Chromatin in a marine picoeukaryote is a disordered assemblage of nucleosomes.

Chromatin organization is central to many conserved biological processes, but it is generally unknown how the underlying nucleosomes are arranged in situ. Here, we have used electron cryotomography to study chromatin in the picoplankton Ostreococcus tauri, the smallest known free-living eukaryote. By visualizing the nucleosome densities directly, we find that O. tauri chromosomes do not arrange into discrete, compact bodies or any other higher level of order. In contrast to the textbook 30-nm fiber model, O. tauri chromatin resembles a disordered assemblage of nucleosomes akin to the polymer melt model. This disorganized nucleosome arrangement has important implications for potentially conserved functions in tiny eukaryotes such as the clustering of nonhomologous chromosomes at the kinetochore during mitosis and the independent regulation of closely positioned adjacent genes.