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1、GEOLOGY, November 2008 879 INTRODUCTION The ability of living microbes to control and direct precipitation of minerals is undoubtedly an important geobiological process. Microbi- ally induced carbonate precipitation has been inferred in continental and marine settings under oxic conditions (Casanova
2、 et al., 1999). Nucleation of the carbonate mineral dolomite CaMg(CO3)2 is an excellent example of how microbes are able to overcome kinetic barriers to facilitate precipitation (Vasconcelos et al., 1995; Roberts et al., 2004; Wright and Wacey, 2005; Snchez-Romn, 2006). Despite comprehensive studies
3、 on dolomite formation at low temperatures, little is known about the role of microorganisms in its forma- tion and nucleation. The observed relation- ships between microbial cells and carbonate minerals in natural environments indicate that microbes directly participate in nucleation processes (Vas
4、concelos et al., 1995; van Lith et al., 2003; Snchez-Romn, 2006). Labora- tory experiments provide evidence supporting this hypothesis (Warthmann et al., 2000; Bosak and Newman, 2003; Snchez-Romn et al., 2007; Bontognali et al., 2008). Microbial cell surfaces and excreted extracellular polymeric sub
5、stances (EPS), which carry a net negative electric charge and have the capacity to bind Ca2+ ions, are frequently cited as being the sites of carbonate nucleation (Rivadeneyra et al., 1996; Dupraz et al., 2004; Snchez-Romn et al., 2007). Aloisi et al. (2006) reported that nucleation of Ca carbonate
6、occurs on nano- globules produced by sulfate-reducing bacteria and hypothesized that nucleation of carbonates on microbial cell material may have been the dominant mode of microbial carbonate forma- tion throughout the geologic record. Here we report the results of an investiga- tion of how microbes
7、 mediate the nucleation and formation of dolomite using aerobic cul- ture experiments at 25 and 35 C. We present a high-resolution transmission electron micros- copy (TEM), atomic force microscopy (AFM), scanning electron microscopy (SEM), and mineralogical study of the microbial dolomite precipitat
8、es. A nanoscale comparative study of modern and ancient dolomite samples was car- ried out to test the validity of the nanoglobule formation process and determine if the process can be observed in geological samples. Because dolomite is: (1) the most stable carbonate min- eral in the oceans, (2) a c
9、ommon mineral in ancient sedimentary rocks, and (3) rarely found to precipitate in modern environments, these observations can provide insight into an impor- tant microbial process in the geologic record. MATERIAL AND METHODS The experiments were designed with two mod- erately halophilic aerobic bac
10、teria, Halomonas meridiana ACAM 246 and Virgibacillus maris- mortui AJ009793 (see the GSA Data Reposi- tory1). These bacterial strains were surface inocu- lated on plates of solid medium D-1 (see the Data Repository) and incubated at 25 and 35 C. In order to detect the presence of precipitates, the
11、plates were examined periodically with light microscopy. We performed pH measurements at the end of mineral formation experiments by direct application of pH-indicator paper (Merck Spezial-Indikatorpapier) on the semisolid sur- face. Parallel control experiments without bac- teria and with dead bact
12、erial cells were run for all conditions. After 30 days of incubation, crystal precipitates were recovered from both cultures. To identify the mineral composition, X-ray diffraction (XRD) patterns of the precipi- tates were produced using a Bruker AXS D8 Advance Bragg-Brentano diffractometer with Cu
13、K radiation. A LEO 1530 scanning elec- tron microscope equipped with an energy dis- persive spectrometer (EDS) was used for imag- ing and elemental analysis of single crystals of dolomite samples from culture experiments and modern and ancient environments (see the Data Repository). To investigate t
14、he involvement of H. meridi- ana in the nucleation of dolomite, we carried out TEM, AFM, and Raman microscopy studies (see the Data Repository). Experimental Results Both V. marismortui and H. meridiana induce the precipitation of dolomite CaMg(CO3)2 and hydromagnesite Mg5(CO3)4(OH)24H2O at 25 and 3
15、5 C (Fig. DR1 in the Data Repository). Dolomite was found to be a major constituent in all culture experiments. No precipitate formation was observed in the control experiments. In all of the culture experiments, the time required for the initiation and extensive precipitation decreased with increas
16、ed temperature (Table DR1). In all cultures, the quantity of crystals increased with increasing incubation time. The rate of crys- Geology, November 2008; v. 36; no. 11; p. 879882; doi: 10.1130/G25013A.1; 3 fi gures; Data Repository item 2008225. 2008 The Geological Society of America. For permissio
17、n to copy, contact Copyright Permissions, GSA, or editinggeosociety.org. 1GSA Data Repository item 2008225, supplemen- tary methods, Table DR1 (biochemical conditions of the culture media), Figure DR1 (X-ray diffractograms of the crystal formed in culture experiments), Figure DR2 (Raman spectrum of
18、the extracellular organic fi lm), and Figure DR3 (scanning electron microscope images of dolomite nanocrystal aggregates show- ing granulated texture), is available online at www. geosociety .org/pubs/ft2008.htm, or on request from editinggeosociety.org or Documents Secretary, GSA, P.O. Box 9140, Bo
19、ulder, CO 80301, USA.*E-mail: monica.sanchezerdw.ethz.ch Aerobic microbial dolomite at the nanometer scale: Implications for the geologic record Mnica Snchez-Romn1*, Crisgono Vasconcelos1, Thomas Schmid2, Maria Dittrich3, Judith A. McKenzie1, Renato Zenobi2, Maria A. Rivadeneyra4 1ETH-Zrich, Geologi
20、cal Institute, 8092 Zrich, Switzerland 2ETH-Zrich, Department of Chemistry and Applied Biosciences, 8093 Zrich, Switzerland 3EAWAG, Swiss Federal Institute of Aquatic Sciences and Technology, 6047 Kastanienbaum, Switzerland 4Department of Microbiology, Faculty of Pharmacy, University of Granada, 180
21、71 Granada, Spain ABSTRACT Microbial experiments are the only proven approach to produce experimental dolomite under Earths surface conditions. Although microbial metabolisms are known to induce dolo- mite precipitation by favoring dolomite growth kinetics, the involvement of microbes in the dolomit
22、e nucleation process is poorly understood. In particular, the nucleation of microbially mediated dolomite remains a matter for investigation because the metabolic diversity involved in this process has not been fully explored. Herein we demonstrate that Halomonas meridiana and Virgibacillus marismor
23、tui, two moderately halophilic aerobic bacteria, mediate primary precipitation of dolomite at low temperatures (25, 35 C). This report emphasizes the bio- mineralogical implications for dolomite formation at the nanometer scale. We describe nucle- ation of dolomite on nanoglobules in intimate associ
24、ation with the bacterial cell surface. A combination of both laboratory culture experiments and natural samples reveals that these nanoglobule structures may be: (1) the initial step for dolomite nucleation, (2) preserved in the geologic record, and (3) used as microbial tracers through time and/or
25、as a proxy for ancient microbial dolomite, as well as other carbonate minerals. 880 GEOLOGY, November 2008 tal growth was higher at 35 C than 25 C, as observed by optical microscopy. A signifi cant increase in pH occurred in the cultures with liv- ing bacteria, from the original pH 7.2 of the D-1 me
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