What Is The Largest Unicellular Organism?

What Is The Largest Unicellular Organism? Biologists used the world’s largest single-celled organism, an aquatic alga called Caulerpa taxifolia, to study the nature of structure and form in plants. It is a single cell that can grow to a length of six to twelve inches. Daniel Chitwood, Ph. What is the smallest unicellular organism? Size.