algae range in form from microscopic unicellular phytopankton to multicellular seaweeds that may grow more than 50 meters long.most are photosynthetic(photoautotrophs),some are facultative or obligate heterotrophs,and a few are phagotrophic!some algae are symbiotic,living on or within animals,fungi,or other plants,and some are parasitic.seagrasses,often confused with seaweeds because both are aquatic,are flowering vascular plants,not algae.