Unraveling Moonmilk's Microbial Mysteries
In the eternal darkness of alpine caves, a mysterious white substance holds clues to some of Earth's most resilient and ancient microbial life.
Deep within the labyrinthine caves of the Austrian Alps, a peculiar substance clings to the walls and ceilings—a soft, white deposit known as moonmilk. For centuries, these mysterious formations were mined for their purported medicinal properties, used as both human and veterinary medicine in the Alpine region as early as the 16th century 8 .
Moonmilk, a secondary calcite deposit found in caves worldwide, serves as a window into Earth's subsurface. These formations represent model systems to investigate the interaction between microorganisms and carbonatic rocks in oligotrophic environments—habitats with extremely low nutrient concentrations 1 4 .
Recent discoveries have revealed that moonmilk deposits contain thriving communities of Archaea, one of the three domains of life, which were once thought to inhabit only extreme environments like hot springs and salt lakes. The study of these cave-dwelling archaea is reshaping our understanding of life in the dark, nutrient-poor depths of our planet 1 6 .
Home to mysterious moonmilk formations
Thriving communities in extreme conditions
Clues to Earth's earliest life forms
For decades, Archaea were considered primarily extremophiles—organisms that thrive in conditions inhospitable to most life forms. However, research in caves has dramatically altered this perception. Archaea are now recognized as abundant and diverse inhabitants of temperate environments, including the moonmilk deposits of alpine caves 1 .
In these subterranean environments, archaea have been found to constitute a surprisingly large portion of the microbial community—sometimes comprising up to 50% of the total microbial populations in moonmilk deposits. Even more remarkably, in the actively forming surface parts of moonmilk, archaea can reach astonishing abundances of around 80% of all microorganisms present 1 2 .
To understand the distribution and ecological role of archaea in moonmilk, a comprehensive study was conducted examining deposits from 11 caves in the Austrian Alps 1 2 .
Moonmilk deposits were aseptically collected from various depths within the cave formations, distinguishing between the actively forming surface layer and the deeper sections extending down to the bedrock 1 .
Quantitative PCR (qPCR) and Denaturing Gradient Gel Electrophoresis (DGGE) sequencing were used to identify and quantify archaeal communities 1 2 .
Archaea were enriched in complex media designed to target different physiological requirements, including methanogenesis and ammonia oxidation 1 .
Researchers analyzed how archaeal distribution correlated with depth, oxygen availability, and other environmental factors 1 .
| Moonmilk Layer | Relative Abundance of Archaea | Dominant Archaeal Groups |
|---|---|---|
| Surface Layer | ~80% of total microbial community | Thaumarchaeota, Moonmilk Euryarchaeota |
| Intermediate Depth | ~50% of total microbial community | Mixed community |
| Near Bedrock | ~5% of total microbial community | Minor archaeal populations |
All moonmilk deposits were characterized by the presence of the same few habitat-specific archaeal species, despite being collected from different caves 1 .
The archaeal community showed distinct stratification based on depth and oxygen availability, with the highest abundances in the surface layers 1 2 .
Through extensive cultivation efforts, researchers successfully enriched the enigmatic Moonmilk Archaea and ammonia-oxidizing archaea (AOA) significantly above the level of bacteria 1 .
| Condition Parameter | Optimal Condition for Cultivation | Significance |
|---|---|---|
| Temperature | Cold temperatures | Reflects natural cave environment |
| Nutrient Availability | Oligotrophic conditions | Mimics nutrient-poor cave habitat |
| Incubation Time | Short incubation times | Accommodates unique growth rates |
| Oxygen Requirements | Anaerobic (microaerophilic) conditions | Matches subsurface oxygen levels |
| Additives | Application of erythromycin | Inhibits bacterial competitors |
Understanding the metabolic capabilities of moonmilk archaea provides insights into how life persists in energy-limited environments and contributes to cave ecosystem functioning.
The discovery that ammonia-oxidizing Thaumarchaeota represent the largest fraction of moonmilk archaea highlights the importance of nitrogen cycling in cave ecosystems 1 . These organisms obtain energy by oxidizing ammonia to nitrite, a process that likely contributes to the complex formation of moonmilk structures.
Contrary to what might be expected, the research revealed that methanogenesis is of marginal importance in these moonmilk deposits 1 . Instead, evidence points to the existence of still undiscovered metabolic pathways that represent vital elements in the archaeal moonmilk biome.
Potential involvement in calcium carbonate precipitation, contributing to moonmilk formation 4 .
As dominant members of the moonmilk microbiome, they likely influence the broader microbial community composition 6 .
| Research Tool/Solution | Application in Moonmilk Research | Function |
|---|---|---|
| qPCR with group-specific primers | Quantification of methanogens and other archaeal groups | Enables precise measurement of specific archaeal populations |
| DGGE Fingerprinting | Microbial community profiling | Separates DNA fragments to visualize community diversity |
| Stable Isotope Probing | Tracking metabolic activity | Identifies active microorganisms in biogeochemical cycling |
| Oligotrophic Culture Media | Cultivation of cave-adapted archaea | Provides low-nutrient conditions mimicking natural habitat |
| Erythromycin Supplementation | Selective enrichment of archaea | Inhibits bacterial growth to isolate archaeal communities |
The study of archaea in moonmilk deposits extends far beyond academic curiosity. Understanding these microbial communities has significant implications for multiple fields:
With caves like Shulgan-Tash in Russia featuring ancient Paleolithic paintings, understanding microbial communities that potentially contribute to biocorrosion is crucial for preservation efforts .
Cave microorganisms have shown potential for drug discovery, producing diverse secondary metabolites with antimicrobial properties 5 .
Cave environments serve as analogs for extraterrestrial habitats, with moonmilk studies informing the search for life on other planets 1 .
Future research will likely focus on cultivating the currently uncultivated archaeal lineages, sequencing their genomes to understand their metabolic potential, and further elucidating their role in speleothem formation and cave ecology.
Moonmilk deposits in alpine caves represent more than just geological curiosities—they are vibrant microbial ecosystems dominated by enigmatic archaea that challenge our understanding of where these microorganisms can thrive. The discovery that archaea can dominate cave microbial communities, forming distinct stratified populations based on environmental conditions, expands the known habitat range of this domain of life.
As research techniques continue to advance, particularly with the application of next-generation sequencing technologies 7 , we stand to uncover even more secrets held within these subterranean formations. The moonmilk biome, with its specialized archaeal communities, reminds us that Earth's least explored frontiers may be right beneath our feet, holding clues to microbial evolution, adaptation, and survival in extreme environments.
Acknowledgement: This article was based on comprehensive research conducted across multiple alpine caves, particularly building on the work published in "Microbial Ecology" (2016) titled "Archael Distribution in Moonmilk Deposits from Alpine Caves and Their Ecophysiological Potential."