11th Class Biology Biological Classification And System Of Classification / जैविक वर्गीकरण और वर्गीकरण की प्रणाली Mode of Nutrition in Bacteria

On the basis of mode of nutrition, bacteria are grouped into two broad categories. First is autotrophic and second is heterotrophic bacteria. 

Autotrophic bacteria : These bacteria are able to synthesize their own food from inorganic substances, as green plants do. Their carbon is derived from carbon dioxide. The hydrogen needed to reduce carbon to organic form comes from sources such as atmospheric \[{{H}_{2}},{{H}_{2}}S\] or \[N{{H}_{3}}.\]These are divided into two categories.

(1) Photoautotrophic bacteria : These bacteria are mostly anaerobic bacteria. They use sunlight as source of energy to synthesize food. 

They possess a pigment called bacteriochlorophyll which is different from the chlorophyll pigment found in higher plants. This is known as anoxygenic photosynthesis. e.g., Green sulphur (Thiothrix) and purple sulphur (Chromatiun) bacteria. They can perform photosynthesis in far-red light. Rhodospirillum bacteria fixes \[C{{O}_{2}}\] into carbohydrate (Photoautotrophic).

Green sulphur bacteria : They are autotrophic. The hydrogen donor is \[{{H}_{2}}S\] and the pigment involved in the process is chlorobium chlorophyll (Bacterioviridin) e.g., Chlorobium.

\[6C{{O}_{2}}+12{{H}_{2}}S\underset{\text{Chlorobium}\,\,\text{chlorophyll}}{\mathop{\xrightarrow{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}}}\,{{C}_{6}}{{H}_{12}}{{O}_{6}}+6{{H}_{2}}O+12S\].

Purple sulphur bacteria : They are also autotrophic. The hydrogen donor is thiosulphate and the pigment involved in photosynthesis is bacteriochlorophyll e.g., Chromatium.

\[6C{{O}_{2}}+15{{H}_{2}}O+3N{{a}_{2}}{{S}_{2}}{{O}_{3}}\underset{\text{Bacteriochlorophyll}}{\mathop{\xrightarrow{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}}}\,{{C}_{6}}{{H}_{12}}{{O}_{6}}+6{{H}_{2}}O+6NaHS{{O}_{4}}.\]

Purple non-sulphur bacteria : They are heterotrophic utilizing succinate or malate or alcohol. e.g., Rhodospirillum, Rhodopseudomonas.

\[6C{{O}_{2}}+12C{{H}_{3}}CHOHC{{H}_{3}}\xrightarrow{{}}{{C}_{6}}{{H}_{12}}{{O}_{6}}+12C{{H}_{3}}COC{{H}_{3}}+6{{H}_{2}}O.\]

(2) Chemoautotrophic bacteria : Some bacteria manufacture organic matter form inorganic raw materials (such as carbon dioxide) and utilize energy liberated by oxidation of inorganic substances present in the external medium such as ammonia, ferrous ion, nitrates, nitrites, molecular hydrogen, etc. The energy liberated from exergonic chemical reactions is trapped in the ATP molecules which is used in carbon assimilation to synthesize organic matter.

Sulphur bacteria : These bacteria derive energy by oxidizing hydrogen sulphide or molecular sulphur. Beggiatoa, a colourless sulphur bacterium oxidises hydrogen sulphide \[({{H}_{2}}S)\] to water and sulphur. The energy released is used up and the sulphur granules are deposited inside or outside the body of bacterial cell.

\[2{{H}_{2}}S+{{O}_{2}}\xrightarrow{{}}2{{H}_{2}}O+2S+\text{Energy}\].

Iron bacteria : Some chemoautotrophic bacteria such as Gallionella, Sphaerotilus, Ferrobacillus, etc, inhabit the environments where irons to ferric form. The Ferric ions are deposited in the form of soluble ferric hydroxide and the energy released during the conversion is used in the production of carbohydrates.

\[4FeC{{O}_{3}}+{{O}_{2}}+6{{H}_{2}}O\xrightarrow{{}}4Fe{{(OH)}_{3}}+4C{{O}_{2}}+\text{Energy}\,\text{(81}\,\text{k}\text{.cal)}\]

Hydrogen bacteria : These bacteria utilize free molecular hydrogen and oxidize to hydrogen into water with the help of either oxygen or oxidize salts e.g., Hydrogenomonas. \[2{{H}_{2}}+{{O}_{2}}\to 2{{H}_{2}}O+\text{Energy}\] (56 kcal).

Amonifying bacteria : They oxidise protein and amino acid into NH3 (ammonia). e.g., Proteus vulgaris, Bacillus mycoids.

Nitrifying bacteria : They oxidise ammonia to nitrites and then into nitrates.

\[N{{H}_{3}}+{{O}_{2}}\xrightarrow{\text{Nitrosomonas}}N{{O}_{2}}+{{H}_{2}}O+\text{Energy}\] and

 \[2N{{O}_{2}}+{{O}_{2}}\xrightarrow{\text{Nitrobacter}}2N{{O}_{3}}+\text{Energy}\].

Denitrifying bacteria : They change nitrogen compound into molecular nitrogen. So that they reduce fertility of soil e.g., Micrococcus denitrificans, Pseudomonas denitrificans.

Methane bacteria : The bacterium Methanomonas utilizes methane as source of carbon and energy.

\[C{{H}_{4}}+2{{O}_{2}}\xrightarrow{{}}C{{O}_{2}}+2{{H}_{2}}O+\text{Energy}\].

Methane producing bacteria : These are spherical or rod shaped bacteria which produce methane \[(C{{H}_{4}})\] from hydrogen gas and carbon dioxide e.g., Methanobacterium.

\[C{{O}_{2}}+4{{H}_{2}}\xrightarrow{{}}C{{H}_{4}}+2{{H}_{2}}O\]

Methane (swamp gas) is produced under anaerobic conditions and can be used as a 'biogas', otherwise it is a pollutant that contributes to the green house effect and global warming.

Carbon bacteria : These bacteria oxidize carbon monoxide into carbon dioxide and use the liberated energy, e.g., Bacillus oligocarbophilus.

\[2C{{O}_{2}}+{{O}_{2}}\xrightarrow{{}}2C{{O}_{2}}+\text{Energy}\].

Heterotrophic bacteria : Most of the bacteria can not synthesize their own organic food. They are dependent on external organic materials and require atleast one organic compound as a source of carbon of their growth and energy. Such bacteria are called heterotrophic bacteria. Heterotrophic bacteria are of three types. Parasites, Saprotrophs and Symbionts.

(1) Parasites : They obtain their organic food or special organic compounds required for their growth from living cells of plants and animals. Some parasitic bacteria are relatively harmless and nonpathogenic, i.e., do not produce disease in hosts. However, majority of parasitic bacteria are pathogenic and cause serious diseases in plants and animals either by exploiting them or by secreting poisonous substances called toxins.

Human diseases

Plant diseases

(2) Saprotrophic bacteria : These bacteria obtain their nutritional requirements from dead organic matter (such as animal excreta, corpses, fallen leaves, bread, fruits, vegetables, jams, jellies, etc.). These bacteria breakdown the complex organic matter into simple soluble forms by secreting exogenous digestive enzymes. Then they absorb the simple nutrient molecules and assimilate them. Aerobic break down of organic matter is called decomposition or decay.

(3) Symbiotic bacteria : Symbiosis is the phenomenon in which the two organisms live in close association in such a way that  both the partners get mutual benefit from this association. For example, a very well known nitrogen fixing bacteria ? Rhizobium forms a symbiotic association with roots of leguminous plants (soyabean, clover, alfalfa, etc.) and producing root nodules.

Another example of symbiosis is the presence of enteric bacterium Escherichia coli (E. Coli) in human intestine. The bacteria shares our food but at the same time checked the growth of harmful putrefying bacteria and releases vitamin K and B12, which help to produce blood components.