Penn State Geosciences > Geomicrobiology Lab

Sulfuric acid has created some of the largest caves on the planet. Many of these, such as the Lechuguilla/Carlsbad cave complex in New Mexico, are "dead" in the sense that sulfuric acid production and limestone corrosion are no longer occurring. Actively-forming sulfidic caves host rich microbial ecosystems that enhance the rate of cave formation by producing sulfuric acid as a waste product. These caves are human-accessible windows into a largely inaccessible, below-ground world. They offer the opportunity to study geological and biological processes that affect large areas of the earth's subsurface, including those that create porosity in carbonate rocks (oil reservoirs) and cause diagenetic effects in carbonate sediments (climate and other records of earth history).

Last but not least, sulfidic caves are extraordinary model systems for ecology and evolution. Like hydrothermal vents and hot springs, they are geochemical "islands" that allow for tests of gene flow between isolated microbial populations. Genetic information encoded in the genomes of extreme acidophiles colonizing sulfuric acid drips on the cave walls offer clues about survival at one of the outer limits of life (pH 0-1). Chemoautotrophic animal-microbe symbioses that we recently discovered in sulfidic caves are similar to those at marine vents and seeps but younger (less evolved), providing a tantalizing glimpse at the ecological and genetic forces at work in the creation of biological partnerships across the domains of life. These any many other aspects of the biology and geology of sulfidic caves remain to be fully explored.

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.

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.