Microbially mediated carbonate precipitation in a hypersaline lake, Big Pond (Eleuthera, Bahamas)

Abstract

Microbial metabolism impacts the degree of carbonate saturation by changing the total alkalinity and calcium availability; this can result in the precipitation of carbonate minerals and thus the formation of microbialites. Here, the microbial metabolic activity, the characteristics and turnover of the extracellular polymeric substances and the physicochemical conditions in the water column and sediments of a hypersaline lake, Big Pond, Bahamas, were determined to identify the driving forces in microbialite formation. A conceptual model for organomineralization within the active part of the microbial mats that cover the lake sediments is presented. Geochemical modelling indicated an oversaturation with respect to carbonates (including calcite, aragonite and dolomite), but these minerals were never observed to precipitate at the mat-water interface. This failure is attributed to the capacity of the water column and upper layers of the microbial mat to bind calcium. A layer of high Mg-calcite was present 4 to 6 mm below the surface of the mat, just beneath the horizons of maximum photosynthesis and aerobic respiration. This carbonate layer was associated with the zone of maximum sulphate reduction. It is postulated that extracellular polymeric substances and low molecular weight organic carbon produced at the surface (i.e. the cyanobacterial layer) of the mat bind calcium. Both aerobic and anaerobic heterotrophic microbes consume extracellular polymeric substances (each process accounting for approximately half of the total consumption) and low molecular weight organic carbon, liberating calcium and producing inorganic carbon. The combination of these geochemical changes can increase the carbonate saturation index, which may result in carbonate precipitation. In conclusion, the formation and degradation of extracellular polymeric substances, as well as sulphate reduction, may play a pivotal role in the formation of microbialites both in marine and hypersaline environments.

Deformed microbial mat structures in a semiarid temperate coastal setting

“…On the other hand, the various metabolic activities of microbes alter the local geochemical and physicochemical conditions of sedimentary systems (Glunk et al, 2011), so microbial mats promote the precipitation or dissolution of some minerals. As the microbial mat has a high percent of organic matter which is rarely preserved in the fossil record, the associated mineralization provides the required lithification for preservation of microbial structures and activities.…”

The Pseudomonas aeruginosa PAO1 Two-Component Regulator CarSR Regulates Calcium Homeostasis and Calcium-Induced Virulence Factor Production through Its Regulatory Targets CarO and CarP

“…However, Ca 2ϩ can be toxic to cells at high concentrations; therefore, bacteria maintain a low-submicromolar intracellular concentration of Ca 2ϩ (6). P. aeruginosa may encounter environments where external Ca 2ϩ levels are in the millimolar range, varying from 10 mM in soil (7) to 40 mM in hypersaline lakes (8). As a plant and human pathogen, P. aeruginosa may be exposed to lower but also varying Ca 2ϩ levels.…”

High Diversity of Culturable Prokaryotes in a Lithifying Hypersaline Microbial Mat

“…Although photosynthetic fixation of CO 2 and associated alkalinization of the surrounding milieu drives CaCO 3 precipitation in poorly buffered environments (Arp et al 2001), precipitation in hypersaline and marine microbial mats is commonly attributed to sulfate-reduction (e.g., Baumgartner et al 2006;Visscher et al 2000) and exopolymer degradation (e.g., Arp et al 1998;Glunk et al 2011;Trichet and D efarge 1995). The relative importance of each of these mechanisms, however, is controversially discussed.…”