Here's why:

* They are made of cells: Algae are made up of one or many cells, which are the basic building blocks of all living organisms.

* They grow and develop: Algae increase in size and complexity over time.

* They reproduce: Algae create more algae through various methods like asexual or sexual reproduction.

* They require energy: Algae need energy to survive and grow, which they obtain through photosynthesis.

* They respond to their environment: Algae can change their behavior in response to changes in light, temperature, or other environmental factors.

Therefore, algae possess all the key characteristics of living organisms.