Abstract EANA2025-109

The Anammox (Anaerobic ammonium oxidation) metabolism represents a unique strategy restricted to a singular group of microorganisms within the Planctomycetota Phylum (1). At present, this metabolism converts ammonium (NH₄⁺) and nitrite (NO₂⁻) to molecular nitrogen (N₂) under anaerobic conditions. This transformation is a highly valued process within the nitrogen biogeochemical cycle, with great interest in the natural environment as well as in waste treatment.

Current literature locates the origin of Anammox metabolism around 2.5 billion years ago, coinciding with the Great Oxygenation Event at the beginning of the Proterozoic Eon (2). Here, we propose and argue about a significant earlier emergence of this unique metabolism, over 1 billion years earlier than previously suggested, during the early Archaean Eon (4.0-3.6 billion years ago) and among the earliest life forms. This ancient metabolism would also allow linking both N and C biogeochemical cycling in early Earth.

Early Earth conditions, around the Archaean Eon, showed intense volcanism and a reducing atmosphere. Ultramafic rocks, like peridotites among others, represented a major component of the emerging crust. In contact with the increasing condensation of water, peridotites could experiment serpentinization. During this process, in early Earth, favourable conditions for anammox metabolism could be present, such as alkaline pH, abundance of H2, N2 and CO2, potential catalytic metals and stable chemical gradients (3, 4, 5).

In this scenario, abiotic H₂ could react with N₂, generating intermediates such as hydrazine (N₂H₄). Hydrazine is a key intermediate of the anammox metabolism. Its key catalytic enzyme, hydrazine dehydrogenase (HDH), would be able to reverse the process to N2. Despite the expected instability of hydrazine, micropores within serpentinized rocks could contribute to its stability, accumulation and availability for the anammox metabolism. Several lines of evidence contribute to support this hypothesis. The catalytic centers in HDH depend on similar metals (Fe) to those assumed to catalyze the serpentinization process and abiotic hydrazine production. In addition, CO2 and the released H2 could lead to the generation of methane and small organic molecules such as acetate and formate. In this respect, anammox bacteria present a C metabolism based on the synthesis of organic C through a previously proposed ancient C synthesis pathway (3), the Wood-Ljungdahl pathway, dependent on H2 reducing power, suggesting a necessary link between biogeochemical cycles of C and N through the biotic compartment on early Earth.

This analysis suggests that anammox represents a unique metabolism linking C and N biotic cycles, preserved through Earth’s evolution within a unique subgroup of the Planctomycetota. Its origin is placed at the very early stages of Earth biome formation, paralleling with early thermophilic methanogenic microorganisms previously located at the early Archaean Eon (4, 5).

1. Cuecas, A., Barrau, M.J. & Gonzalez, J.M. (2023). Front. Microbiol., 15, 1355780.

2. Liao, T., Wang, S. et al. (2022). Mol. Biol. Evol., 39(8), msac170.

3. Lane, N., Allen, J.F. & Martin, W. (2010). BioEssays, 32(4), 271–280.

4. Moore, E., Jelen, B. et al. (2017). Nat. Geosci., 10(8), 629–636.

5. Helmbrecht, V., Reichelt, R. et al. (2025). Nat. Ecol. Evol., 9, 769–778.