Most algae are in fact unicellular organisms that produce their own food as plants do by photosynthesizing, using energy from the sun to convert carbon dioxide, water, and other chemicals to form more of itself.
They’re different from plants, though, because of how simple the reproductive structures are. They are eukaryotes, meaning they have a nucleus and membrane-bound organelles (like chloroplasts). Each type of algae is classified taxonomically by color and what chemicals make up their cell walls.
Algae can be either unicellular or multicellular. Even huge ocean kelp are classified as algae, their cell walls are used for things like agarose for scientists to use in microbiology as staining gel or a food medium to feed growing cultures of bacteria or fungi. Unicellular algae are found all over the place like ponds, streams, lakes and oceans.
There are also unicellular algae that have unique silica-based structures. These are called diatoms. Their glass-like cell walls are like shells that protect them from herbivorous animals or protists.
FUN FACT: Unicellular algae produce most of the world’s oxygen, this is actually just a byproduct of photosynthesis!