Read the given passage and answer the question that follows. Algae are photosynthetic organisms containing pigments like chlorophyll that help in phot...

Read the given passage and answer the question that follows.

Algae are photosynthetic organisms containing pigments like chlorophyll that help in photosynthesis. They lack true stems, roots, or leaves found in vascular plants. Some algae are multicellular, while others are unicellular and may form colonies. Most algae live in aquatic environments, though some are terrestrial, found on rocks, trees, and moist soil. Certain algae species form symbiotic relationships, such as lichens, which are associations between green or blue-green algae and fungi. Algae belong to a polyphyletic group, meaning they do not share a common ancestor and are not necessarily closely related. They lack the morphoanatomical features typical of vascular plants but share the characteristic of being eukaryotes with chlorophyll as the main pigment for photosynthesis. The study of algae is called phycology. Blue-green algae are sometimes excluded from algae classification because they are prokaryotes.

What is the central theme of the passage?