[Solved] In a closed universe, if you could send out a powerful enough light, it would eventually return from behind you. Zooxanthellae: Photosynthetic algae found in close symbiotic relationships with marine invertebrates such as corals and sea anemones. Microscopic Algae: Tiny, often unicellular algae, not visible to the naked eye, that live in water and can perform photosynthesis. Phytoplankton: Microscopic floating algae and cyanobacteria that are the base of most aquatic food webs. Compare with zooplankton. See plankton and nanoplankton. Classification: The systematic organization of living things into categories based on shared attributes or evolutionary ancestry.