Why is algae classified in the protist kingdom and not the plant kingdom even though they are photosynthetic? research why some scientists feel they should be classified as plants.

Algae (Euglena) do photosynthesis as plants do. They also move around and eat, as do animals. But they are unicellular. In order to be classified as a plant or animal, an organism has to be multicellular, made of more than one cell. Since it is a unicellular organism with some plant and animal characteristics, it is called a protist. Plant cells have walls while algae does't have one, so it is a protozoan. Algae resemble the protozoa, so they are put into the Protist Kingdom. 

That would be a working theory.