There are two lectures covering microorganisms: 

The first video concerns prokaryotes, single-celled organisms without a nucleus. 

Prokaryotes are the most abundant organisms on Earth. This is an introductory lecture that describes their diversity and phylogeny.

The second video covers single-celled microorganisms with a nucleus, known as protists.

Protists are unicellular, eukaryotic organisms that are the predecessors of plants, animals, and fungi.