How to know that living things can be unicellular or multicelluar?

Unicellular-the body of the cell is singular.
                  division of labor is at the organelle level.
                  the single cell carries out of it's life processes

Multicellular-the body of the organism is made up of many cells
                    division of labor is at cellular, tissue, organ, and organ system level
                    different cells are specialized to perform different functions in the organism

I'm am pretty sure it's both bc we did this today

This is what i found on google “phytoplankton, tiny organisms that live in the ocean, are autotrophs. some types of bacteria are autotrophs. most autotrophs use a process called photosynthesis to make their food.” i hope this you for what your looking for