Movile Cave () is a cave near Mangalia, Constanța County, Romania discovered in 1986 by Cristian Lascu during construction work a few kilometers from the Black Sea coast. It is notable for its unique subterranean groundwater ecosystem abundant in hydrogen sulfide and carbon dioxide, but low in oxygen. Life in the cave has been separated from the outside for the past 5.5 million years and it is based completely on chemosynthesis. Due to its extreme environment, access to Movile Cave is strictly controlled, and a limited number of researchers have permission to study its conditions.
Similar caves where life partly or fully depends on chemosynthesis have been found in Ein-Nur Cave and Ayalon Cave (Israel), Frasassi Caves (Italy), Melissotrypa Cave (Elassona municipality, Greece), Tashan Cave (Iran), caves in the Sharo-Argun Valley in the Caucasus Mountains, Lower Kane Cave and Cesspool Cave (Wyoming and Alleghany County, VA, USA), and Villa Luz Cave (Mexico).Sârbu, Şerban M. "The fascinating biology of stinky caves", - ARPHA Conference Abstracts, 25th International Conference on Subterranean Biology, Cluj-Napoca, 18–22 July 2022. Re-accessed 23 March 2024.Chiciudean, I., Russo, G., Bogdan, D.F. et al. "Competition-cooperation in the chemoautotrophic ecosystem of Movile Cave: first metagenomic approach on sediments." Environmental Microbiome 17, 44 (2022). Permanent . Re-accessed 23 March 2024.
Description Movile Cave is a network of paths in limestone that are approximately long, with portions that are partially or fully submerged by hydrothermal waters. The temperature of the air and water is a constant 21 °C (70 °F) and the relative humidity is about 100%. Access to the cave is limited to a few researchers per year, to minimize external impact on the delicate ecosystem.
The air in the cave is extremely different from the outer atmosphere. The level of oxygen is only a third to half of the concentration found in open air (7–10% O2 in the cave atmosphere, compared to 21% O2 in air), and about 100 times more carbon dioxide (2–3.5% CO2 in the cave atmosphere, versus 0.04% CO2 in air). It also contains 1–2% methane (CH4) and both the air and waters of the cave contain high concentrations of hydrogen sulfide (H2S) and ammonia (NH3). The lake waters only contain as little as 9–16 μM dissolved oxygen at the surface; <1 μM below 3–4 cm. As depth increases in the lake, the water becomes completely anoxic.
Biogeochemical cycling Movile Cave is chemically characterized by high concentrations of sulfide, with levels reaching up to 500 μM. This highly anaerobic, sulfur-rich environment shares several similarities with deep-sea hydrothermal vents, where sulfur oxidation plays a central role in energy production. The cave's biochemical processes are driven by aerobic elemental sulfur-oxidizing bacteria, which serve as the primary producers of biomass for the ecosystem.
Though they are less abundant, methylotrophs and diazotrophs are also important primary producers that feed on methane and contribute to the cave's carbon and nitrogen cycling.
Geology Movile Cave is classified as a karst cave that formed through weathering and dissolution of carbonate rocks in view of acidic groundwater over millennia. A term for this is hypogene speleogenesis, which would allow the formation of an entire system of caves isolated from surface influences for about 5.5 million years. Most caves have at least some surface exposure, but Movile Cave is sealed, establishing an extreme but stable ecosystem. Such geological isolation is what sustains a chemosynthesis-based ecosystem, which is unique.
Biology Movile Cave supports a complex ecosystem built upon chemosynthetic bacteria. These bacteria derive energy from oxidizing hydrogen sulfide and methane, forming the base of the food web. This makes the cave one of the few known ecosystems that do not rely on sunlight as a source of primary production. Similar chemosynthetic ecosystems have been found in Ayalon Cave and Villa Luz Cave.
The cave is known to contain 57 animal species,GESS Lab. (2020). Movile Cave - a unique ecosystem. Gesslab. https://www.gesslab.org/movile-caveGESS Lab. (2020a). List of invertebrate species encountered and described in the Movile Cave ecosystem. among them leeches, spiders, pseudoscorpions, woodlice,Karen Graham: Movile Cave in Romania has an ecosystem unlike any other on Earth. In: DigitalJournal. 25 May 2016. Source: Science centipedes (Cryptops speleorex),* Varpu Vahtera, Pavel Stoev, Nesrine Akkari: Five million years in the darkness: A new troglomorphic species of Cryptops Leach, 1814 (Chilopoda, Scolopendromorpha) from Movile Cave, Romania. On: ZooKeys 1004: pp 1-26. doi:10.3897/zookeys.1004.58537 (16 December 2020). See also: David Nield: Meet The 'King' of a Toxic Underground Ecosystem Unlike Anywhere Else on Earth. On: sciencealert. 19 December 2020. About Cryptops speleorex, the cave dwelling sister species of C. hortensis, Cryptopidae New Centipede Discovered on Top of Food Chain in Hellish Ecosystem of a Sulfur-Soaked Romanian Cave. On: SciTechDaily. December 16, 2020 water scorpions (Nepa anophthalma), and also snails.Andrzej Falniowski, Magdalena Szarowska, Ioan Sirbu, Alexandra Hillebrand, Mihai Baciu: Heleobia dobrogica (Grossu & Negrea, 1989)(Gastropoda: Rissooidea: Cochliopidae) and the estimated time of its isolation in a continental analogue of hydrothermal vents. In: Molluscan Research 28(3): pp 165-170. 22 Dec 2008. ISSN 1323-5818 Of these, 37 are endemic. While animals have lived in the cave for 5.5 million years, not all of them arrived simultaneously. One of the most recent animals recorded is the cave's only species of snail, Heleobia dobrogica, which has inhabited the cave for slightly more than 2 million years.
Adaptations to extreme environment The organisms within Movile Cave have gained unique adaptations in order to survive its extreme chemosynthetic environment. Many of these species lack functional eyes, as vision is not necessary in complete darkness. As a replacement, many species develop enhanced mechanosensory and chemosensory awareness that enables species to detect food and move in the darkness.
Due to the high concentration of toxic gases, many organisms have developed physiological mechanisms in order to tolerate the elevated carbon dioxide and hydrogen sulfide levels. Some species exhibit specialized respiratory adaptations that increase the efficiency of oxygen extraction from the cave's low oxygen atmosphere.
Key species and food web Among the key species found in Movile Cave are leeches, troglobitic (species that only live in caves and have fully adapted to it) centipedes, and cave-adapted spiders. Many of these organisms are reliant on the microbial production of chemosynthetic bacteria, with grazers feeding on bacterial biofilms and higher level consumers preying on the primary consumers.
Aquatic fauna
Aquatic invertebrates in Movile Cave include:
Meiofauna species: rotifers, nematodes, polychaetes. Crustaceans: copepods, ostracods, amphipods, and isopods. Gastropods: Heleobia dobrogica from family Moitessieriidae. True bugs: Nepa anophthalma from family Nepidae (water scorpions) – the only cave-adapted water scorpion in the world. Leeches: Haemopis caeca. Earthworms: Helodrilus sp. Flatworms: Dendrocoelum obstinatum
Due to the sealed environment of the cave, trophic chains present are extremely simple in comparison to comparable food webs aboveground. Its base is made up of bacterial biofilms, which are taken up by
Terrestrial – Archiboreoiulus serbansarbui, Trachelipus troglobius, Armadillidium tabacarui, Aquatic – Helodrilus sp., Niphargus racovitzai, Niphargus dancaui.
These primary consumers are then predated on by secondary consumers and top predators, including:
Terrestrial – Medon dobrogicus, Agraecina cristiani, Cryptops speleorex Aquatic – Nepa anophthalma, Haemopis caeca
Terrestrial fauna The terrestrial ecosystem present within the cave is composed of a variety of isopod, spider, pseudoscorpions, acarian, chilopods, millipedes, springtails, dipluran, and beetle species. The largest invertebrate and top predator in the cave is the centipede Cryptops speleorex, which constantly roams the cave for prey, from collembolan or coleoptera species to isopods. Other centipedes (Geophilus sp., Clinopodes carinthiacus) are also amongst the top predators in the cave.
The largest species diversity and density within the cave is found within the "Lake Room", containing many species of millipedes (Archiboreoiulus serbansarbui, Strongylosoma jaqueti), isopods (Trachelipus troglobius), and water scorpions (Nepa anophthalma), likely due to the presence of O2 in the chamber. In contrast, primarily isopods (Caucasonethes vandeli pygmaeus, Armadillidium tabacarui) are found in oxygen absent chambers.
Microbiology The Movile Cave represents a distinct habitat that shelters a multidisciplinary community of microbial eukaryotes adapted to very specific low-oxygen, high-sulfide, and methane-saturated environments. Their significance in ecosystem stability is as a result of their communities and association with chemosynthetic bacteria and archaea.
Prokaryotes The Movile Cave's unique groundwater system supports a complex community of chemoautotrophic primary producers. The different "rooms" of the cave have distinctive chemo-physiological conditions, allowing for the cultivation of unique bacterial genera in each environment.
Biofilm-associated bacterial community The surface waters and most of the cave walls are covered in varying sizes of bacterial biofilms ranging from small, white floating patches in the Lake Room and Air-bell I to yellowish biofilms up to thick found in Air-bell II.
Because of differing atmospheric conditions throughout the cave, the sizes and community compositions of biofilms differ significantly by location, resulting in metabolically-related microbial communities forming at specific cave sites. Preliminary studies of cave microbial mats revealed metabolically active methylotrophs and sulfur oxidizers, suggesting a lithotrophy-dominated ecosystem.
Kumaseran et al. discovered the representative species Ca. Methylomas sp. LWB in microbial mats, presenting evidence for aerobic methylotrophy in the cave.
Similar studies by Aerts et al. found complex groups of unique genera in biofilm samples collected at 3 sub-locations:
(1) from floating (surface) biofilms in Air-bell II (2) from wall biofilms in Air-bell II (3) from submerged biofilms on rocks below Air-Bell I
The identified microbial community from those samples are dominated by unique chemoautotrophic genera:
(1) Air-bell II surface (floating) biofilm: Aquimonas – promotes denitrification, keystone genus to determine biofilm stability Methylophilaceae – methanol and methylamine reducer Rhodomicrobium – iron oxidizing purple non-sulfur bacteria, using HS- as an alternate electron acceptor (2) Air-bell II wall biofilm: Woodsholea – halophilic bacteria, unclear metabolism (3) Air-bell I submerged biofilm: Nitrospiraceae – genus of ammonia and nitrite oxidizers
Cave water-associated bacterial community