The process of manufacture of food from inorganic raw materials by using chemical energy is termed as chemosynthesis. There are some organisms that are able to manufacture all their organic food from inorganic raw materials in the absence of light. They get chemical energy for this purpose by oxidizing different types of substances present in their medium. The reaction is exergonic (i.e., energy releasing).
Winogradsky (1887) discovered a group of organisms that can assimilate carbon dioxide in the dark. He discovered another group of organisms which grew in the presence of carbon dioxide and NH3 . Both the group of organisms were colourless and although they grew in the dark and in the environment free of organic matter, they were autotrophic.
The energy required for the reduction of carbon dioxide was obtained from simultaneous oxidation of simple inorganic compound such as hydrogen sulphide (H2S), ammonia (NH3) . These groups were bacteria which released energy from chemical reaction mostly in the form of ATP. This energy is much less than required for the reduction of one molecule of carbon dioxide and only a part of energy released becomes available in the form of energy-rich phosphate group (ATP). Such an energy must, therefore, be pooled before being used.
In chemosynthesis carbon dioxide is reduced to form carbohydrate . This occurs without the participation of light energy and chlorophyll but by utilizing the chemical energy released during oxidation of inorganic substances. Chemosynthesis occurs only in case of some colourless aerobic bacteria termed as chemoautotrophic bacteria such as; Nitrifying bacteria (Nitrosomonas and Nitrobacter), Sulphur bacteria (for example, Beggiatoa), Iron bacteria (for axample, Ferrobacillus) etc.
Chemosynthetic bacteria are colourless sulphur bacteria, nitrifiers, iron bacteria, hydrogen bacteria, carbon bacteria and methane oxidizers. Following are some of the chemosynthetic bacteria performing chemosynthesis.
Colourless sulphur bacteria:
These bacteria are classified into the following three groups:
a.) Sulphur oxidizing bacteria:
Sulphur oxidizing bacteria are able to grow with sulphur thiosulphates or thiocyanates. They are obligate aerobes, obligate autotrophs and are acid tolerant for example Thiobacillus thiooxidans, Thiobacillus thioparus, etc. Following are the exergonic reactions carried out by these bacteria:
2S+3O2+2H2O → 2H2S04+energy
Na2S2O3+H2O+2O2 → Na2SO4+H2SO4+energy
b.) Sulphide oxidizing bacteria:
Many Thiobacilli oxidize sulphides to sulphur. These bacteria are entirely dependent for their development on the oxidation of sulphides (e.g., Beggiatoa). The exergonic reactions are as follows:
2Na2S+O2+H2O → 2S+4NaOH+energy
2H2S+O2 → 2S+2H2O+energy
c). Sulphate reducing bacteria:
These obligate anaerobes use sulphates instead of oxygen to oxidize their food (e.g., Desulphovibrio).
C6H12O6+3H2SO4 → 3H2S+6CO2+6H2O+energy
These bacteria oxidize ammonia (NH3) into nitrite(NO2) and nitrate(NO3). They belong to the following two groups:
a) Nitrosomonas, Nitrosocystis, Nitrospira which oxidize ammonia(NH3) to nitrite (NO2).
2NH3+3O2 → 2HNO2+2H2O+energy
b). Nitrobacter which oxidizes nitrite (NO2) to nitrate(NO3).
2KNO2+O2 → 2KNO3+energy
Most of these bacteria have been earlier put under the genus Hydrogenomonas. It is a mixed group. These bacteria oxidize hydrogen gas to form water. Thus the chemosynthetic forms get energy by oxidizing hydrogen to water.
2H2+O2 → 2H2O+energy
These bacteria oxidize carbon monoxide to carbon dioxide (e.g., Bacillius oligocarbophilus).
O2+2CO → 2CO2+energy
A number of bacteria have the property of converting ferrous salt to ferric salt. They are termed as iron bacteria. Some of the iron bacterias are autotrophic for example, Thiobacillus ferro-oxidans and Ferrobacillus ferro-oxidans . The other iron bacteria are not completely autotrophic for example, Leptothrix ochracea, Spirophyllum ferrugineum . The reaction is exergonic.
4FeCO3+6H2O+O2 → 4Fe(OH)2+4CO3+energy
These bacteria oxidize methane (e.g., Methanomonas).
CH4+2O2 → CO2+2H2O+energy
Comparision between photosynthesis and chemosynthesis
1. Photosynthesis and chemosynthesis are both processes by which organisms produce food; photosynthesis requires sunlight while chemosynthesis runs on chemical energy.
2. Chemosynthesis occurs at the heart of deep-sea communities, sustaining life in absolute darkness, where sunlight cannot penetrate. Chemosynthetic bacterial communities live in springs, on lands and on the seafloor around hydrothermal vents, cold seeps and also in sunken ships. Photosynthesis occurs in plants and also in some bacteria, wherever there is sufficient sunlight on land, in shallow water, even inside and below clear ice.
3. Chemosynthesis takes place in bacterial cells whereas photosynthesis occurs inside chloroplast.
4. The reactions in photosynthesis are endothermic whereas the reactions in chemosynthesis are exothermic. The energy available in photosynthesis is more than that of chemosynthesis.
5. All photosynthetic organisms use solar energy to turn carbon dioxide and water into glucose and oxygen. All chemosynthetic organisms use the energy released by chemical reactions to make food, but different species use different pathways.
6. End products of chemosynthesis are sulphur and water whereas oxygen is the end product of photosynthesis.
Significance of Chemosynthesis
1. Chemosynthesis allows organisms to live without using the energy of sunlight or depending on other organisms for food.
2. Microbes living deep in the oceans use chemosynthesis.
3. A few multicellular organisms live in symbiotic relationship with chemosynthetic bacteria, making them a partial energy source.
Some green and purple bacteria are capable of fixing atmospheric carbon dioxide to organic matter in presence of light. These are known as photosynthetic bacteria and the process is termed as bacterial photosynthesis.
Photosynthetic bacteria are divided into three main groups; Green Sulphur bacteria e.g., Chlorobium, Purple Sulphur bacteria e.g., Chromatium and Non-Sulphur bacteria e.g, Rhodospirillum. Photosynthetic bacteria have special pigments which act as photosynthetic units. These pigments include bacteriochlorophylls, bacterioviridin and carotenoids.
Following are some of the characteristic features of photosynthetic bacteria:
- Photosynthetic bacterias do not evolve O2 as they do not use water as a source of reducing power. They use reduced organic and inorganic compounds.
- They have only one pigment system
- The coenzyme NAD+ is reduced instead of NADP+
Following are the overall reactions of carbon assimilation in different bacteria :
Green Sulphur Bacteria:
6CO2+12H2S → C6H12O6+6H2O+Energy
Purple Sulphur Bacteria:
6CO2+15 H2O+3Na2S2O2 → C6H12O6+6H2O+6NaHSO2+Energy