Quantifying the Carboxyl Group Density of Microbial Cell Surfaces as a Function of Salinity: Insights Into Microbial Precipitation of Low-Temperature Dolomite

Abstract

Recent laboratory experiments have documented microbial mediation of low temperature dolomite precipitation via nucleation on microbial surfaces that have high (>0.06 groups Å0.06 groups Å-2) carboxyl group densities. It is hypothesized that carboxyl groups form a cell wall complex with Mg2+, dewatering the magnesium ion and overcoming kinetic barriers that allow dolomite formation at low temperature. Three microorganisms, two that precipitate dolomite in laboratory and field settings (Desulfovibrio brasiliensis; Haloferax sulfurifontis) and a control organism, not associated with dolomite precipitation (Shewanella putrefaciens), were selected for this research and were grown in media of varying salinities. Acid-base titrations were performed on the microorganisms, revealing an increase in buffering capacity of microorganisms grown in higher salinity growth conditions. Using ProtoFit 2.1 (Rev 1), site density values (mol/kg) were calculated for various functional groups (carboxyl, phosphoryl, and amine). For D. brasiliensis and H. sulfurifontis a 175% increase in carboxyl group density was measured (when doubling ionic strength from 0.5 M to 1.0 M) and a decrease in carboxyl group density by 47% (when reducing ionic strength from 3.2 M to 0.8 M), respectively. S. putrefaciens, the control specimen, also increased carboxyl group density by 70% (when increasing ionic strength from 0.1 M to 2.0 M). Calculated carboxyl group density, normalized to surface area (Å2), revealed 5.56 x 10-1 sites Å-2 for D. brasiliensis (at 1.0 M ionic strength), 1.15 x 10-1 sites Å-2 for S. putrefaciens (at 2.0 M ionic strength), and 1.53 sites Å-2 for H. sulfurifontis (at 3.2 M ionic strength), the cell wall of all three microorganisms show an increased carboxyl group density at higher salinities, and a lower carboxyl group density when grown at lower salinities. These data demonstrate that increases of carboxyl group density are evidence for environmental control on microorganisms. Microorganisms respond to increasing salinities by modifying their exterior cell wall as a coping mechanism to high ionic strength and osmotic pressure. Under these conditions, typical of mixing zones of fresh and marine waters, sabkha and hypersaline lagoon environments, microbial cell walls may serve as nucleation sites for low temperature dolomite when geochemical conditions for dolomite are favored.