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The widespread development of Permian/Triassic boundary microbialites is a key biosedimentary response to the mass extinction event. The microbial communities that precipitated these structures remain, however, poorly resolved. Here, we report abundant columnar microfossils from five Permian‐Triassic successions in South China. Four phases of the microfossil life cycle are recorded: baeocytes, juvenile column, mature column, and baeocyte‐splitting, which are interpreted as Pleurocapsales (cyanobacterial clade that reproduces by multiple fission). The great thickness (up to 13 m) of these microbial deposits and Raman spectrum analysis indicate a strong calcification capacity of the Pleurocapsales. Pronounced peaks of Mg/(Mg + Ca) ratios from microbialite and distinct Mg‐enrichment in microfossils indicate that Pleurocapsales blooms may also have facilitated dolomitization of the microbialites. Common occurrence, strong biomineralization capacity, baffling framework, and the close association with carbonate minerals suggest that Pleurocapsales were key constructors of post‐extinction microbialites. Plain Language Summary: As a biosedimentary response to the end‐Permian mass extinction, the widespread occurrence of microbialites has attracted increasing levels of interests from geologists worldwide. However, the major constituents of microbial communities that constructed the microbialites have long been disputed. Our new study describes morphological features, growth direction and reproduction characteristic of columnar microfossils obtained from the microbialites, and all characteristics indicate that these microfossils are assignable to a type of photosynthesizing bacteria, known as Pleurocapsales cyanobacteria. Both biomineralization and dolomitization of these Pleurocapsales are evaluated based on in‐situ observations and geochemical analyses. The new results show that the cluster‐branching morphologies, together with common occurrence and strong calcification capacity indicate that Pleurocapsales were key constructors of the microbialites. High Mg/(Mg + Ca) ratios coinciding with the microbialites and Mg‐enrichment in the microfossils suggest that the Pleurocapsales may have catalyzed dolomitization of the microbialites after the end‐Permian mass extinction. Key Points: Four phases of a microbe life cycle are recognized from fossils within microbialites reflecting Pleurocapsales asexual reproductionCommon occurrence and baffling framework indicate that Pleurocapsales cyanobacteria are important in constructing microbialitesPleurocapsales cyanobacteria were an important component of microbialite calcification and magnesium enrichment [ABSTRACT FROM AUTHOR]

Redox variations across the Permian‐Triassic boundary (PTB) have long been debated, especially during the proliferation of PTB microbialites. Here, we report redox fluctuations across the PTB to evaluate links between the two based on pyrite framboid analysis from basin to platform settings in South China. During the end‐Permian extinction, abundant framboids indicate a widespread anoxia that was likely a direct cause of extinction. In the earliest Triassic (Hindeodus parvus conodont zone), pyrite framboids were absent in ramp to basin and shallow, nonmicrobialite platform sections. In contrast, the coeval microbialites yield abundant framboids indicative of dysoxia. The fact that framboids were only confined to PTB microbialites and absent in other habitats indicates that microbe bloom may have stimulated dysoxic watermass and triggered the framboid growth within microbe aggregates. Thus, microbialites were not built in reducing settings, but instead, microbial proliferation caused local, dysoxia within shallow oxygenated platforms after the extinction. Plain Language Summary: The widespread occurrence of microbialites immediately after the end‐Permian extinction has attracted interest about their formation conditions. Microbialites are generally believed to be formed during microbial bloom in reducing conditions linked to the mass extinction. However, the cause‐and‐effect relationship between microbe bloom and reducing states of seawater has long been disputed. Our new results based on analyses of 17,306 pyrite framboid diameters and morphologies derived from various Permian‐Triassic boundary sections in South China show that microbialite formation (microbial bloom) was not triggered by reducing waters upwelled from deeper water masses, instead, it caused local, dysoxic areas within oxygenated platforms after the end‐Permian crisis. Key Points: A total of 17,306 pyrite framboids are analyzed in size and morphology from the Permian‐Triassic boundary beds of 26 sections, South ChinaFramboids were absent in ramp to basin and shallow, nonmicrobialite platforms after the extinction, but occurred in coeval microbialitesMicrobe bloom/microbialite growth was not triggered by reducing condition, but stimulated dysoxic water mass and triggered framboid growth [ABSTRACT FROM AUTHOR]