Which one of the following is an incorrect statement?

Options

A. Anabaena and Nostoc are capable of fixing nitrogen in free living state also
B. Root nodule forming nitrogen fixers live as aerobes under free living conditions
C. Phosphorous is a constituent of cell membranes certain nucleic acids and all proteins
D. Nitrosomonas and Nitrobacter are chemoautotrophs

Answer

To determine which statement is incorrect among the given options, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Analyze Statement A**:
- "Anabena and Nostoc are capable of fixing nitrogen in free living state also."
- **Evaluation**: This statement is correct. Anabena and Nostoc are indeed capable of fixing nitrogen in a free-living state, aided by specialized cells called heterocysts.

2. **Analyze Statement B**:
- "Root nodule forming nitrogen fixers live as aerobes under free living conditions."
- **Evaluation**: This statement is partially correct but misleading. While root nodule forming nitrogen fixers like Rhizobium are aerobic in free-living conditions, they can survive in anaerobic conditions when in symbiotic association with plants. Therefore, the statement is not entirely accurate.

3. **Analyze Statement C**:
- "Phosphorus is a constituent of cell membranes, certain nucleic acids and all proteins."
- **Evaluation**: This statement is incorrect. Phosphorus is indeed a component of cell membranes (as phospholipids) and all nucleic acids (as phosphates), but it is NOT a constituent of all proteins. Therefore, this statement is incorrect.

4. **Analyze Statement D**:
- "Nitrosomonas and nitrovector are chemoautotrophs."
- **Evaluation**: This statement is correct. Nitrosomonas and Nitrovector are indeed chemoautotrophic bacteria involved in the nitrification process.

### Conclusion:
Based on the analysis, the incorrect statement is **C**: "Phosphorus is a constituent of cell membranes, certain nucleic acids and all proteins."