Cyanobacteria are

Options

A. Bacteria using cyanide for nutrition
B. Coloured fungi
C. Algae having blue-green cells
D. Viruses affecting bacterial growth.

Answer

### Step-by-Step Text Solution:

1. **Definition of Cyanobacteria**:
Cyanobacteria are commonly known as blue-green algae. They are a group of photosynthetic microorganisms that are technically classified as bacteria.

2. **Photosynthetic Nature**:
Despite being bacteria, cyanobacteria are included in the category of algae because they perform photosynthesis. This process allows them to convert sunlight into energy, similar to plants.

3. **Coloration**:
The term "blue-green" comes from the fact that dense populations of these organisms can turn water a blue-green color. This is due to the pigments they contain, such as chlorophyll and phycocyanin.

4. **Ecological Importance**:
Cyanobacteria play a crucial role in ecosystems. They are primary producers, meaning they create food through photosynthesis, which supports various heterotrophic organisms, including small animals and zooplankton.

5. **Population Dynamics**:
While cyanobacteria are essential for the food web, they can also reproduce rapidly under certain conditions, leading to population explosions known as algal blooms. These blooms can occur when nutrient levels in the water are high.

6. **Environmental Impact**:
Although cyanobacteria are beneficial, excessive growth can lead to negative consequences for aquatic ecosystems, such as reduced oxygen levels and the production of toxins.