Stromatolitic knobs in Storr's Lake (San Salvador, Bahamas): a model system for formation and alteration of laminae

Abstract

The initial lamination in young, metabolically active Scytonema knobs developing in Storr's Lake (Bahamas) results from the iterative succession of two different stages of microbial growth at the top of this microbialite. Stage 1 is dominated by vertically oriented cyanobacterial filaments and is characterized by a high porosity of the fabric. Stage 2 shows a higher microbial density with the filaments oriented horizontally and with higher carbonate content. The more developed, dense microbial community associated with Stage 2 of the Scytonema knobs rapidly degrades extracellular organic matter (EOM) and coupled to this, precipitates carbonate. The initial nucleation forms high-Mg calcite nanospheroids that progressively replace the EOM. No precipitation is observed within the thick sheath of the Scytonema filaments, possibly because of strong cross-linking of calcium and EOM (forming EOM-Ca-EOM complexes), which renders Ca unavailable for carbonate nucleation (inhibition process). Eventually, organominerals precipitate and form an initial lamina through physicochemical and microbial processes, including high rates of photosynthetic activity that lead to (13) C-enriched DIC available for initial nucleation. As this lamina moves downward by the iterative production of new laminae at the top of the microbialite, increased heterotrophic activity further alters the initial mineral product at depth. Although some rare relic preservation of 'Stage 1-Stage 2' laminae in subfossil knobs exists, the very fine primary lamination is considerably altered and almost completely lost when the knobs develop into larger and more complex morphologies due to the increased accommodation space and related physicochemical and/or biological alteration. Despite considerable differences in microstructure, the emerging ecological model of community succession leading to laminae formation described here for the Scytonema knobs can be applied to the formation of coarse-grained, open marine stromatolites. Therefore, both fine- and coarse-grained extant stromatolites can be used as model systems to understand the formation of microbialites in the fossil record.

Changing Microspatial Patterns of Sulfate-Reducing Microorganisms (SRM) during Cycling of Marine Stromatolite Mats

“…This has posited an important role of the SRM in the precipitation of laminae in Type-2 stromatolite mats. A similar role for SRM in precipitation of carbonate laminae has been described in lithifying hypersaline mats [22–24]. …”

Microbial Extracellular Polymeric Substances (EPSs) in Ocean Systems

“…These systems, therefore, offer excellent platforms from which to study how EPS may influence the precipitation of carbonate minerals. Thrombolites (Mobberley et al, 2015) and tufa deposits (Zippel and Neu, 2011; Dupraz et al, 2013) are other forms of microbialites.…”

External controls on the distribution, fabrics and mineralization of modern microbial mats in a coastal hypersaline lagoon, Cayo Coco (Cuba)

“…Previous works (e.g. Arp et al ., ; Dupraz & Visscher, ; Dupraz et al ., , ; Glunk et al ., ) have proposed conceptual geomicrobiological models for the lithification of mats resulting in laminated microbialites. These models suggest the succession of microbial communities as a dominant feature and include kinetic considerations for early nucleation and precipitation of laminae, resulting in the preservation of subfossil microbialites.…”