SULFIDIC KARST SYSTEMS
Of all the known caves in the world, very few formed from sulfuric acid speleogenesis, and even fewer are actually forming today by this process. While these caves are themselves quite interesting phenomena from a geological standpoint, these systems also have many different microhabitats occupied by a wide variety of microorganisms. In general, the composition of microbial communities in the caves is controlled by the chemistry of the groundwater, the sources and types of reduced sulfur compounds and other essential nutrients, as well as the mineralogy of the host bedrock. The following examples are from some of the known and well-characterized sulfidic cave systems.
| Movile Cave is located approximately
3 km from the Black Sea coast in Southern Dobrogea. The cave has 200 m of dry passages,
and ~40 m of submerged passages and airbells (shown here is an airbell). The cave and its
strange ecosystem were discovered in 1986, although sulfidic springs have been known from
the region since the Roman times. To date there are about 30+ new species of terrestial
and aquatic animals that have been found in the Movile Cave and surrounding sulfidic
groundwater system. The ecosystem was the first suggested terrestrial system based
chemosynthesis, similar to the deep sea hydrothermal vents (Sarbu et al. 1996). The
chemosynthetic microorganisms can be found in the lower part of the cave, in one of three
types of microbial mats. Here are the wall and floating mats. A submerged mat (not shown)
covers rock and sediment surfaces at the bottom of the water column. Below several
millimeters in the mats, the water becomes anoxic, and consequently the submerged mat
consists of mostly anaerobes, especially methanogens. The wall mat consists of
predominately fungi, but there are sulfur-oxidizers and other bacteria found, as well as
moist organic material adhered to the rock surface, about 2-3 cm thick in places. The
floating mat is quite unique to the cave. This mat is a complex microbial habitat,
comprised of fungal hypae and an assortment of bacteria. The mat is very thin (like wet
tissue paper) and is kept afloat due to air bubbles. For more information: Movile Cave Project - compiled by Romanian researchers
Karst Water's Institute
2nd Annual Top Ten List of Endangered |
Airbell in Movile Cave, Romania, with floating mats and
wall mats. |
| Cesspool Cave is located in
Allegheny County, Virginia, near the West Virginia border. The cave is developed in
travertine, formed as a result of travertine-depositing streams common to the area. The
cave only has 20 m of passage so there is only total darkness at the farthest points in
the cave. Hubbard et al. (1990) reported the presence of sulfur-oxidizing bacteria in the
cave, and suggested that these microbes were responsible for cave formation through the
generation of sulfuric acid. Webs of microbial mat float on the sediment surfaces with
organic material that gets washed into the cave. Porter (1999) found that the predominate
microorgniams are chemoautotrophs based on primary productivity measurements. While
thiobacilli were most commonly observed microscopically, molecular genetic data suggest
that most of the microbes are Thiothrix spp. and Thiovulum spp. 16S rDNA
gene sequences of pure cultures of sulfur-oxidzing bacteria reveal the presence of Thiomonas
thermosulfatus (syn. Thiobacillus thermosulfata) (Engel et al.,
2001).
|
Thin, delicate filamentous mats in Cesspool Cave,
Virginia, consisting of |
FRASASSI CAVES
(GROTTO GRANDE DEL VENTO - GROTTA DEL FIUME SYSTEM), ITALY
| The Frasassi Caves have been studied
extensively by Italian speleologists, and Galdenzi (1990) proposed that the system was
formed from sulfuric acid corrosion due to the active sulfidic stream that flows through
portions of the Grotta del Fiume cave, and abundant gypsum and sulfur accumulations
throughout the entire cave system. Where the cave passages intercept the sulfur stream,
there is a range of microbial mats. On the cave walls, sulfur and gypsum crystals grow on
and through a thick paste of milky gypsum and corroded limestone. Microbial draperies (the
consistancy of mucous) hang from crystals and wall protrusions, where some droplets are
about 5 cm in length. The average is 2 cm long, however. Acid droplets form at the tips of
the draperies and have a pH of 0 to 1 (Vlasceanu et al. 2000). The microbial community is
dominated by acidophilic, sulfur-oxidizers, especially Thiobacillus spp. Microbes
are not restricted to the wall surfaces, as there are thick filamentous mats in the stream
passages. For more information: The Frasassi Caves Federation, Genga, Italy |
Microbial draperies, or "snottites", in Grotta del Fiume, Italy,
associated |
| Lower Kane Cave is part of a
multi-cave system that formed by sulfuric acid speleogenesis along the Bighorn River in
north-central Wyoming. While Lower Kane Cave has several hydrogen sulfide-rich springs
that discharge into the passages, several caves in the area are no longer active but
retain some indications of having formed through sulfuric acid dissolution. The caves were
studied by Egemeier (1981), who found that oxidation of H2S to sulfuric acid
contributed to karstification by a "replacement-solution" mechanism. However,
this process was thought of as strictly abiotic, and that sulfur-oxidizing microorganisms
were not considered major players in the speleogenesis process. More recent investigations
in Lower Kane Cave and others indicate that diverse microbial populations proliferate the
caves, forming dense microbial mats and biofilms in both aqueous and subaerial settings.
Lower Kane and others are currently being investigated. (Photo by Annette Summers Engel,
2000) For more information: Lower Kane Cave Project, maintained by University of Texas at Austin, Geomicrobiology Research Group |
Sampling water in Lower Kane Cave, Wyoming. White area in front of Megan Porter is thick filamentous white microbial mats. (photo by Annette Summers Engel, 2000) |
Cueva de Villa Luz, or the "Cave of the Sulfur-Eaters", is located in Tabasco, Mexico, and has received quite a bit of media attention over the years. This exciting cave is full of stinky sulfide springs and microbial draperies reaching nearly 0.5 m in length. Like reported from other sulfidic caves, these snottites have acid drops at their tips, with a pH ~ 0, and are associated with gypsum and elemental sulfur deposits. The cave systems is currently being investigated by a large group of researchers at various institutions (Hose et al. 2000). For more information: Biological Investigations in Cueva de
Villa Luz, multiple investigators' |
Snottites in Cueva de Villa Luz. Image by Jim Pisarowicz, from 1998 Field trip website. |
Some other sulfidic (active and ancient) cave systems:
Lechuguilla Cave (site about cave microbiology, by Diana Northup and others), Lechugullia Cave links
Carlsbad Caverns, New Mexico (National Parks Site)
Cupp-Coutunn Cave, Turkmenistan (mostly images) and informative article regarding cave's geology and mineralogy, both by Vladimir Maltsev
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Last modified:07/22/04
