Harmful activity of Blue green algae is :-

Options

A. Denitrification
B. Water-bloom
C. Increase alkalinity of soil
D. Decrease fertility of soil

Answer

**Step-by-Step Solution:**

1. **Understanding Blue Green Algae**: Blue green algae, also known as cyanobacteria, are prokaryotic organisms that exhibit algal morphology. They play a significant role in aquatic ecosystems.

2. **Identifying the Harmful Activities**: The question asks for the harmful activity of blue green algae. The options provided are:
- A) Denitrification
- B) Water bloom
- C) Increase alkalinity of soil
- D) Decrease fertility of soil

3. **Analyzing Each Option**:
- **Option A: Denitrification**: Blue green algae are nitrogen fixers, which means they convert atmospheric nitrogen into a form usable by plants. Denitrification is the process where nitrates are converted back to nitrogen gas, which is not a function of blue green algae. Therefore, this option is incorrect.

- **Option B: Water Bloom**: Blue green algae can proliferate rapidly in nutrient-rich waters, leading to a phenomenon known as algal bloom or water bloom. This can deplete oxygen in the water when the algae die and decompose, harming aquatic life. This is a harmful activity associated with blue green algae.

- **Option C: Increase Alkalinity of Soil**: Blue green algae prefer slightly alkaline conditions for nitrogen fixation, but they do not increase the alkalinity of soil. Instead, they can help in maintaining or slightly decreasing it. Thus, this option is incorrect.

- **Option D: Decrease Fertility of Soil**: Since blue green algae are nitrogen fixers, they actually increase the nitrogen content of the soil, which enhances soil fertility. Therefore, this option is also incorrect.

4. **Conclusion**: After analyzing all the options, the harmful activity of blue green algae is best described by **Option B: Water bloom**.

**Final Answer**: The harmful activity of blue green algae is water bloom.

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