1 1 Geomicrobiology Journal 20: 15-24 (2003) Iron-rich diagenetic minerals are biomarkers of microbial activity in Antarctic rocks Short title: Iron-rich minerals are biomarkers of microbial activity. Jacek Wierzchos, Carmen Ascaso 1 , Leopoldo García Sancho 2 and Allan Green 3 Servei de Microscopia Electronica, Universitat de Lleida, c/ Rovira Roure, 44, 25198 Lleida, Spain. jacekw@suic-me.udl.es ( correspondence address). 1 Centro de Ciencias Medioambientales, CSIC, c/ Serrano, 115 bis, 28006 Madrid, Spain. 2 Fac. Farmacia, Universidad Complutense, 28040 Madrid, Spain. 3 Biological Sciences, Waikato University, Hamilton, New Zealand. The cold, dry ecosystems of Antarctica have been shown to harbour traces left behind by microbial activity within certain types of rocks, but only two indirect biomarkers of cryptoendolithic activity in the Antarctic cold desert zone have been described to date. These are the geophysical and geochemical bioweathering patterns macroscopically observed in sandstone rock. Here we show that in this extreme environment, minerals are biologically transformed, and as a result, Fe-rich diagenetic minerals in the form of iron hydroxide nanocrystals and biogenic clays are deposited around chasmoendolithic hyphae and bacterial cells. Thus, when microbial life decays, these characteristic, neocrystalised minerals act as distinct biomarkers of previous endolithic activity. The ability to recognise these traces may have potential astrobiological implications since the Antarctic Ross Desert is considered a terrestrial analogue of a possible ecosystem on early Mars. Keywords: Antarctic granite, biomarkers, cryptoendoliths, microfossils One of the most outstanding features of life on the Antarctic continent is the predominance of rocks as substrates for living organisms (Friedmann 1982; Friedmann et al. 1988; Kappen 1993). Any discrete, unfavourable change in external conditions can result in the death of microscopic organisms, which may be followed by their disappearance possibly leading to trace microfossil formation. The sequence of events leading to the extinction of life in the Antarctic desert is considered to be a terrestrial analogue of the disappearance of possible life on early Mars (McKay et al. 1992). On these grounds, it is proposed that the investigation of such ecosystems could serve as a starting point for the development of methodologies, preparative techniques and new technologies aimed at detecting and evaluating traces of life such as microfossils and biomarkers. To date, only two biomarkers of the past activity of cryptoendoliths have been described: one is the geophysical bioweathering of rock surfaces forming characteristic exfoliation mosaic patterns (Kappen 1993; Sun and Friedmann 1999) and the other, the geochemical bioweathering patterns resulting from iron leaching observed in the surface layers of sandstone rocks (Friedmann and Weed 1987). However, both of these biomarkers have been observed macroscopically. It is clear that much further work is required in the detection of traces of past life in Antarctic rocks. Although the study of microorganisms colonising the inside of lithic materials confronts with considerable difficulties, the detailed mineralogical and biological characterisation of endolithic niches is essential for understanding the dynamic relationships between rock-dwelling microorganisms and their microhabitats. Recently, scanning electron microscopy with backscattered electron imaging (SEM-BSE) was successfully combined with microanalytical procedures (such as energy dispersive X-ray spectroscopy - EDS) for the in situ study of endolithic microorganisms (Wierzchos and Ascaso 1994; Ascaso and Wierzchos 1994; Ascaso et al. 1995). Such an approach also permits the chemical characterisation of mineral features (Wierzchos and Ascaso 1996). Here we report the use of SEM-BSE combined with EDS visualisation and microanalytical strategies to provide substantial information on biologically transformed minerals in granitic rock from Antarctica. Using high resolution transmission electron microscopy (HRTEM) on previously SEM-BSE-EDS examined microzones separated from the polished blocks, we were able to observe relationships between microorganisms and minerals around chasmoendolithic communities on the spatial nanometer scale (Wierzchos and Ascaso 1998).