Sulfuric acid has created some of the largest limestone caves known. Many of these, such as the Lechuguilla/Carlsbad cave complex in New Mexico, are "dead" in the sense that sulfuric acid production and carbonate dissolution are no longer occurring. Sulfidic caves which are actively forming host rich sulfur-cycling microbial ecosystems that likely enhance the rate of acid production. Sulfidic caves are human-accessible windows into widespread subsurface processes which create porosity in carbonate rocks and cause diagenetic effects in carbonate sediments.


Sulfidic caves are model ecosystems for microbial ecology and evolution, including biogeography and gene flow between isolated microbial populations. Last but not least, sulfidic caves host microbial communities living at extremely low pH (0-1). Extreme acidophiles have the potential to show us the mechanisms and rates of evolution which allow microorganisms to adapt to conditions in "geochemical islands" radically different from the surrounding (neutral-pH) environment.

Sulfidic Caves in Italy
The Frasassi cave system is a large, actively-forming sulfidic cave located in central Italy. The lowest levels of the cave complex host an isolated ecosystem powered by sulfur-cycling microorganisms. We have initiated genetic, microbiological, geochemical and isotopic studies of both acidophilic and neutrophilic microbial communities at Frasassi in collaboration with Italian colleagues. Frasassi also hosts stratified lakes with sulfidic bottom waters, providing a modern analog for microbial biogeochemistry in stratified oceans in earth's past. Numerous other sulfidic caves are accessible in the Italian Apennines, providing an opportunity to study the time scale for microbial evolution among physically and geochemically isolated microbial populations.

A recent expedition to Grotta Nuova di Rio Garrafo, a hot sulfidic cave near Acquasanta Terme, revealed "snot bubbles" and pH 0-1 microbial "curtains" as well as snottites on cave walls.

A gallery of Italy cave fieldwork photos can be viewed here. Images are the property of the Macalady Geomicrobiology Group (PSU) and the Coldigioco Cave Science Consortium. Photo credits vary, please contact me for more information about specific images. Caves shown include many parts of the sulfidic cave systems developed in the Frasassi and Rio Garrafo gorges.

Sulfidic karst in Mexico
See Caves of Tabasco Project



Images of Frasassi cave biota:

Snottites and gypsum crusts
	Acidic slime drips ("snottites") hanging from gypsum wall crusts typically have pH 0-1. The drips contain dense assemblages of microorganisms including fungi, bacteria and archaea. Learn more.
	Snottites with elemental sulfur on gypsum crusts.
	Optical micrograph of snottite sample showing fungal hyphae, fungal spores, and dense assemblage of prokaryotic cells. Scale bar 10 um.
	Fluorescence in situ hybridization (FISH) micrograph of snottite sample. Cells are ~ 1 um diameter. Green = EUB338+, blue = DAPI.
	SEM micrograph of gypsum crust.
People
	Sandro Galdenzi at a cave entrance.
	Laura Cleaveland, Sandro Mariani, and Elisabetta Ferroni prepare for a long trip.
	Annaliese Eipert filtering cave stream water in pursuit of an indpendent project while on the Carleton Geology Program in Italy.
	Sandro Mariani (left) and Simone Cerioni filter cave lake water.
	Bess Koffman takes note. Bess did her senior thesis (Geology, Carleton College) on the phylogeny of Frasassi stream biofilms.
	Sandro Mariani (right) and Giorgio Roscioni (left) prepare to sample a remote cave lake.
Clay vermiculations
	Gravity defying network of clay vermiculations (dark areas) on cave walls. The vermiculations are ~ 3 cm wide. Learn more.
	SEM micrograph of clay vermiculation sample showing filamentous (tube-shaped) microbial cell envelopes. Long intertwined filaments (> 100 um) may play a role in forming or stabilizing the vermiculations.
Stream biofilms
	White biofilms in shallow cave stream. Water is flowing towards top. Stream channel ~ 2 m wide. Learn more.
	FISH micrograph of stream biofilm shown above. Large filaments are ~ 8 um diamter. Green = EUB338+, blue = DAPI.
	FISH micrograph of stream bioflim. Thin filaments are ~2 um diameter. Green = EUB338+, red = Eps402a, blue = DAPI.

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Coldigioco Geological Observatory



The Frasassi Gorge

Cave scientist and collaborator Sandro Galdenzi cleans his equipment.

Italian Cavers

The Frasassi caves were discovered in 1971 by a group of Italian cavers in the Gruppo Speleologico Marchigiano CAI. Caver Alessandro Montanari (shown above) is now Director of the Osservatorio Geologico di Coldigioco, a world class independent research facility for earth science research and education. Some parts of the Frasassi complex can be visited by the public, and are among the largest show caves in Europe.

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