Question

Which among the following is used by purple and green sulphur bacteria during photosynthesis?

• A. ${\text{H}}_2\text{S}$
• B. $\text{HCl}$
• C. $\text{HF}$
• D. $\text{HBr}$

Answer 1

The correct option is A ${\text{H}}_2\text{S}$

In purple and green sulfur bacteria, sulfur is used as an electron donor in the form of sulfide or thiosulphate. Usually, ${\text{H}}_2\text{S}$ is the hydrogen donor instead of water. By giving away the electrons, the sulfur in hydrogen sulfide gets oxidized to form elemental sulfur or sulphate that can be further oxidized to form sulphuric acid. Since they do not use water as their reducing agent, they do not release oxygen as their by-product. They are called purple and green sulfur bacteria since they use sulphur as their electron donors. $\text{HCl}$,$\text{HF}$ and $\text{HBr}$ do not serve the same purpose.