What are the cyanobacteria ?

Explanation

**Step-by-Step Solution:**

1. **Definition of Cyanobacteria**: Cyanobacteria are a group of microorganisms that are classified as gram-negative photosynthetic prokaryotes.

**Hint**: Remember that "gram-negative" refers to the type of cell wall structure they possess, which can be identified using the Gram stain technique.

2. **Characteristics of Cyanobacteria**: They are capable of photosynthesis, meaning they can synthesize their own food using sunlight, carbon dioxide, and water. This process is facilitated by the presence of chlorophyll in their cells.

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