Select incorrect statement for gas vacuoles, which usually occur in cyanobacteria, purple and green photosynthetic bacteria :

Options

A. The are basically aggregates of a number of small, hollow cylindrical gas vesicles
B. Gas vesicles are permeable to water but are not permeable to atmospheric gases.
C. Because of gas vacuoles, these bacteria keep floating on or near the surface of water.
D. These vacuoles help the organisms in positioning themselves in the water column for trapping sunlight for photosynthesis , or for avoiding intense sunlight

Answer

### Step-by-Step Solution:

1. **Understanding Gas Vacuoles**:
- Gas vacuoles are small, hollow structures found in certain bacteria, including cyanobacteria and purple and green photosynthetic bacteria. They play a crucial role in buoyancy, allowing these organisms to float in water.

2. **Analyzing the Statements**:
- The question requires us to identify the incorrect statement regarding gas vacuoles. We will evaluate each statement provided in the options.

3. **Evaluating the First Statement**:
- "They are basically aggregates of a number of small hollow cylindrical gas vesicles."
- This statement is **correct**. Gas vacuoles are indeed composed of multiple gas vesicles that are hollow and cylindrical in shape.

4. **Evaluating the Second Statement**:
- "Gas vesicles are permeable to water but not permeable to atmospheric gases."
- This statement is **incorrect**. Gas vesicles are not permeable to water; they are designed to be impermeable to water while allowing gases to enter and exit. This helps maintain buoyancy and pressure.

5. **Evaluating the Third Statement**:
- "Because of gas vacuole, this bacteria keep floating on and near the surface of water."
- This statement is **correct**. Gas vacuoles help bacteria maintain buoyancy, allowing them to float near the water's surface.

6. **Evaluating the Fourth Statement**:
- "This vacuole helps the organism in positioning themselves in the water column for trapping sunlight, for photosynthesis, or for avoiding intense sunlight."
- This statement is also **correct**. The buoyancy provided by gas vacuoles allows these organisms to position themselves optimally for photosynthesis.

7. **Conclusion**:
- After evaluating all the statements, the **incorrect statement** regarding gas vacuoles is the second one: "Gas vesicles are permeable to water but not permeable to atmospheric gases."