by the anaerobic bacteria concerned in the process. As a result the complex protein compounds are reduced to proteosis, peptones, polypeptides, peptides and amino acids.
Some of these substances have foul or unpleasant odour. The later stages of decomposition usually require oxygen. The amino acids are further decomposed by various saprophytic bacteria resulting in the production of much simpler substances such as methane (marsh gas), carbon dioxide, hydrogen sulphide, water, nitrogen, hydrogen and ammonia.
The formation of ammonia resulting from the decomposition of amino acids is called ammonification. A few species of saprohytic bacteria are able to decompose fats into glycerine and fatty acids.
A few putrefying bacteria are phosphorescent. They produce light causing rotten fish, meat or wood to shine in the dark.
(b) Parasitic Bacteria:
They live on or within living organisms both plants and animals. They obtain their organic food from the host on which they grow. Not all parasitic bacteria cause disease.
Some may be harmless to their hosts and others cause serious diseases in them. To the former category belong the symbiotic species which give something useful to the host and receive shelter and food in return.
The numerous coli bacteria inhabiting the intestines in man and most other organisms are the typical examples of this kind. They do not cause disease or any other injury to the host but help in the digestion of cellulose by breaking down the cell walls through the secretion of various enzymes.
The also deposit vitamins in the intestines which can be absorbed and used by the host. In nature they live on the otherwise unusable food. The harmless parasitic bacteria are said to be non-pathogenic and the disease causing ones pathogenic.
Many diseases of plants and animals including man are caused by the pathogenic bacteria. The disease is the result of either the direct attack of the parasite upon the host tissues or liberation of toxic substances which may be either caustic in effect or a protein to which the body reacts.
This disturbs the host metabolism. The common examples of human diseases which are bacterial in origin are cholera, pneumonia, diphtheria, tuberculosis, tetanus, typhoid and many others. There are relatively few bacterial pathogens of plants.
The unicellular condition of the bacterial cells is the handicap. It cannot be readily transported inside the multicellular plant body as it lacks the circulatory system. Cabbage rot, pear blight, citrus canker and a few others are common bacterial diseases of plants.