# Airborne Halophilic and Non‐Halophilic Microbial Communities in an Underground Salt Mine Affected by Seasonal Environmental Fluctuations

## TL;DR

This study explores how seasonal changes affect airborne halophilic and non-halophilic microbes in a salt mine, showing that halophilic microbes decline significantly in winter due to lower humidity.

## Contribution

The study provides the first evidence of seasonal fluctuations in airborne halophilic archaea in a subterranean salt mine.

## Key findings

- Halophilic archaeal densities dropped by at least 13-fold in winter compared to summer.
- Only two halophilic taxa, Halalkalicoccus subterraneus and Halococcus hamelinensis, persisted through winter.
- Non-halophilic bacterial loads varied seasonally, with skin-associated bacteria dominating in visitor-intensive areas.

## Abstract

The UNESCO‐listed Bochnia Salt Mine in Poland is a heritage site whose galleries remain coupled to surface air, generating warm, humid, salt‐laden summers and cool, dry, ion‐poor winters. Airborne halophilic archaea were recently reported here but their seasonal behaviour remained unknown. We quantified physicochemical parameters and airborne microbes at three underground stations and an outdoor control site. Winter measurements of air samples were compared with previously published summer data from the same transect. Winter relative humidity (~42%–63%) was lower than summer values (~65%–77%). Furthest from the intake shaft, the combined Na+ + Cl− aerosol load fell from 2665 to 1280 μg/m3 (~52%), and cultivable halophilic archaea dropped from 7164 to 528 colony‐forming units/m3 (~93%). Averaged across the underground stations, halophilic archaeal densities declined by at least 13‐fold between seasons. Halophilic richness contracted from seven to two taxa. Only Halalkalicoccus subterraneus and
Halococcus hamelinensis
persisted through winter. Non‐halophilic bacterial loads varied by 3.5‐fold but species richness was relatively high. The visitor‐intensive chamber was dominated by skin‐associated bacteria, underscoring anthropogenic control of non‐halophilic communities. This first demonstration of seasonally responsive airborne halophilic archaea shows that their abundance and diversity are strongly influenced by microclimatic water availability in subterranean salt environments.

This study tests seasonal controls on airborne microbes in the UNESCO‐listed Bochnia Salt Mine. In summer, relative humidity exceeded the halite deliquescence threshold, coinciding with higher NaCl aerosol and greater densities of halophilic microbes; in winter, humidity stayed below deliquescence and halophilic microbes declined at least 13‐fold while halophilic archaea richness fell from seven to two taxa, with only Halalkalicoccus subterraneus and
Halococcus hamelinensis
persisting. Thus, halophilic archaea track in mine air moisture and saline aerosol loading.