Red tide is mostly caused by

Options

A. Noctiluca
B. Gymnodinium
C. Symbiodinium
D. Nematodinium.

Answer

### Step-by-Step Solution:

1. **Understanding Red Tide**:
- Red tide refers to a phenomenon where the surface of the sea becomes discolored, typically appearing red. This discoloration is caused by the rapid growth or blooming of certain microorganisms in the water.

2. **Identifying the Cause**:
- The question asks which organism is primarily responsible for causing red tides from the given options: Noctiluca, Gymnodinium, Symbiodinium, or Nematodinium.

3. **Analyzing the Options**:
- **Noctiluca**: Known to cause green tides, not red tides.
- **Gymnodinium**: Specifically, Gymnodinium breve (now known as Karenia brevis) is a unicellular photosynthetic dinoflagellate that is known to cause red tides.
- **Symbiodinium**: This organism is primarily associated with coral reefs and does not cause red tides.
- **Nematodinium**: A light-sensitive organism that is not known for causing red tides.

4. **Conclusion**:
- Based on the analysis, the organism responsible for causing red tides is **Gymnodinium** (specifically Gymnodinium breve or Karenia brevis). Therefore, the correct answer is option number 2.

### Final Answer:
Red tide is mostly caused by **Gymnodinium**.

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