Abstract
Abstract. The search for microfossils in the geological record has been a long-term challenge. Part of the problem comes from the difficulty of identifying such microfossils unambiguously, since they can be morphologically confused with abiotic biomorphs. One route to improve our ability to correctly identify microfossils involves studying fossilization processes affecting bacteria in modern settings. We studied the initial stages of fossilization of cyanobacterial cells in modern microbialites from Lake Alchichica (Mexico), a Mg-rich hyperalkaline crater lake (pH 8.9) hosting currently growing stromatolites composed of aragonite [CaCO3] and hydromagnesite [Mg5(CO3)4(OH)2 · 4(H2O)]. Most of the biomass associated with the microbialites is composed of cyanobacteria. Scanning electron microscopy analyses coupled with confocal laser scanning microscopy observations were conducted to co-localize cyanobacterial cells and associated minerals. These observations showed that cyanobacterial cells affiliated with the order Pleurocapsales become specifically encrusted within aragonite with an apparent preservation of cell morphology. Encrustation gradients from non-encrusted to totally encrusted cells spanning distances of a few hundred micrometers were observed. Cells exhibiting increased levels of encrustation along this gradient were studied down to the nm scale using a combination of focused ion beam (FIB) milling, transmission electron microscopy (TEM) and scanning transmission x-ray microscopy (STXM) at the C, O and N K-edges. Two different types of aragonite crystals were observed: one type was composed of needle-shaped nano-crystals growing outward from the cell body with a crystallographic orientation perpendicular to the cell wall, and another type was composed of larger crystals that progressively filled the cell interior. Exopolymeric substances (EPS), initially co-localized with the cells, decreased in concentration and dispersed away from the cells while crystal growth occurred. As encrustation developed, EPS progressively disappeared, but remaining EPS showed the same spectroscopic signature. In the most advanced stages of fossilization, only the textural organization of the two types of aragonite recorded the initial cell morphology and spatial distribution.
Highlights
The section of the microbialite sample collected at a 4 m depth and analyzed by confocal laser scanning microscopy (CLSM) was coated with goldpalladium and observed by scanning electron microscopy (SEM)
Bulk x-ray diffraction (XRD) analyses (Fig. S1) showed that Alchichica microbialites collected at 4 m were composed of two main phases: aragonite (CaCO3) and hydromagnesite (Mg5(CO3)4(OH)2 · 4H2O). These two mineral phases could be clearly discriminated by SEM in the backscattered electron mode (BSE): hydromagnesite appeared as light grey areas and composed the major part of the samples (85 % of the section observed), while aragonite appeared as bright discontinuous patches located preferentially at the surface of the samples in contact with microbial biofilms
A previous analysis of 16S rRNA genes showed that at least 34 phylotypes of Cyanobacteria were present in Alchichica microbialites, including 5 phylotypes of Pleurocapsales (Couradeau et al, 2011)
Summary
The microbialite sample analyzed in this study was collected at a depth of 4 m in Lake Alchichica in 2007 (Kazmierczak et al, 2011) and placed in a sterile zip plastic bag. Samples were dehydrated through a graded series of ethanol solutions (i.e., ethanol/water volume ratios at 30 %, 50 %, 70 %, 90 %, and 100 %), and progressively embedded in hard grade LR-white resin (Polysciences, Inc.). This was followed by incubation at 4 ◦C for 18 h in (1/1) (2/1) mixture of LR-white/ethanol and in pure LR-white resin. Polished sections were observed using an Olympus FluoViewTM FV1000 confocal laser scanning microscope. Despite the possible occurrence of crosstalks between DAPI and autofluorescence when using simultaneous excitation, Gerard et al (2013) have shown the efficiency of this approach on Lake Alchichica microbialites to image diverse cyanobacteria, including Pleurocapsales
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
