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In the Apennine Mountains many examples of hypogene caves are known, generally related to present or past rise of sulfidic water that, mixing with oxygenated water of shallow flow systems, causes the sulfuric acid dissolution of limestone. The hypogene caves are generally located in small limestone outcrops covered by rocks of low permeability that in?uence the groundwater flowpaths. Some caves, however, are known also in hydrogeological massifs, where epigenic caves prevail. The hypogene caves show different patterns, ranging from phreatic to pure water table caves. The former prevail when karst evolved below the water table in structures almost completely covered by low permeability units; the latter occur in zones where a fast recharge of freshwater can reach the sulfidic water from the karst surface. The progressive lowering, thinning and removal of the low-permeability covers by non-karstic erosion processes can cause the progressive evolution from phreatic to water table caves. Active speleogenetic processes due to H2 S oxidation can be directly observed in different hydrogeologic settings: in highly permeable aquifers with ready recharge of freshwater (Frasassi caves), in thermal caves, below low permeability cover (Acquasanta Terme), or in marine thermal caves with salt water intrusion (Capo Palinuro).
Although ecosystems thriving in the absence of photosynthetic processes are no longer considered unique phenomena, we have yet to understand how these ecosystems are energetically sustained via chemosynthesis. Ecosystem energetics were measured in microbial mats from active sulfidic caves (Movile Cave, Romania; Frasassi Caves, Italy; Lower Kane Cave, Wyoming, USA; and Cesspool Cave, Virginia, USA) using radiotracer techniques. We also estimated bacterial diversity using 16S rRNA sequences to relate the productivity measurements to the composition of the microbial communities. All of the microbial communities investigated were dominated by chemolithoautotrophic productivity, with the highest rates from Movile Cave at 281 g C/m2/yr. Heterotrophic productivities were at least one order of magnitude less than autotrophy from all of the caves. We generated 414 new 16S rRNA gene sequences that represented 173 operational taxonomic units (OTUs) with 99% sequence similarity. Although 13% of these OTUs were found in more than one cave, the compositions of each community were significantly different from each other (P≤0.001). Autotrophic productivity was positively correlated with overall species richness and with the number of bacterial OTUs affiliated with the Epsilonproteobacteria, a group known for sulfur cycling and chemolithoautotrophy. Higher rates of autotrophy were also strongly positively correlated to available metabolic energy sources, and specifically to dissolved sulfide concentrations. The relationship of autotrophic productivity and heterotrophic cycling rates to bacterial species richness can significantly impact the diversity of higher trophic levels in chemolithoautotrophically-based cave ecosystems, with the systems possessing the highest productivity supporting abundant and diverse macro-invertebrate communities.
Most caves owe their origin to carbonic acid generated in the soil. In contrast, sulfuric acid caves are produced by the oxidation of sulfides beneath the surface. Although sulfuric acid caves are relatively few, they include some large and well-known examples, such as Carlsbad Cavern, New Mexico. They also provide evidence for a variety of deep-seated processes that are important to petroleum geology, ore geology, tectonic history, and the nascent field of karst geomicrobiology.
The measurement of the weight loss in limestone tablets placed in the Grotta del Fiume (Frasassi, Italy) provided data on the rate of limestone dissolution due to the sulfidic water and on the influence of local environmental conditions.
A linear average corrosion rate of 24 mm ka-1 was measured in stagnant water, while the values were higher (68-119 mm ka-1) where the hydrologic conditions facilitate water movement and gas exchanges. In these zones the increase in water aggressivity is due to mixing with descending, O2-rich, seepage water and is also favored by easier gas exchange between ground-water and the cave atmosphere. Very intense corrosion was due to weakly turbulent flow, which caused evident changes in the tablets shape in few months.
A comparison between the measured corrosion rates and the cave features showed that the values measured in the pools with stagnant water are too low to account for the largest solutional cave development, while the average values measured in the zones with moving water are compatible with the dimension of the cave rooms in the main cave levels, that must have developed when the base level was stable and hydrologic conditions favored the increase of water aggressivity.
Rich, diverse assemblages comprising a total (live + dead) of twenty-one ostracod species belonging to fifteen genera were recovered from phreatic waters of the hypogenic Frasassi Cave system and the adjacent Frasassi sulfidic spring and Sentino River in the Marche region of the northeastern Apennines of Italy. Specimens were recovered from ten sites, eight of which were in the phreatic waters of the cave system and sampled at different times of the year over a period of five years. Approximately 6900 specimens were recovered, the vast majority of which were disarticulated valves; live ostracods were also collected. The most abundant species in the sulfidic spring and Sentino River were Prionocypris zenkeri, Herpetocypris chevreuxi, and Cypridopsis vidua, while the phreatic waters of the cave system were dominated by two putatively new stygobitic species of Mixtacandona and Pseudolimnocythere and a species that was also abundant in the sulfidic spring, Fabaeformiscandona ex gr. F. fabaeformis.
Pseudocandona ex gr. P. eremita, likely another new stygobitic species, is recorded for
the first time in Italy. The relatively high diversity of the ostracod assemblages at Frasassi
could be attributed to the heterogeneity of groundwater and associated habitats or to
niche partitioning promoted by the creation of a chemoautotrophic ecosystem based on
sulfur-oxidizing bacteria. Other possible factors are the geologic age and hydrologic
conditions of the cave and karst aquifer system that possibly originated in the early–
middle Pleistocene when topographic uplift and incision enabled deep sulfidic waters to
reach the local carbonate aquifer. Flooding or active migration would have introduced
the invertebrates that now inhabit the Frasassi Cave system
Two species of the crustacean amphipod genus Niphargus inhabit the sulfidic groundwaters of the Frasassi caves in central Italy, and both harbor filamentous, sulfide-oxidizing Thiothrix ectosymbionts. As sulfide is toxic to most aerobic organisms, it appeared possible that the ectosymbionts could help their Niphargus hosts with detoxification processes. In this study, mortality due to sulfide was compared between Niphargus individuals with ectosymbionts and individuals whose ectosymbionts had been killed by antibiotic treatment. Both Frasassi- dwelling Niphargus species revealed exceptionally high tolerances to sulfide compared to other amphipod species studied so far. Niphargus individuals without viable ectosymbionts tolerated sulfide levels exceeding those occurring in Frasassi cave waters. Thus, the amphipods may employ Thiothrix-independent mechanisms for sulfide resistance.
Hypogean speleogenesis is the main cave formation process in the Frasassi area. The carbon flux represents an important proxy for the evalution of the different speleogenetic processes. The main sources of CO2 in the underground karst system are related to endogenic fluid emissions due to crustal regional degassing. Another important CO2 source is hydrogen sulfide oxidation. A small amount of CO2 is also contributed by visitors to the parts of the cave open to the public.
The condensation of acidic waters on subaerial carbonate surfaces (condensation corrosion) can be an important speleogenetic agent under certain conditions (Cigna and Forti, 1986; Sarbu and Lascu, 1997). Specific morphologies associated with condensation corrosion include notches, niches, cupolas, megascallops and domes (Audra, 2009), and have been recognized in many caves from different regions of the world and from different geologic settings. Condensation corrosion can be particularly important in thermal caves, where temperature differences facilitate air convection and water condensation, as well as in sulphidic caves, where degassing and subsequent oxidation of hydrogen sulphide (H2S) gas provides a ready source of acidity to the subaerial cave environment.
In pioneering studies on the formation of sulphidic caves, condensation corrosion via H2S degassing and oxidation to sulphuric acid was considered the primary mechanism for speleogenesis (Principi, 1931; Egemeier, 1981). However, recent research has cast doubt on the importance of subaerial H2S oxidation for sulphidic cave formation (Engel et al., 2004). In the Frasassi cave system, Italy, morphological evidence for both subaerial and subaqueous limestone dissolution has been extensively documented (Galdenzi, 1990; Galdenzi and Maruoka, 2003). In particular, corrosion above the water table has resulted in the formation of massive gypsum deposits as well as specific passage morphologies. Measured rates by Galdenzi et al. (1997) corroborated morphological evidence that condensation corrosion is important at least under certain conditions. Therefore, in order to better define the role of subaerial processes in the Frasassi cave system, we quantified sulphide flux to the cave atmosphere in the modern cave environment, and documented morphological evidence for subaerial corrosion in the past
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