Merry Youle from the Small Things Considered blog ponders the potential size a virus can be:
"With such fascinating stories being told by Mimivirus and the other giants, people are now looking for them in more environments. Modified techniques are called for, as those used previously to spot viruses may have excluded many of them. For example, when collecting marine samples for viral metagenomes, researchers often use filters with 0.16-0.2 µm pores to catch the "microbial" fraction and allow the "viral" fraction to pass through. Realizing that many NCLDVs are apt to be caught with the microbes, Monier and colleagues searched the "microbial" sequences from the Sorcerer II Global Ocean Sampling (GOS) Expedition for NCLDVs using their conserved DNA polymerase sequences as a handle. They found Mimivirus sequences in 86% of the samples and chlorella viruses in a third.
Claverie and colleagues see no limitations that would preclude the existence of even larger viruses. Unlike cellular organisms, there are no metabolism-based constraints on particle volume. Of course, a virus must be smaller than its host, and Mimivirus is