Collenia is genus of fossil cyanobacteria that form a particular type of stromatolites.

[2] Collenia are stromatolites made up of convex layers flattened in the center, forming columnar colonies.

The microorganisms involved were likely photosynthetic bacteria expiring oxygen.

[3] Collenia stromatolites were very common in the Paleoproterozoic era during the Siderian and Rhyacian periods, about 2.2 to 2.4 billion years ago.