Question

Which one of the following are Phosphate-solubilizing bacteria?

• A. Azospirillum
• B. Penicillium
• C. Bacillus
• D. Azotobacter

Hint:

To identify biofertilizers:
Phosphate Solubilizing Bacteria (PSB) $\rightarrow$ Bacillus and Pseudomonas.
Nitrogen Fixing Bacteria $\rightarrow$ Azotobacter, Azospirillum, and Rhizobium.
Phosphate Solubilizing Fungi $\rightarrow$ Aspergillus and Penicillium.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Phosphate-solubilizing microorganisms (PSMs) are beneficial microbes capable of hydrolyzing organic and inorganic phosphorus compounds from insoluble pools, converting them into bioavailable orthophosphate forms ($H_2PO_4^-$ and $HPO_4^{2-}$) that plants can absorb.

Step 2: Detailed Explanation:
Several soil bacteria and fungi secrete organic acids (such as citric, oxalic, and gluconic acids) that chelate bound cations (like $\text{Ca}^{2+}$, $\text{Fe}^{3+}$, and $\text{Al}^{3+}$), releasing associated phosphate into the soil solution.
Among soil bacteria, species belonging to the genus Bacillus (such as Bacillus megaterium var. phosphaticum, Bacillus polymyxa, and Bacillus subtilis) and Pseudomonas are highly effective phosphate-solubilizing bacteria (PSB).
Let us analyze the other microorganisms listed in the options:
1. Azospirillum and Azotobacter: Are free-living, associative, or diazotrophic nitrogen-fixing bacteria that convert atmospheric nitrogen ($\text{N}_2$) into ammonia, but they are not primarily classified as phosphate solubilizers.
2. Penicillium: Is a genus of soil fungi that can solubilize phosphate, but the question specifically asks for phosphate-solubilizing bacteria.
Therefore, Bacillus is the correct bacterial genus.

Step 3: Final Answer:
The phosphate-solubilizing bacteria among the choices is Bacillus, which corresponds to Option (C).