Growth of algae and fungi on the tree trunk imparts black colour to the tree trunk.

Explanation

### Step-by-Step Solution:

1. **Understanding the Organisms**: Algae and fungi are microorganisms that can grow on various surfaces, including tree trunks. They derive their nutrients from the environment, often feeding on decaying organic matter.

2. **Nature of Growth**: These organisms are typically classified as saprophytes or parasites. Saprophytes feed on dead or decaying material, while parasites derive nutrients from living hosts.

3. **Color Change on Tree Trunks**: When algae and fungi grow on tree trunks, they can cause a change in color. However, the common change observed is not black but rather a grayish hue.

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