A recent study by researchers from Yonsei University (led by Jaekyung Yoon and Jihyun F. Kim) and collaborators proposes that microbes capable of oxidizing iron in complete darkness and without oxygen could explain some banded iron formations (BIFs) from Snowball Earth periods.

The work centers on Holocene (roughly the last ~11,700 years) sediments recovered from beneath Antarctica’s Larsen C Ice Shelf. A sediment core (GC16B) from ~324 m water depth provided a pristine, largely undisturbed archive sealed under ice since the Last Glacial Maximum.

Microbial community profiling (16S rRNA amplicon sequencing across many layers) showed distinct phases that track geological facies: more diverse communities in shallower, occasionally open-marine intervals versus chemolithoautotroph-dominated assemblages in deeper anoxic, aphotic sub- ice- shelf layers.

A keystone taxon in the sub- ice-shelf community is an uncultured member of the Thermodesulfovibrionia (phylum Nitrospirota). Metagenome -assembled genomes from these layers revealed a novel candidate order (Candidatus Mariimomonadales). The organism was provisionally named Candidatus Mariimomonas ferrooxydans.

Its genome encodes Cyc2, a fused porin– cytochrome outer -membrane protein previously linked to Fe (II) oxidation in other bacteria, along with other cytochromes, iron- sulfur proteins, and nitrate- related machinery that could support anaerobic iron oxidation coupled (directly or indirectly) to nitrate reduction.

Functional confirmation came from expressing the cyc2 gene (codon- optimized) in Escherichia coli: the protein incorporated heme, showed peroxidase activity, and significantly accelerated ferrous iron oxidation relative to controls.

Fluorescence in situ hybridization also visualized the cells in the sediment. Elevated iron- to- titanium ratios in the anoxic intervals roughly coincide with the distribution of these bacteria, consistent with in situ iron mineralization.

These dark, anoxic, oligotrophic, ice- covered sediments are presented as a modern analogue for synglacial BIFs deposited during Neoproterozoic Snowball Earth events (when ice reached low latitudes and iron- rich layers accumulated under or near ice).

Classic models emphasize photosynthetic cyanobacteria or anoxygenic photoferrotrophs that require light. The Larsen C findings supply genomic and experimental evidence that chemolithotrophic iron oxidizers can drive substantial Fe (III) precipitation without sunlight or free oxygen, potentially resolving how iron formations could accumulate under global ice cover.

The authors note this pathway may also have operated earlier in Earth’s history and has implications for interpreting iron minerals on other worlds (e.g., Mars) or icy- ocean moons.

The peer- reviewed paper is “Novel Antarctic chemolithotroph drives iron biomineralization” (Yoon et al., Microbiome, 2026).

A related preprint appeared earlier on bioRxiv under a similar title. Limitations include reliance on whole-genome amplification (due to very low DNA yields in deeper layers) and the still- open question of whether iron oxidation is directly coupled to nitrate reduction or involves abiotic steps. Overall, the work expands the known diversity of iron- cycling microbes and challenges purely phototroph- centered explanations for certain ancient iron deposits.

Novel Antarctic chemolithotroph drives iron biomineralization

Background

Iron is the most abundant redox- active metal in Earth’s crust and is tightly linked to many biogeochemical cycles, yet the organisms and mechanisms of iron oxidation are still incompletely understood. Banded iron formations (BIFs) are a major Precambrian iron- ore reservoir, but the biological contribution to their formation remains debated. Although large- scale BIF deposition largely stopped after the Proterozoic, modern Holocene sediments under fluctuating redox conditions offer analogues for studying similar iron-cycling processes.

Results

Sediments from beneath the Larsen C Ice Shelf (LCIS), Antarctica, record microbial community shifts that closely track environmental changes through the Holocene. Metagenomic analyses identified three dominant microbial phases that align with geological facies boundaries:

• Phase A (shallower, occasionally open-marine conditions) showed higher taxonomic richness.
• Phases B and C (deeper sub- ice-shelf sediments) were largely anoxic and dominated by diverse chemolithoautotrophic metabolisms.

A keystone taxon among the uncultured Thermodesulfovibrionia (visualized by fluorescence in situ hybridization) was designated Candidatus Mariimomonas ferrooxydans. It forms a novel clade within the phylum Nitrospirota. Metagenome- assembled genome analysis identified a putative outer- membrane Fe (II) oxidase, Cyc2, whose iron-oxidation activity was experimentally confirmed.

Conclusion

The LCIS sediments preserve a dynamic record of microbiome, environment interactions. The findings provide insights into microbial iron mineralization that parallel the biogeochemistry of the geologic past, particularly synglacial iron formation during the Neoproterozoic Snowball Earth.

Key additional points from the paper

• Core GC16B (236 cm long) was recovered from ~324 m water depth in the Larsen C embayment (north-western Weddell Sea). It preserves an essentially undisturbed Holocene record sealed under ice since the Last Glacial Maximum.
• Lithological units range from glacial till diamicton at the base, through parallel- and cross- laminated muds under the floating ice shelf, to sandy mud with ice-rafted debris near the top.
• 16S rRNA amplicon sequencing (V5- V8) of 37 layers and shotgun metagenomics (with whole-genome amplification due to very low DNA yields) revealed the community structure and metabolic potential.
• Elevated iron- to- titanium ratios in the anoxic intervals roughly coincide with the distribution of the Thermodesulfovibrionia, supporting in situ microbial iron mineralization under dark, anoxic conditions.
• Heterologous expression of the cyc2 gene in Escherichia coli confirmed its ability to catalyze Fe (II) oxidation.

The study positions these modern sub-ice-shelf sediments as a living analogue for how chemolithotrophic iron oxidizers (rather than solely light- dependent phototrophs) could have contributed to iron deposition under global ice cover during Snowball Earth events.

Published:  Microbiome, 14, Article 225, 8 October 2026 (open access)

DOI: 10.1186/s40168-026-02536-0

Provided: Yonsei University

Authors: Jaekyung Yoon,
Boyoung Lee,
Kyu-Cheul Yoo,
Min-Jung Kwak,
Hae Jung Song,
Chung Yeon Hwang,
Yusook Chung,
Kitae Kim,
Soon-Kyeong Kwon,
Ju Yeon Song,
Hwan Su Yoon&
Jihyun F. Kim

Background

Iron, the most abundant redox-active metal in the Earth’s crust, is coupled to numerous biogeochemical cycles. However, the mechanisms of iron oxidation and the organisms involved remain incompletely understood. Banded iron formations (BIFs) are a major reservoir of iron ore in the Precambrian sedimentary record, yet the biological contribution to their genesis remains a subject of unresolved debate. While large-scale BIF deposition largely ceased after the Proterozoic, microbial activity in modern Holocene sediments under fluctuating redox conditions provides a unique opportunity to examine mechanisms reminiscent of ancient iron cycling. Here, we report the stratigraphic sequence of microbiome profiles recorded in laminated facies with iron-containing crystalline illite in the embayment sediments beneath the Larsen C Ice Shelf (LCIS) in Antarctica during the Holocene.