Which one of the following is not a diatom?

Options

A. Nostoc
B. Navicula
C. Cyclotella
D. Pinnularia

Answer

To determine which of the following options is not a diatom, we need to understand the characteristics of diatoms and compare them with the given options. Here’s a step-by-step solution:

### Step 1: Understand Diatoms
Diatoms are a group of microalgae known for their unique cell wall structure. The key features of diatoms include:
- **Cell Wall Composition**: Diatoms have a silica-based cell wall, which is made of two halves known as theca (epitheca and hypotheca).
- **Symmetry**: Diatoms exhibit bilateral symmetry.
- **Habitat**: They are primarily aquatic and contribute significantly to oxygen production in water bodies.

### Step 2: Analyze the Options
Now, let's analyze the options provided to identify which one does not belong to the diatom category.

1. **Nostoc**: This is a type of blue-green algae (BGA). Its cell wall is made of peptidoglycan, not silica, which is a key characteristic of diatoms. Additionally, Nostoc is known for its ability to fix nitrogen and has structures called heterocysts.

2. **Nevicula**: This is a diatom that has a boat-shaped structure and belongs to the class Bacillariophyceae. It has a silica-based cell wall.

3. **Cyclotella**: This is also a diatom within the class Bacillariophyceae. It is known for its polymorphism and has a silica-based cell wall.

4. **Penularia**: This is a freshwater diatom, and like the others, it has a silica-based cell wall.

### Step 3: Identify the Non-Diatom
From the analysis:
- **Nostoc** is the only organism that does not fit the criteria of a diatom because it has a peptidoglycan cell wall and is classified as blue-green algae.

### Conclusion
The organism that is not a diatom is **Nostoc**.

---