Microbe–mineral interactions: early carbonate precipitation in a hypersaline lake (Eleuthera Island, Bahamas)

Abstract

Microbialites (benthic microbial carbonate deposits) were discovered in a hypersaline alkaline lake on Eleuthera Island (Bahamas). From the edge towards the centre of the lake, four main zones of precipitation could be distinguished: (1) millimetre‐sized clumps of Mg‐calcite on a thin microbial mat; (2) thicker and continuous carbonate crusts with columnar morphologies; (3) isolated patches of carbonate crust separated by a dark non‐calcified gelatinous mat; and (4) a dark microbial mat without precipitation. In thin section, the precipitate displayed a micropeloidal structure characterized by micritic micropeloids (strong autofluorescence) surrounded by microspar and spar cement (no fluorescence). Observations using scanning electron microscopy (SEM) equipped with a cryotransfer system indicate that micrite nucleation is initiated within a polymer biofilm that embeds microbial communities. These extracellular polymeric substances (EPS) are progressively replaced with high‐Mg calcite. Discontinuous EPS calcification generates a micropeloidal structure of the micrite, possibly resulting from the presence of clusters of coccoid or remnants of filamentous bacteria. At high magnification, the microstructure of the initial precipitate consists of 200–500 nm spheres. No precipitation is observed in or on the sheaths of cyanobacteria, and only a negligible amount of precipitation is directly associated with the well‐organized and active filamentous cyanobacteria (in deeper layers of the mat), indicating that carbonate precipitation is not associated with CO2 uptake during photosynthesis. Instead, the precipitation occurs at the uppermost layer of the mat, which is composed of EPS, empty filamentous bacteria and coccoids (Gloeocapsa spp.). Two‐dimensional mapping of sulphate reduction shows high activity in close association with the carbonate precipitate at the top of the microbial mat. In combination, these findings suggest that net precipitation of calcium carbonate results from a temporal and spatial decoupling of the various microbial metabolic processes responsible for CaCO3 precipitation and dissolution. Theoretically, partial degradation of EPS by aerobic heterotrophs or UV fuels sulphate‐reducing activity, which increases alkalinity in microdomains, inducing CaCO3 precipitation. This degradation could also be responsible for EPS decarboxylation, which eliminates Ca2+‐binding capacity of the EPS and releases Ca2+ ions that were originally bound by carboxyl groups. At the end of these processes, the EPS biofilm is calcified and exhibits a micritic micropeloidal structure. The EPS‐free precipitate subsequently serves as a substrate for physico‐chemical precipitation of spar cement from the alkaline water of the lake. The micropeloidal structure has an intimate mixture of micrite and microspar comparable to microstructures of some fossil microbialites.

Microbial communities in a coastal cave: Cova des Pas de Vallgornera (Mallorca, Western Mediterranean)

“…Biologically induced mineralization of calcium carbonate can take place passively (metabolically driven changes in the chemistry around the living organisms) or actively (when the organism or its metabolic byproducts provide nucleation sites which allow the carbonate molecules to become particularly aligned in order to promote mineralization) (Northup & Lavoie, 2001;Jones, 2010). The bacterial surface (S-layers, specific proteins) has been suggested as possible nucleation sites that are probably just a side effect of their shape (Van Lith et al, 2003;Dupraz et al, 2004;Aloisi et al, 2006). These peculiarities can explain why some strains in a species are able to promote crystal formation and others not.…”

Micron-scale mapping of sulfur cycling across the oxycline of a cyanobacterial mat: a paired nanoSIMS and CARD-FISH approach

“…This method is an adaptation of a technique previously used in 35 S-radiolabel studies (Visscher et al, 2000;Dupraz et al, 2004;Wieland et al, 2005), where silver foil was coated with 35 SO 4. After incubation, radiography of the precipitated silver sulfide documented the presence of any 35 S in the silver sulfide, which was attributed to sulfate reduction during the incubation (Visscher et al, 2000).…”

“…The abundance of microbially laminated sedimentary strata attests to the ubiquity of such mats and highlights their potential to preserve biosignatures over geologic time (Grotzinger and Knoll, 1999). Although the laminated architecture of microbial mats is thought to result from strong spatial gradients in light intensity and redox in their uppermost few millimeters, there is evidence for ongoing, localized metabolic activity at depth (Visscher et al, 2000;Dupraz et al, 2004;Wieland et al, 2005). Despite many decades of investigation, the relationship between microbial activity (on the micron scale) and the resulting isotopic geochemical profiles remains poorly understood.…”

Origin and significance of lamination in Lower Cretaceous stromatolites and proposal for a quantitative approach

“…5-9). Both clotted and clotted-peloidal microfabrics are common in ancient and modem microbial carbonates, and they are widely attrib uted to calcification of microbial mats induced by heterotrophic bacteria (Dalrymple, 1965;Chafetz, 1986;Chafetz and Buczynski, 1992;Reitner, 1993;Dupraz et al, 2004;Riding and Tomas, 2006;Heindel et al, 2010;Spadafora et al, 201 0). Micritic fabrics similar to these in Leza coarse grained stromatolites also occur in present-day coarse-grained carbon ate stromatolites and thrombolites, generally filling the intergranular space of grain-rich microfabrics but not typically as relatively thick micritic laminae (Reid and Browne, 1991;Reid et al, 1995Reid et al, , 2003Feldmann, 1997;Feldmann and McKenzie, 1998;Planavsky and Ginsburg, 2009;Browne, 2011;CoUins, 2011, 2012), which is the case in Leza coarse-grained stromato lites, where micrite-rich laminae typically alternate with grain-rich lam inae and both display similar mm-scale thicknesses.…”