SEM_BSE image and EDS elemental distribution mapsof Na, Al, Si, S, K, Mn and Fe of the cells

C

This story is told with the help of a green, a blue and a yellow arrow.Moving from top to bottom, the green arrow shows how we can identify perfectly mineralized Trebouxia-like lichen thallus photobionts inside the pores of Mount Fleming sandstones. If we follow the blue arrow, we see how these lichen photobionts share the space within the pores of the same sandstones with mineralized diatoms. The yellow arrow shows sacs containing diatoms in the walls of a pore in the same rock. The EDS technique reveals the elemental composition of these three zones including both the fossilized microorganisms and their environment within the pore.The EDS maps of the green and blue arrows show how the mineralized lichen photobionts mainly contain silicon. However, the yellow arrow indicates how the diatoms have lost their silicon content and are mineralized by S, K and Fe. Some even eventually become pure iron oxide. The sacs that envelop the diatoms are rich in S, K and Fe and the environment surrounding the photobionts transformed into silicon (green and blue arrows), is also rich in S, K and Fe. Raman spectroscopy reveals that the zones where EDS indicates the presence of S, K and Fe, are unequivocally comprised of jarosite.The questions we should ask ourselves are: why do the photobionts of lichen thalli become transformed into essentially silicon structures while the diatoms that originally had a silica frustule lose their silicon? If most diatoms now contain S, K and Fe, why do some diatoms only show the element iron? Why do lichen photobionts and diatoms often share the same pores? and Why do photobionts and diatoms always appear enveloped in a jarosite matrix?

C. Ascaso* 1, V. Souza-Egipsy 2, A. de los Ríos 1, C. Domingo 3 and J. Wierzchos 1

1. Instituto de Recursos Naturales. Centro de Ciencias Medioambientales. CSIC. Madrid, Spain. 2. Facultad de Ciencias. Departamento de Biología Vegetal, Universidad de Málaga. Spain. 3.Instituto de Estructura de la Materia. CSIC. Madrid.

ascaso@ccma.csic.es

*

*

Analysis and interpretation of microbial fossils in the extreme environment of the Dry Valley rocks, AntarticaAnalysis and interpretation of microbial fossils in the

extreme environment of the Dry Valley rocks, Antartica

50 nm

[1] Friedmann E.I. (1992) Science 215, 26: 1045-1053. [2] Wierzchos J, Ascaso C. (2001) Polar Biol. 24, 863-868.

However, if extensive biomobilization of elements occurs when microorganisms are biologically active and/or after their

decay, then microorganism fossil formation may be observed [3].

[3] Wierzchos J, Ascaso C. (2002) Int. J. of Astrobiology 1, 51-59.

Fossilized microorganisms, can be interpreted only through careful observation. SEM-BSE images revealing the ultrastructural features of mineralized cells. Raman spectroscopy had confirmed previous analysis obtained with EDS regarding to the composition of the mineralized cell and their mineralized environment. Thanks to Raman application in this study, the presence of jarosite filling the pore in samples of sandstone from Mount Fleming ( 77º33`S, 160º 06`E,2200 m

altitude) is definitively confirmed in all the analized cases.

BIOMINERALIZATION (formation of calcium oxalates and silica gel)

The EDS technique coupled to SEM-BSEpermitts the recognition of biotransformation,biomovilization and biomineralization processes in the substrate.

FUNGI

Live algal cell and fungi

Mineralized Trebouxia-like cells (algae) and fungi.

Mineralized diatoms cellsSome of the ultrastructural

features of living microorganisms, have been frequently observed in the fossilized microorganisms

SEM_BSE image and EDS distribution mapsof Al, Si, S, K and Fe in the mineralized cells and in the pore

* Mount Fleiming 77º33`S, 160º06È,2200 m altitude

SEM_BSE image and EDS elemental distribution mapsof Na, Al, Si, P, S, K, Ca and Fe of the sacs containing diatoms

Detail of a pore

Sacs containing diatoms

Acknowledgments:We thanks Ana Burton for revising the english text and Fernando Pinto, Sara Paniagua, M. Castillejo, J. M. Hontoria and Teresa Carnota for technical support.Prof Friedman provided the rock samples from Mount Fleming. The study was founded by the Ministerio de Ciencia e Innovación. Grants CGL-2006-04658 and CGL- 2007-62875/BOS

Cross section of a sandstonesample showing empty and fill pores

The presence of indigenous microbiota colonising the inside of Antarctic desert rocks [1] is a landmark for extreme environment microbial ecology. Some minerals

in Antarctic rocks are formed by biomineralization and/or transformed by microbial activity [2]. These minerals are examples of inorganic Biomarkers i.e., traces left by living microorganisms due to their biological activity.

Mineralized algal cell.

A

Diatoms within a sac

B

Raman spectra of *jarosite* deposits surrounding microbial fossils within the pores of sandstone from the Mount Fleming, Dry Valleys acquired at A, B and C points and marked on SEM-BSE images respectively;785 nm excitation, wavenumber range 200–1600 cm-1.

Mineralized photobiont cell of a lichen thallus

Conclusions: Only when we are really clear about the factscan we elaborate accurate theories

SEM_BSE image and EDS elemental distribution mapsof Al, Si, S, K, Ca and Fe of the sacs containing diatoms

SEM_BSE image and EDS elemental distribution mapsof Al, Si, S, K and Fe of the mineralized cells and pore.