Algae and green plants are the first organisms in a food chain, also known as producers.

Here's why:

* They make their own food: Algae and green plants use sunlight, water, and carbon dioxide to create their own food through photosynthesis. This process is the foundation of most food chains.

* They are the base: All other organisms in a food chain rely on these producers for energy. Herbivores eat plants, carnivores eat herbivores, and so on.

So, without producers like algae and green plants, the rest of the food chain would collapse.