Evidencing the role of carbonic anhydrase in the formation of carbonate minerals by bacterial strains isolated from extreme environments in Qatar

Abstract

Calcium carbonate, one of the most abundant minerals in the geological records is considered as primary source of the carbon reservoir. The role of microorganisms in the biotic precipitation of calcium carbonate has been extensively investigated, especially at extreme life conditions. In Qatar, Sabkhas which are microbial ecosystems housing biomineralizing bacteria, have been carefully studied as unique sites of microbial dolomite formation. Dolomite (CaMg(CO₃)₂ is an important carbonate mineral forming oil reservoir rocks; however, dolomite is rarely formed in modern environments. The enzyme carbonic anhydrase is present in many living organisms, performs interconversion between CO₂ and the bicarbonate ion. Thus, carbonic anhydrase is expected to accelerate both carbonate rock dissolution and CO₂ uptake at the same time, serving as carbonite source to carbonites-forming bacteria. This study gathered cross-linked data on the potential role of the carbonic anhydrase excreted by mineral-forming bacteria, isolated from two different extreme environments in Qatar. Dohat Faishakh Sabkha, is a hypersaline coastal Sabkha, from where various strains of the bacterium Virgibacillus were isolated. Virgibacillus can -not only-mediate carbonate mineral formation, but also contributes to magnesium incorporation into the carbonate minerals, leading to the formation of high magnesium calcite. The latter is considered as precursor for dolomite formation. In addition, bacterial strains isolated from marine sediments, surrounding coral reef in Qatar sea, would provide additional knowledge on the role of carbonic anhydrase in mineral formation. Here, the quantification of the two mostly described activities of carbonic anhydrase; esterase and hydration reactions were performed. Mineral-forming strains were shown to exhibit high activities as opposed to the non-forming minerals, which confirms the relation between the presence of active carbonic anhydrase combined with elevated metabolic activity and the biomineralizing potential of the bacterial strains. The highest specific intracellular carbonic anhydrase activity; as both esterase and hydration (i.e., 66 ± 3 and 583000 ± 39000 WAU/10⁸ cells respectively), was evidenced in mineral-forming strains as opposed to non-mineral forming strains (i.e., 6 ±. 0.5 and 1223 ± 61 WAU/10⁸cells) respectively. These findings would contribute to the understanding of the mechanism of microbially mediated carbonate precipitation. This role may be both in capturing CO₂ as source of carbonate, and partial solubilization of the formed minerals allowing incorporation of Mg instead of calcium, before catalyzing again the formation of more deposition of carbonates.

Keywords: Carbonate formation; Carbonic anhydrase; Extreme environments; Marine sediments; Sabkha; Virgibacillus.

Graphical abstract

Figure 1

Map of Qatar showing sampling point.

Figure 2

SEM/EDS analysis of crystals formed by A) Vibrio alginolyticus (RS3) on MD1, B) and Vibrio alginolyticus (RS3) on MD1-. Bacterial cells are shown to be surrounding the crystal with only part of it appearing to be smooth and spherical, C) Psychrobacter sp. (RS5) strains on MD3, the crystal formed appears to be a perfect sphere, partially covered with bacterial cells, D) by Psychrobacter sp. (RS5) strains on MD2.

Figure 3

XRD patterns of minerals formed by the strain of Psychrobacter sp. (RS5) on MD1 medium compared to minerals formed by Virgibacillus reference strains. H: Halite, MHC: Monohydrocalcite, HMC: High Magnesium Calcite.

Figure 4

Highest intracellular and extracellular esterase specific activities for the studied strains recorded after 72 h of incubation.

Figure 5

Highest intracellular and extracellular hydration specific activities for the studied strains recorded after 72 h of incubation.