A unicellular organism is one composed of one cell. There are unicellular organisms in all three domains (eukaryotes, prokaryotes, archaebacteria ) of life. Unicellular organisms can be found anywhere on Earth, and they pursue strategies of survival ranging from photosynthesis, chemotrophy, and heterotrophy. Many unicellular organisms can be seen through high powered compound microscopes; however others require electron microscopes to even be viewed. In the picture above is yeast at the bottom of a glass vase. Yeast is an example of a eukaryotic unicellular organism.