Role and lifestyle of calcified cyanobacteria (Stanieria) in Permian–Triassic boundary microbialites

•The coccoid microfossils in the PTB microbialites in southern China are calcified sheaths of the extant unicellular cyanobacterium Stanieria;•The PTB Stanieria attached themselves only to densely packed phytoplanktons in bloom, in a “hitch-hiking” lifestyle;•Sheath calcification of Stanieria cells depends on high carbonate saturation of ambient water caused by phytoplanktonic blooms;•Calcified fossils of Stanieria cells can be indicative of ancient phytoplanktonic blooms;•Anoxia caused by phytoplanktonic blooms is favorable to preservation of Stanieria cells as fossils.

Calcified coccoid fossils in the Permian–Triassic boundary microbialites were widely considered to represent the remains of the organisms that formed the microbialites. However, this role has been recently attributed to the fossil casts of Microcystis cyanobacterial colonies, leaving the ecological role of the coccoid fossils, as well as their affinities, uncertain. Here we propose that these coccoid fossils bear morphological similarity and are related to the extant epiphytic cyanobacterium Stanieria, and that they lived in surface waters by attaching themselves to bloom-forming phytoplankton, in a hitch-hiking lifestyle. Calcification of the envelopes of these interpreted Stanieria cells produced the small thin-walled spherical calcareous coccoid fossils. Different from ordinary pseudoplanktons, Stanieria cells cannot attach themselves to sparse planktons, but only to densely packed phytoplanktons in bloom. The dependence of these calcified Stanieria fossils on this lifestyle makes them possible indicators of ancient planktonic blooms.