Algae from the Lower Devonian Rhynie chert: Harpericystis verecunda gen. et sp. nov., a probable green alga (Chlorophyta) that forms few-celled colonies

The Lower Devonian Rhynie chert preserves a remarkable diversity of microorganisms from an early terrestrial ecosystem. However, eukaryotic microalgae are rarely reported. This study describes Harpericystis verecunda gen. et sp. nov., a probable green alga (Chlorophyta) from the Rhynie chert, using more than 50 specimens, all of which are enclosed in a degraded land plant axis. Specimens include single cells up to 8(− 10) μm in diameter, dyads, decussate and tetrahedral tetrads, and small colonies comprised of 8 or 16 cells. Other specimens consist of both larger cells and tetrads of smaller cells, which either are the result of successive cell divisions (autosporogenesis), or have formed from multinucleate parent cells by directed cytokinesis. Cells, cell groups, and colonies are surrounded by a colorless mucilage. In some cases, it appears that the constituent cells of the colonies are interconnected by thickened plates of their envelopes or short processes. Harpericystis verecunda was non-motile and most likely planktonic. It shows similarities with various present-day colony-forming green algae, in particular with Coelastrum (Sphaeropleales) and Sphaerocystis (Chlamydomonadales), but its systematic affinity remains unresolved. This discovery is further evidence that intra-plant environments facilitated the preservation of fragile microbial life in the Rhynie setting.