Blue-green alga that causes red bloom is

Options

A. Anabaena
B. Gleocapsa
C. Trichodesmium
D. Nostoc.

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the blue-green alga that causes a red bloom. Blue-green algae are scientifically known as cyanobacteria, which belong to the kingdom Monera (the prokaryotes).

2. **Identifying the Options**: The options provided in the question likely include various types of cyanobacteria. We need to identify which one is known for causing red blooms.

3. **Researching Cyanobacteria**: Among the cyanobacteria, some are known to cause blooms in aquatic environments. One notable genus is Trichodesmium, which is specifically known for its red blooms.

4. **Confirming the Correct Answer**: The full name of the cyanobacterium responsible for the red bloom is **Trichodesmium erythreum**. This organism is often referred to as "sea sawdust" and is found in nutrient-poor tropical and subtropical waters.

5. **Understanding the Habitat**: Trichodesmium is primarily found in marine environments, particularly around Australia and in the Red Sea. This distinguishes it from other cyanobacteria like Anabaena and Nostoc, which are more common in terrestrial ecosystems.

6. **Differentiating Between Terms**: It’s important to note that red blooms caused by Trichodesmium should not be confused with red tides, which are caused by dinoflagellates such as Gonyaulax.

7. **Final Conclusion**: Based on the information gathered, the blue-green alga that causes red bloom is **Trichodesmium erythreum**.

### Final Answer:
The blue-green alga that causes red bloom is **Trichodesmium erythreum**.

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