Co-enrichment of Ce and organics in microbe-like structures at the deep-sea ferromanganese crust surface.

Cerium is among the most important elements for understanding the redox state of the ocean throughout the Earth's history. Deep-sea ferromanganese crust is expected to be a major sink of Cerium in the oxygenated ocean, where Cerium enrichment from seawater is conventionally explained by inorganic oxidation on the surface of Mn oxide. As microbial colonization was demonstrated at the crust surface by recent studies, Cerium accumulation might result from microbial activities. Here we show microbe-like structures mineralized with Fe-Mn oxides at the deep-sea ferromanganese crust surface. Coordinated nanoscale solid analyses revealed that the microbe-like structures are enriched with Cerium and organic matter. The organic matter is enriched with aromatic and carboxylic functional groups rather than amide groups that are characteristic of living cells. The occurrence of Cerium (III) in the surface pit indicates the local establishment of conditions, under which Cerium (III) stably accumulated in the microbe-like structures. This new process found in the deep-sea environment with limited supply of photosynthetic organics could be relevant to ferromanganese oxides formed on ancient Earth and Mars. This article is part of the theme issue 'Planetary Protection for sustainable space exploration'.