One of two cellular nuclei found in the single-celled ciliated protozoan "tetrahymena." This nucleus contains a full set of the organism's genes, but is not used except under stressful conditions (such as lack of food). At that time, the micronucleus takes over and directs the tetrahymena to undergo sexual reproduction with another stressed tetrahymena, producing four offspring identical to each other with a mix of the parental genes.