Which one is alga

Options

A. Rhodospirillum
B. Cynobacteria
C. Purple bacteria
D. Green bacteria.

Answer

To determine which option is classified as an alga, we will analyze each of the provided options step by step.

### Step-by-Step Solution:

1. **Identify the Options**: The options given are:
- Rhodospirillum
- Cyanobacteria
- Purple Bacteria
- Green Bacteria

2. **Analyze Rhodospirillum**:
- Rhodospirillum is identified as a purple non-sulfur bacterium.
- It is not classified as an alga.

3. **Analyze Cyanobacteria**:
- Cyanobacteria are also known as blue-green algae.
- They contain chlorophyll similar to that found in green plants and are capable of photosynthesis.
- They can exist in unicellular, colonial, or filamentous forms and can be found in freshwater, marine, or terrestrial environments.
- Some cyanobacteria can fix atmospheric nitrogen, which is a characteristic of certain algae.
- Therefore, this option is classified as an alga.

4. **Analyze Purple Bacteria**:
- Purple bacteria are gram-negative proteobacteria that are phototrophic.
- They can produce their own food via photosynthesis, but they are not classified as algae.

5. **Analyze Green Bacteria**:
- Green bacteria, such as Chlorobium tepidum, are anaerobic and photoautotrophic.
- While they perform photosynthesis, they are not classified as algae.

6. **Conclusion**:
- After analyzing all the options, it is clear that **Cyanobacteria** is the only option that is classified as an alga.

### Final Answer:
The correct answer is **Cyanobacteria**.

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