Thank you for the opportunity to help you with your question!
algae are plants because they posses chlorophyll; a pigment necessary for the production of food(glucose) in plants. So the algae can be categorized as photosynthetic creatures.
The key to habitats for algae is light for photosynthesis--the
ability to synthesize carbohydrates using energy from the sun.
The critical aspects are light, water quality and temperature, availability
of carbon and other essential nutrients, and depth or tidal exposure.
Macroalgae thrive in cold temperate oceans around the world
and in polar oceans, including the Antarctic and Arctic. In the tropics, algae grow on hard substrata
like beachrock, coral reefs and on the roots of mangrove trees.
Please let me know if you need any clarification. I'm always happy to answer your questions.