Unicellular organisms that lack a membrane-bound nucleus are called prokaryotes.

Here's why:

* Prokaryotes are a broad category of single-celled organisms that lack a true nucleus and other membrane-bound organelles. Their genetic material (DNA) is located in a region called the nucleoid, which is not enclosed by a membrane.

* Eukaryotes, on the other hand, are organisms whose cells have a membrane-bound nucleus containing their DNA, as well as other membrane-bound organelles.

Examples of prokaryotes include:

* Bacteria

* Archaea

Let me know if you'd like more information about prokaryotes!