# Millimetre‐Scale Stratification of Microbial Communities in Hydrothermal Sediments

## TL;DR

This study uses high-resolution imaging to reveal how microbial communities are arranged in millimeter-scale layers in hydrothermal sediments shaped by extreme conditions.

## Contribution

The study introduces high-resolution mass spectrometry imaging to uncover millimeter-scale microbial community stratification in hydrothermal sediments.

## Key findings

- Lipid zonation is tightly compressed to a 5-cm segment below the sediment–water interface.
- Anaerobic methane-oxidizing archaea and sulfate-reducing bacteria dominate a specific anoxic zone.
- Molecular signals from active microbial communities are limited to a siliceous concretion at depth.

## Abstract

Resolving the spatial organisation of microbial populations in environments shaped by steep thermal and geochemical gradients remains a challenge in environmental biogeochemistry. Conventional molecular biomarker or gene‐based approaches typically require large volumes of homogenised samples, limiting their ability to depict spatially structured microbial ecosystems, where critical microbial processes occur on millimetre scales. To overcome these limitations, we applied high‐resolution mass spectrometry imaging (MSI) to an 11.5 cm long sediment section from the hydrothermal Cathedral Hill mat complex in the Guaymas Basin, known for its extreme temperatures and sharp geochemical gradients. The μm‐scaled spatial resolution unveiled a nuanced lipidome zonation tightly compressed to a narrow 5‐cm segment below the sediment–water interface. The surface layer (above 1.1 cmbsf) hosts molecular patterns primarily shaped by opposing oxygen and sulphide gradients, followed by a near‐seamless transition to an anoxic zone dominated by anaerobic methane‐oxidising archaea (ANME) and sulphate‐reducing bacteria (SRB). At greater depth, molecular signals indicative of active microbial communities remained below the detection limit except for diverse, potentially ANME‐ and SRB‐related lipids concentrated within a siliceous concretion. The sharp transitions in lipid zonation hint at persistent redox zones and resilient microbial niches under intense fluid flow and dynamic geochemical gradients.

Zooming into the millimetre‐scale spatial distribution of organic molecules in hydrothermally impacted sediments using mass spectrometry imaging reveals a distinct lipid zonation with abrupt transitions at critical redox interfaces. This zonation reflects a pronounced stratification of diverse microbial communities and highlights surprisingly persistent microbial niches in this dynamic environment.