Which of the following statements about cyanobacteria is incorrect ?

Options

A. Cyanobacteria like Nostoc and Anabaena are chemosynthetic autotrophs.
B. Cyanobacterial colonies are generally surrounded by a gelatinous sheath.
C. They are unicellular, filamentous and terrestrial/marine organisms.
D. Some of these organisms can fix nitrogen in specialized cells called heterocysts.

Answer

To determine which statement about cyanobacteria is incorrect, we will analyze each provided statement one by one.

### Step-by-Step Solution:

1. **Statement 1: Cyanobacteria like Nostoc, Anabaena are chemosynthetic autotrophs.**
- **Analysis:** This statement claims that cyanobacteria are chemosynthetic autotrophs. However, cyanobacteria are primarily photosynthetic autotrophs, meaning they use sunlight to produce their food through photosynthesis. Therefore, this statement is incorrect.

2. **Statement 2: Cyanobacteria colonies are generally surrounded by a gelatinous sheet.**
- **Analysis:** This statement is true. Cyanobacteria often form colonies that are surrounded by a gelatinous sheath, which provides protection and helps retain moisture.

3. **Statement 3: They are unicellular, filamentous, terrestrial, and marine.**
- **Analysis:** This statement is also true. Cyanobacteria can exist as unicellular organisms or in filamentous forms, and they can be found in both terrestrial and marine environments.

4. **Statement 4: Some of the organisms can fix nitrogen in specialized cells called heterocysts.**
- **Analysis:** This statement is true as well. Certain cyanobacteria, like Nostoc and Anabaena, have specialized cells known as heterocysts that are capable of fixing atmospheric nitrogen, which is beneficial for plants.

### Conclusion:
The incorrect statement about cyanobacteria is **Statement 1**, which incorrectly categorizes them as chemosynthetic autotrophs instead of photosynthetic autotrophs.

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