EXTREMOPHILES as Astrobiological Models. Группа авторов

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СКАЧАТЬ experimental approaches and applications.

      In active steam caves, meteoric waters characteristically descend through porous and fractured surface lava and eventually meet upward convection of heat and rising volcanic gases. Heat generates abundant steam that rises as a vapor-gas mixture through fractured lava crevices and fissures towards the surface. Near the surface, passageways—both horizontal and vertical—allow the vapors to condense on the cooler cave ceilings and walls, depositing characteristic chemical signatures and forming a matrix-like material rich in nutrients for extreme microorganisms. The steam exits the cave or vent opening and provides a diffused smoke-like flow known as a fumarole. Venting steam commonly proceeds in an artesian or burst flow. With large volumes of steam, visibility of the cave interior or vent opening can be obscured. Steam cave/vent openings can be small on the order of a few centimeters or large, one to three meters or larger for wide steam vents or vertical caves. Our selection of steam caves was based on the idea that while we and others [1.7] [1.10] [1.12] [1.16] had already investigated and identified organisms, especially Archaea in steam vents, there was not much information on organisms from those steam caves/vents regarding their in-situ appearance and some of the sites where they have previously been isolated left gaps in our information on the physical appearance of steam vent microorganisms at the structural level.

1.3 Steam Cave and Vent Sample Collection

      The steam cave and vent sites were selected for their chemical properties. These were initially recognized in flowing hot springs as iron and sulfur. Here, deposition of iron occurred in the anoxic zone of the spring and at the point of disappearance of iron at the anoxic/oxic interface, sulfur deposition took place and finally iron deposits formed on the existing sulfur in the anoxic/oxic zone. The same phenomenon can be seen in very small sulfur caves where the interior has no sulfur, the cave opening is small and sulfur deposition occurs only at the anoxic/oxic interface or on the outer oxic surface, sometimes along with iron deposits over sulfur. The other categories, nonsulfur and salt (the latter so far only seen in Hawai’i), represent a departure from the flowing hot spring analogy. Nonsulfur steam caves have not yet been fully categorized, although unusual thin filamentous organisms have been enriched from the H5 salt cave, and from sulfur caves such as SW 3 in Lassen. Nonsulfur caves farther below are considered for their solid surface chemistry by means of energy dispersive X-ray microanalysis (EDX). Sampling of steam cave or vent ceilings depends on the nature of the surface. With highly acidic solfatara areas, cave ceilings can be delicate and sometimes only careful hand sampling is essential. With deeper nonsulfur caves sometimes encountered in Hawai’i, this is less of a problem because the cave ceilings are formed of hard lava and only shallow samples are obtained. However, sampling through a small opening into a deep cave interior is not without its challenges, principally resulting from the high level of heat encountered, with or without leather gloves.

Table 1.1 Fumarole sampling locations and physical properties.

Location	pH	Temp (°C)	Type	Chemistry
Hawai’i Volcanoes National Park1
Hawai’i (1)	5.5	65.0	Cave	Nonsulfur
Hawai’i (2)	5.2	68.0	Cave	Nonsulfur
Hawai’i (3)	3.0	82.0	Cave	Sulfur
Hawai’i (4)	3.0	82.0	Cave	Iron/sulfur
Hawai’i (5)	5.0	68.0	Cave	Salt
Hawai’i (6)	4.5	66.0	Cave	Salt/sulfur
Lassen Volcanic National Park2
Sulphur Works, lower (1)	4.5	87.0	Cave	Nonsulfur
Sulphur Works, lower (2)	3.2	93.0	Vent	Sulfur
Sulphur Works, lower (3)	4.0	91.5	Cave	Sulfur
Sulphur Works, upper (4)	3.0	85.5	Vent	Iron
Sulphur Works, upper (5)	2.5	81.2	Cave	Iron
Valles Caldera National Preserve3
Sulphur Springs, New Mexico	2.0	89.0	Cave	Nonsulfur
Pozzuoli, Italy4
Solfatara Crater	3.0	94.0	Vent	Sulfur

¹ June 2016 sampling

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