Discovery of microfossil in China from the 518-million-year-old Qingjiang biota sheds light on adaptive evolution

Microbial sulfate reduction dating back to the Paleoarchean plays a crucial role in driving global carbon and sulfur cycles in ancient and modern Earth. Over 150 species of sulfate reducers from bacterial and archaeal phyla ...
Recently, a 518-million-year-old microbial fossil from China identified as an ancient sulfate-reducing bacterium has shed light on the adaptive evolution of sulfate-reducing in response to Earth's oxygenation events. This new fossil, named as Qingjiangonema cambria, is reported by a research team led by Prof. Xingliang Zhang from the Shaanxi Key Laboratory of Early Life and Environments at Northwest University, Prof. Jinhua Li from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS) and Prof. Yinzhao Wang from the School of Life Sciences and Biotechnology at Shanghai Jiao Tong University.
The study is published in the journal Science Bulletin.

Qingjiangonema was discovered in black shales of the Shuijingtuo Formation that yields the Qingjiang biota, an early Cambrian Burgess Shale–type (BST) fossil Lagerstätte of South China. And it appears as a long filament comprising hundreds of rod-shaped cells. Cells are constricted at junctions, ∼1 to 3 μm wide and ∼0.8 to 11.0 μm long.

Each cell is externally enveloped by a trilaminar ultrathin film and internally filled by equimorphic and equidimensional pyrite microcrystals. The unique chain-like morphology and its presence in black shales (cemented anoxic mud), provide crucial clues to determine the biological affinity of Qingjiangonema. The faithful replication of cell morphology by pyrite microcrystals infilling suggests that Qingjiangonema was able to precipitate minerals intracellularly when it was alive.

(a) Optical photograph of a soft-bodied macrofossil (specimen No. cy266) of unknown affinity, with abundant filamentous microfossils of Q. cambria (b–g) present on the surface. (b) SEM image of rod-shaped cells, photographed after acid maceration, showing equimorphic and equidimensional microcrystals within each cell. (c) SEM image, showing a cell embedded in claystone sediments. (d, e) BSEM images, showing multicellular filaments. (f) SEM image of a four-cell filament, showing elongated cells with a division groove at the mid length (arrows). (i) BSEM image of a filament composed of cells of variable length; note constrictions between neighboring cells. Credit: Science China Press
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(a, b) SEM images of a Q. cambria on the macrofossil in Fig. 2a, indicating FIB milling positions of longitudinal and cross sections, respectively. (c) STEM-HAADF image of the ultrathin cross section in (b), showing microcrystals tightly packed within a thin envelope. (d) STEM-HAADF image of the ultrathin longitudinal section in (a), showing that neighboring cells are disconnected with intercellular boundaries, note that microcrystals are much smaller in the smaller cell than those in the larger neighboring cells. (e) Close-up of the boxed area in (c), showing the ultra-thin, multilayered film comparable to the typical cell envelope of Gram-negative bacteria, which comprises a cytoplasmic membrane (CM), a periplasmic space (PS) and an outer membrane (OM). Credit: Science China Press
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(a) Time tree of the major lineages of the phylum Desulfobacterota, with the sulfate-reducing Desulfonema and sulfide-oxidizing cable bacteria falling in two separate linages. (b) Evolution of Earth's atmospheric oxygen content through time, PAL = present atmospheric level. (c) Simplified estimate for the history of seawater sulfate concentrations. Shaded areas crossing (a) to (c) represent time intervals of GOE and NOE, respectively, highlighting the coincidence between phylogenetic evolution of the sulfate-reducing bacteria and major increases of Earth's atmospheric oxygen content as well as oceanic sulfate concentrations. Credit: Science China Press
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