It's tough to say definitively which is the *absolute* smallest microorganism visible only by electron microscope, as new discoveries are constantly being made. However, some contenders for this title include:

* Viruses: These are generally considered the smallest infectious agents and are often too small to be seen with a light microscope. The picoviruses (such as poliovirus) are among the smallest known viruses, with diameters around 20-30 nanometers.

* Nanobacteria: These are controversial entities that some scientists believe are extremely small bacteria, possibly measuring only 50-100 nanometers in diameter. However, their existence and classification as bacteria are still debated.

* Viroids: These are even smaller than viruses, consisting of only a short strand of RNA without any protein coat. They can be as small as 246 nucleotides, which translates to roughly 10 nanometers in length.

It's important to note that these are just a few examples, and the ongoing research in microbiology constantly reveals new and smaller microorganisms.