Algae use sunlight as their primary energy source. This process is called photosynthesis, where they convert light energy into chemical energy in the form of sugars.

Here's how it works:

* Sunlight: Algae contain chlorophyll, a green pigment that absorbs light energy.

* Carbon Dioxide: They take in carbon dioxide from the air or water.

* Water: They absorb water from their surroundings.

* Photosynthesis: Using the energy from sunlight, algae combine carbon dioxide and water to produce sugars (glucose) and oxygen.

This glucose is then used as fuel for growth and other life processes.

However, some algae are heterotrophic meaning they cannot perform photosynthesis and instead get their energy from consuming organic matter, like other organisms.