From the paper: https://www.biorxiv.org/content/10.1101/2025.05.02.651781v1

> ... we report the discovery of Candidatus Sukunaarchaeum mirabile, a novel archaeon with an unprecedentedly small genome of only 238 kbp —less than half the size of the smallest previously known archaeal genome— from a dinoflagellate-associated microbial community.

For comparison, the smallest bacteria genome, nasuia deltocephalinicola, is 139 kbp.