Exopolymeric substances of sulfate‐reducing bacteria: Interactions with calcium at alkaline pH and implication for formation of carbonate minerals

Abstract

Sulfate‐reducing bacteria (SRB) have been recognized as key players in the precipitation of calcium carbonate in lithifying microbial communities. These bacteria increase the alkalinity by reducing sulfate ions, and consuming organic acids. SRB also produce copious amounts of exopolymeric substances (EPS). All of these processes influence the morphology and mineralogy of the carbonate minerals. Interactions of EPS with metals, calcium in particular, are believed to be the main processes through which the extracellular matrix controls the precipitation of the carbonate minerals. SRB exopolymers were purified from lithifying mat and type cultures, and their potential role in CaCO3 precipitation was determined from acid‐base titrations and calcium‐binding experiments. Major EPS characteristics were established using infrared spectroscopy and gas chromatography to characterize the chemical functional groups and the sugar monomers composition. Our results demonstrate that all of the three SRB strains tested were able to produce large amounts of EPS. This EPS exhibited three main buffering capacities, which correspond to carboxylic acids (pKa = 3.0), sulfur‐containing groups (thiols, sulfonic and sulfinic acids – pKa = 7.0–7.1) and amino groups (pKa = 8.4–9.2). The calcium‐binding capacity of these exopolymers in solution at pH 9.0 ranged from 0.12gCa gEPS−1–0.15 gCa gEPS−1. These results suggest that SRB could play a critical role in the formation of CaCO3 in lithifying microbial mats. The unusually high sulfur content, which has not been reported for EPS before, indicates a possible strong interaction with iron. In addition to changing the saturation index through metabolic activity, our results imply that SRB affect the rock record through EPS production and its effect on the CaCO3 precipitation. Furthermore, EPS produced by SRB may account for the incorporation of metals (e.g. Sr, Fe, Mg) associated with carbonate minerals in the rock record.

Effects of Elevated Carbon Dioxide and Salinity on the Microbial Diversity in Lithifying Microbial Mats

“…In addition to cyanobacteria, sulfate-reducing could have produced a significant part of the EPS material. These organisms have been shown to produce copious amounts of EPS material, especially under hypersaline conditions [87]. Sequences recovered from microbialites exposed to elevated salinity also showed enrichments of taxa with high similarity to sulfate-reducing bacteria of the orders Desulfarculales and Desulfobacterales.…”

“…ions with the surrounding environment [87]. EPS content is an important factor to consider as it has been shown to have a significant impact on levels of free Ca 2+ potentially altering CaCO 3 precipitation rates (19).…”

Specific carbonate–microbe interactions in the modern microbialites of Lake Alchichica (Mexico)

“…EPSs are mainly composed of carbohydrates with small amounts of nucleic acids and proteins, and may contain functional groups such as phosphates and sulfates, which may bind divalent cations. These may be locally concentrated and then released during the degradation of decaying cyanobacterial colonies by heterotrophic bacteria and/or physicochemical degradation (for example, by UV irradiation or changes in pH values), resulting in increased local supersaturation and carbonate precipitation (Decho et al, 2005;Dupraz and Visscher, 2005;Braissant et al, 2007Braissant et al, , 2009Dupraz et al, 2009). This process may be operating in Alchichica microbialites as well.…”

“…Some metabolisms, such as photosynthesis and sulfate reduction, are known to promote carbonate precipitation by local alkalinization (that is, pH increase), whereas others, such as aerobic heterotrophy or fermentation, promote carbonate dissolution by acidification (Dupraz and Visscher, 2005;Dupraz et al, 2009). Exopolymeric substances (EPSs), consisting a mixture of carbohydrates, proteins and nucleic acids, excreted in various amounts, are also known to have a negative or positive role in carbonate formation, depending on their divalent cation binding capacity, rendering Ca 2 þ or Mg 2 þ , available or not, for precipitation (Braissant et al, 2007Dupraz et al, 2009). In addition, microbial activity may lead to the formation of particular mineralogical structures, which, when associated with remnants of cells or organic polymers, could be used as biosignatures of the past life on Earth and/or, possibly, other planets (for example, Benzerara et al, 2006;Benzerara and Menguy, 2009;Dupraz et al, 2009).…”

Magnesium isotope fractionation during hydrous magnesium carbonate precipitation with and without cyanobacteria

“…This conclusion is supported by observations at Salda Lake (Shirokova et al, 2011); Mg fractionation between lake water and hydromagnesite at Salda Lake is equal to that measured in this experimental study. The only exception to this might be Mg-carbonate formation in biofilms that exhibit a high cell:mineral ratio, as cyanobacteria can store Ca 2+ and Mg 2+ ions in organic envelopes (Braissant et al, 2003(Braissant et al, , 2007. In such systems, "organic" Mg originated from cell decay and lysis would be isotopically lighter than the bulk fluid phase.…”