The following points highlight the four major nutritional types of bacteria. The types are: 1. Photoautotrophs 2. Photoheterotrophs 3. Chemoautotrophs 4. Chemoheterotrophs.
Type # 1. Photoautotrophs:
These bacteria capture the energy of sun light and transform it into the chemical energy. In this process CO2 is reduced to carbohydrates. The hydrogen donor is water and the process produce free oxygen. Chlorophyll is present in the cell and its main function is to capture sun light e.g., Cyanobacteria.
The reaction produces free oxygen. However, in some bacterial photosynthesis hydrogen donor is a substance other than water, hence, oxygen is never produced. This is called an oxygenic photosynthesis and is found in purple sulphur bacteria and green sulphur bacteria.
This photoautotrophic bacteria are anaerobes and have bacteriochlorophyll and bacteriovirdin pigments respectively. In the process of photosynthesis these pigments absorb light and reduce carbon dioxide to form organic compounds. Carbon dioxide is taken from the atmosphere and hydrogen from sources except water.
Purple Sulphur Bacteria:
These bacteria have the pigment bacteriochlorophyll located on the intracytoplasmic membrane i.e., thylakoids. These bacteria obtain energy from sulphur compounds e.g., Chromatiiun. Theopedia rosea, Thiospirilium.
Green Sulphur Bacteria:
These bacteria used hydrogen sulphide (H2S) as hydrogen donor. The reaction takes place in the presence of light and pigment termed as bacteriovirdin or bacteriopheophytin or chlorobium chlorophyll e.g., Chlorobium limicola, Chlorobacterium etc.
These bacteria take hydrogen from inorganic sources like sulphides and thiosulphates. Therefore, these bacteria are also known as photolithographs.
Type # 2. Photoheterotrophs (Gr., Photo = light; hetero = (an), other; troph = nourishment):
These bacteria can trap light energy but cannot use carbon dioxide as their sole caron source. They use organic compounds from the environment to satisfy their carbon and electron requirements. They use organic compounds such as carbohydrates, fatty acids and alcohols as their organic food. The pigment is bacteriochlorophyll e.g., Purple non-sulphur bacteria (Rhodospirillum, Rhodomicrobium, Rhodopseudomonas palustris).
Type # 3. Chemoautotrophs:
These bacteria do not require light (lack the light phase but have the dark phase of photosynthesis) and pigment for their nutrition. These bacteria oxidize certain inorganic substances with the help of atmospheric oxygen. This reaction releases the energy (exothermic) which is used to drive the synthetic processes of the cell.
The source of carbon is carbon dioxide. In the absence of light synthesis of organic food from inorganic substances by utilizing chemical energy is also known as chemosynthesis. The chemoautotrophic bacteria play a very important role in recycling inorganic nutrients. These bacteria are commonly named after the structure of the compound which is utilized as the source of energy viz.
(a) Nitrifying Bacteria:
These bacteria obtain energy by oxidizing ammonia into nitrate. The process occurs in two steps and each step is carried out by a specialized group of bacteria.
In the first step ammonia is oxidized into nitrites by the bacteria Nitrosomonas, Nitrococcus:
In the second step, the nitrites are converted into nitrate.
This is brought about by the bacteria Nitrobacter, Nitrocystis:
2HNO2 + O2 → 2HNO3 + 21.6 k.cal
These nitrifying bacteria are present in the soil and are of considerable economic importance. When these two groups of bacteria work together, ammonia in the soil is oxidized to nitrate in a process called nitrification. Energy released upon the oxidation of both ammonia and nitrite is used for chemosynthesis by these bacteria (to make ATP by oxidative phosphorylation).
(b) Sulphur Bacteria:
These bacteria obtains energy either by oxidation of elemental sulphur or H2S.
(a) Elemental Sulphur Oxidising Bacteria:
Denitrifying sulphur bacteria e.g., Thiobacillus denitrificans oxidize elemental sulphur to sulphuric acid and utilize energy produced in this process
2S + 2H2O + 3O2 → 2H2SO4 + 126 kcal.
(b) Sulphide Oxidizing Bacteria:
These bacteria oxidizes H2S and release the sulphur e.g., Beggiatoa.
2H2S 4 – O2 → 2H2O + 2S + 141.8 cal
(c) Iron Bacteria:
These bacteria inhabit waters that contain inorganic iron compounds and oxidize ferrous compounds to ferric forms e.g., Thiobacillus ferroxidans, Ferro bacillus, Leptothrix.
4FeCo3 + 6H2O + O2 → 4Fe (OH)3 + 4CO2 + 81 kcal.
(d) Hydrogen Bacteria:
These bacteria oxidizes hydrogen into water e.g. Hydrogenomonas.
2H2 + O2 → 2H2O + 55 kcal.
4H2 + CO2 → 2H2O + CH4 + Energy
(e) Carbon Bacteria:
These bacteria oxidizes CO into CO2 e.g., Bacillus oligocarbophillous, Oligotropha carboxydovorans
2CO + O2 → 2CO2 + Energy
Type # 4. Chemoheterotrophs:
These bacteria obtain both carbon and energy from organic compounds such as carbohydrates, lipids and proteins. The carbon source as well as the source of energy are mostly the same for these bacteria. Most of the bacteria are chemo heterotrophs.
Glucose or Monosaccharide [(CH2O)n] + O2 → CO2 + H2O + Energy
chemo heterotrophs may belong to one of the three main categories that differ in how they obtain their organic nutrients.
These bacteria obtain their food from living hosts on which these grow. Parasites which cause diseases are known as pathogens e.g., Clostridium, Mycobacterium etc.
These bacteria obtain their food from dead and organic remains like fruits, vegetables, leaves,meat, faeces, corpses and other non-living products. The anaerobic breakdown of carbohydrates is fermentation while that of proteins is called putrefaction, e.g., Putrefying bacteria like Bacillus mycoides, B. ramosus etc.
These bacteria live in close association with organs of other organisms (higher plants and animals) in such a way that both the concerned organism receive mutual benefit from this association. This is called symbiosis for e.g., Rhizobium leguminosarum in the root nodules of the leguminous plants.
This bacteria fix free atmospheric nitrogen into nitrogenous compounds which are utilized by the plants. In return, the plant provides nutrients and protection to the bacteria. In the stomach of the cows and goats bacteria digest cellulose enabling these animals to feed on grass. Our own intestine contains a number of harmless bacteria e.g., Escherichia coli.