What is the simplest that can eat like plants?

Most plants are autotrophic. This means that they receive organic matter during photosynthesis. Organic molecules are created from inorganic substances. There are not so many representatives of the protist kingdom, which have the ability to photosynthesize. The main representative is green euglena. It has chloroplasts in its cell structure, which make the process of photosynthesis possible. But euglena green is not only able to eat autotrophically, but can also eat like a heterotroph in the dark.