Qingjiangonema cambria

 Fossil of Qingjiangonema cambria（圖片來源：Cui L et al. (2024)，採用 CC BY 4.0 授權）

Age

Cambrian（"Stage 3"）

518 Ma

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Taxonomy

Domain: Bacteria

Kingdom: Pseudomonadati

Phylum: Desulfobacterota

Genus: Qingjiangonema

Species: Qingjiangonema cambria

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Morphological description

Qingjiangonema cambria is preserved as abundant straight, unbranched, and extremely slender filaments. Each filament is approximately 1–3 μm in diameter and can reach lengths of up to about 1 mm. These thread-like structures maintain a constant width and consist of hundreds to thousands of cells linked end to end, forming elongated multicellular chains.

Individual cells are typically rod-shaped, measuring roughly 1–3 μm in width and 0.8–11 μm in length. The boundaries between adjacent cells are often marked by a slight constriction. Within a single filament the cell width remains highly uniform, whereas cell length varies considerably, reflecting cells captured at different stages of division. Some cells display a central constriction interpreted as a division furrow, indicating that longitudinal cell division was occurring.

Under electron microscopy, the interiors of the cells are filled with pyrite microcrystals. These crystals range from approximately 150 to 1,100 nm in size and are tightly packed within each cell. Within a single cell the crystals are equant, equimorphic, and equidimensional, while differences in crystal size between neighboring cells correspond to differences in cell volume. The morphology of these microcrystals—cubic, octahedral, or intermediate forms—resembles that of pyrite framboids found in the same strata. However, unlike framboids that form spherical aggregates, these crystals closely conform to the shape of individual cells and replicate their internal morphology. The tight packing of microcrystals within each cell faithfully preserves cellular structure at microscopic scale.

Outside the cells, a very thin envelope approximately 9–50 nm thick is preserved. This envelope consists of three distinct layers comparable to the cell envelope structure of Gram-negative bacteria.

Pyrite crystals（圖片來源：Cui L et al. (2024)，採用 CC BY 4.0 授權）

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Etymology

The genus name Qingjiangonema combines “Qingjiang,” referring to the Qingjiang biota from which the fossils were discovered, with the Greek-derived suffix “nema,” meaning “thread,” highlighting the organism's filamentous form.

The species name cambria refers to the Cambrian Period, indicating the geological age of the fossil.

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Biological description

Qingjiangonema cambria represents a multicellular filamentous microorganism that lived within seafloor sediments during the early Cambrian. The cells preserved in the fossil are filled with equant pyrite microcrystals. Sulfur isotope measurements of these crystals display strong isotopic fractionation consistent with microbial sulfate reduction, with an average Δ³⁴S of approximately 40.83‰. This value closely matches the fractionation produced by the modern sulfate-reducing bacterium Desulfonema magnum under optimal growth conditions, indicating that the metabolism of this fossil organism involved sulfate reduction.

The fossils occur in black shale deposited in an anoxic marine environment rich in sulfate. Bottom waters were persistently oxygen-depleted, and microbial activity in the sediments produced abundant framboidal pyrite. Geological and geochemical evidence suggests that these microorganisms inhabited soft, oxygen-poor mud on the seafloor. After marine animals died and their bodies settled into the sediment, the microbes likely colonized the decaying tissues. Their metabolic activity promoted rapid precipitation of pyrite within the cells, preserving the filamentous structures three-dimensionally within the rock.

The overall morphology of Qingjiangonema cambria strongly resembles that of the modern filamentous sulfate-reducing genus Desulfonema. Both possess multicellular filaments composed of uniform cell chains and occupy anoxic sedimentary environments. On this basis, researchers infer that Qingjiangonema cambria represents an early multicellular member of the bacterial phylum Desulfobacterota.

This fossil indicates that multicellular filamentous sulfate-reducing bacteria had already appeared by approximately 518 million years ago during the Cambrian Period.

（Author: Shui-Ye You）

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Reference

Cui L et al. (2024). The Cambrian microfossil Qingjiangonema reveals the co-evolution of sulfate-reducing bacteria and the oxygenation of Earth's surface. Science Bulletin.

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