Difference Between Photoautotrophic and Chemoautotrophic Nutrition

In bacteria their is mainly two modes of nutrition autotrophic and hetertrophic. The autotrophes are further divided into types by their ability. They make make food by utilizing different resources. So on the basis of nutrition, the bacteria is divided into following types.

What is Metabolism?

All chemical reactions that occur in (living body) organisms is called metabolism.

Metabolism is further classified into following types:
Anabolism: Synthesis of complex molecules (large molecules) from simpler ones (small molecules) in living organisms is called anabolism.

Catabolism: Breakdown of complex molecules (large molecules) into simpler ones (small molecules) in living organisms is called catabolism.

Autotrophs	Heterotrophs
They make their own food by using inorganic carbon CO­­­­­2.	They use only organic carbon (organic compound).
They do not depend on other organisms for gaining food. The obtain their energy from following sources. Light energy and chemical energy.	They are dependent on other organisms for gaining organic carbon from organic compounds.

The autotrophs are further divided into following types by their nutrition ability.

Photoautotrophs/Photolithotrophs	Chemoautotrophs
They use light energy to utilize their food.	They use chemical energy to utilize their food.
Electron excitation takes place in photoautotrophs	Electron excitation also takes place in chemoautotrophs

Nutrition in Photoautotrophs/Photolithotrophs

Photoautotrophs include different types of bacteria purple Sulphur bacteria and green Sulphur bacteria.

PSB	GSB
PSB stands for Purple Sulfur Bacteria	GSB stands for Green Sulfur Bacteria
PSB have only one photosystem.	They also have one photosystem.
They absorb light of wavelength of 890nm. Also called PS890.	They absorb light of wavelength of 840nm. Also called PS840.
Light absorbing pigment is bacterio-purpurin.	Light absorbing pigment is bacterio-viridin.
Electron main donor is Sodium Thiosulphate	Electron main donor is Hydrogen Sulphide.

Due to utilization of inorganic e^– and H⁺ donor that is sodium thiosulphate and H₂S (Hydrogen Sulphide) during photosynthesis for making ATP and NADPH through ETS (Electron Transport System). These two bioenergy molecules are further on utilized for CO_2­­­ fixation for making food. Therefore, photoautotrophs are also called Photolithoautotrophs.

 

 

Why photoautotrophs are also called Photo-lithoautotrophs?

They are called photolithoautotrophs due to following reasons:
1. They have ability to utilize light energy.
2. They have ability to use e^– and H⁺ donor compounds of lither.
3. They use inorganic compound (CO_2­­­) for fixation to make food.

 

In case of plants, e^– and H⁺ donor is 2H₂O and after photolysis. There is production of oxygen which shows oxygenic photosynthesis. But in case of these bacteria, there is no utilization of H₂O. So, release of O₂ during this type of photosynthesis is absent. Therefore, is known as An-oxygenic photosynthesis.

Bacteria	Plant
There is involvement of one photosystem PS-I	There is involvement of two photosystem PS-I and PS-II.
Sulphurous compounds: Sodium thiosulphate and Hydrogen Sulphide act as e– and H+ donor.	Due to splitting of H2S there is release of oxygen, that’s why known as oxygenic photosynthetic.
Release of oxygen is absent. Therefore, known as An-oxygenic photosynthesis.	They absorb light of wavelength of 840nm. Also called PS840.
Light absorbing pigment is bacterio-purpurin.	Light absorbing pigment is bacterio-viridin.
Electron main donor is Sodium Thiosulphate	Electron main donor is Hydrogen Sulphide.

Nutrition in Chemoautotrophs

Those bacteria which utilize chemical energy to utilize their food are called chemoautotrophs. To explain this, we can take example of Nitrifying bacteria present in the soil. They are involved in the conversion of

Nitrosomonas                   Nitrobacter
Nitrococcus                      Nitrocystis

NH₃ ——————–> No₂ ——————–> No₃

Ammonia                             Nitrite                               Nitrate

At the end this is synthesis of Nitrate from Ammonia, that’s why these bacteria are also called as nitrifying bacteria.

Is the above-mentioned reaction oxidative (oxidation reaction)?

Yes, the above-mentioned reaction is oxidation reaction. Because there is addition of oxygen.

During this oxidation reaction there is loss or release of the e^–. This e^– is picked up by bacteria and is utilized in ETS. In the ETS, with the help of ATPase complex. There is synthesis of ATP, that is utilized for CO_2­­­ fixation through which there is synthesis of sugar in the form of food. So, in this type of bacteria there is synthesis of food by utilizing energy that is obtained from chemical reaction. That’s why this type of bacteria is known as chemoautotrophs.

Iron Bacteria

They can convert ferrous into ferric by losing electron.

Fe⁺²     Ferrobacillus        Fe⁺³

Ferrous ——————–> Ferric
Salts                                           Salts

Again, there is oxidation (loss of electron). We can simplify by the following chain.

Hydrogen Bacteria

The hydrogen bacteria convert H₂ into H₂O

  Hydrogenomonas

H₂ ——————————–> H₂O