Abundant acritarch fossils, some giant-sized by microstandards, are thought to be fossil cysts or other life stages of algae. Although we cannot discern exactly "who" they are, they speak to us of life's diversity over millions of years.
Categories are maps, but maps are not the territory. In the game "20 Questions," the basic categories of animal, vegetable and mineral underestimate the richness of life forms. Similarly, basic categories such as "small," "large," "single-celled" and "multicellular" can confuse us.