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| Titel |
High resolution characterization of stromatolitic iron oxidizing microbial mats from a subterranean deep biosphere |
| VerfasserIn |
Barbara Zippel, Christine Heim, Danny Ionescu, Thomas R. Neu, Dirk de Beer, Joachim Reitner  , Volker Thiel |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250055676
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| Zusammenfassung |
| Stromatolitic iron oxidizing microbial mats form on many rock surfaces in the Äspö Hard
Rock Laboratory (Southern Sweden). Most intense growth of these mineralizing mats is
observed at depths more than 150m below the surface, wherever groundwater drops from the
tunnel ceiling. The microbial consortia harbour highly diverse chemolithotrophic microbial
communities consisting mainly of iron oxidizing bacteria (Mariprofundus sp., Gallionella
ferruginea), ammonia and nitrite oxidizing bacteria (Candidatus Nitrotoga, Nitrosomonas
europaea, Nitrospira moscoviensis), magnetotactic bacteria (Magnetospirillum magneticum),
and crenarchaeota. Cross-sections of mineralized microbial mats showed a distinctive
stromatolitic character, consisting of laminae with internal dendritic structures. These textures
strikingly resemble the enigmatic Frutexites structures reported from particular
Paleozoic carbonate rocks. The mineralized portions of these mats consist of iron
hydroxides, iron oxides, and, less abundant, manganese oxides. Small amounts
of siderite, calcite, and siliceous material occur along with iron and manganese
oxides.
A key role in the biomineralization process of chemolithotrophic microorganisms is assigned
to negatively charged organic surfaces like cell surfaces and extracellular polymeric
substances (EPS). These reactive interfaces act as “mineralizing templates” via cation
binding, complex formation and thus mineral nucleation (Konhauser, 2007; Westall,
2008).
Structural characterization and localization of microbial cells and associated EPS structures
within the mineralized mats were achieved by reflected light microscopy and confocal Laser
Scanning Microscopy (LSM). LSM provides simultaneous information about the
3-dimensional structure of living, fully hydrated and complex environmental communities.
Distribution of microorganisms in the mats was examined using nucleic acid specific
fluorochromes (Syto64; Sybr Green) and protein-specific fluorochromes (Sypro-Red,
Sypro-Orange). In addition, fluorescence lectin-binding analysis was employed for the
characterization of EPS glycoconjugates. To study the vertical distribution of cellular and
polymeric constituents within the mats, fresh samples were embedded in special cryomedium
(SCEM) and cryosectioned using cryofilm (Finetec). Subsequently, microbial mat sections
where stained with nucleic acid and protein-specific fluorochromes as well as selected
lectins.
A variety of morphologically distinct bacterial structures was localized within the mats,
including cocci-, rod-shaped and filamentous bacteria. In cases, especially single cells were
surrounded by a layer stained by protein-specific fluorochromes. Fluorescence lectin-binding
analysis resulted in binding of 13 out of 35 lectins tested. They preferentially bound to (i)
diffuse (“cloud-like”) and (ii) cell-associated EPS glycoconjugates. Diffuse EPS
glycoconjugates were visualized by amino sugar – specific lectins only (TL, UDA, PSA, and
WFA). They showed a rather homogeneous distribution within the upper 500 μm of the
mineralized mats. Cell-associated EPS glycoconjugates were visualized by amino sugar –
specific lectins and also reacted with lectins specific for fucose, lactose and galactose
residues. This second type of EPS glycoconjugates was found preferentially in
deeper parts of the mats and frequently formed large aggregates with bacterial cells
inside.
Noticeably, a mineralized thin layer characterized by very dense reflection signals was
localized approximately 20 μm below the crust surface. In some cases, a “street”-like
colonization by microbes was observed within this thin subsurface layer, whose origin and
function is as yet unclear.
The combination of laser scanning microscopy with differential fluorescence staining and
lectin-binding using adequate preparation protocols represents a useful tool providing deeper
insight into the internal architecture of mineralizing biofilms and their component organic
matrices and mineralization templates. Such knowledge will be essential for the interpretation
of related morphological and chemical signatures of microbial life in ancient sedimentary
rocks. This project is part of the Research Unit 571 “Geobiology of Organo- and Biofilms”,
funded by the German Research Foundation (DFG-FOR 571; publication #54). |
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